ECLIPSES
The total or partial obscuration of light from a celestial body as it passes through the shadow of another body is known as eclipse. On earth we are familiar with the solar and lunar eclipses. The moon, as a satellite of the earth, revolves round it; in the process it is bound to come in- between the sun and the earth at times. When both sun and moon are on the same side of the earth (i.e., in conjunction) so that all three bodies lie approximately on a straight line, the possibility exists for an eclipse of the sun, or solar eclipse.
It is a rare occurrence because the moon is so small and the plane of its orbit is tilted about 5° with respect to the plane of the ecliptic. It is for this reason that eclipses do not occur every month.' (Ectiptic is the apparent track of the sun throughout the year as a result of the earth's motion around it. The plane of the ecliptic is the plane passing through this path coincident with the place of the earth's orbit, and is imagined to be horizontal, passing through the globe's centre.) }Vhen the moon and sun are on the opposite sides of the earth (Le., in opposition), the possibility exists for an eclipse of the moon, or lunar eclipse. In this case the earth's shadow falls on the moon partly or completely covering it for a short while.
A solar eclipse occurs between sunrise and sunset at new moon; a lunar eclipse occurs at full moon. The chances of our seeing a lunar eclipse from a given place on earth are much better than for seeing a solar eclipse. In one year, up to se:ven eclipses can occur; either five solar and two lunar, or four solar and three lunar.
Incidentally, in a solar eclipse, the shadow of the moon
is some 45 km wide and crosses a belt of the earth several kilometres long.
In a lunar eclipse the earth's shadow is far larger than the moon and can cover the moon's surface for about 3 hours. A total lunar eclipse may last up to 1 hour 40 minutes. The moon does not become completely dark during most lunar eclipses. In many cases, it becomes reddish. The earth's atmosphere bends part of the sun's light around the earth and towards the moon.' This light is red because the atmosphere scatters the other colours present in sunlight in greater amounts than it does red, In the case of a total solar eclipse, the totally darkened period may be as long as 7 minutes and 40 seconds, but the average is about 2Vz minutes. The path of totality is wide but not wider than about 274 kilometres.
Tuesday, October 27, 2009
Perigean and Apogean Tides
Perigean and Apogean Tides When the moon is nearest to the earth in its orbit (at perigee), its tide-producing power is greater than average, resulting in perigean tides. These are 15-20 per cent greater than average. When the moon is farthest from the earth (in apogee), the tides are
called apogean tides, which are about 15-20 per cent les~ than average. Coincidence of spring and perigean tides results in an abnormally great tidal range, while when neap and apogean tides coincide the range is abnormally small.
RIvER TIDES Tides are experienced in the lower parts of many of the great rivers. These are known as tidal rivers, where either the coastal area has recently subsided or the ocean level has risen causing the lower part of the river to be drowned. Such water bodies are, actually, extensions of the sea itself, or estuaries. River tides are distinguishable from ocean tides by one characteristic: the interval between a low tide and the next high tide is shorter than the interval between a high tide and the next low tide.
TIDAL BORES When a tidal wave meets a tidal river, or estuary, a tidal bore is formed: where the outgoing river currents are strong and the tidal river rather shallow and funnel-shaped, the rapidly rising high water advances upstream lik~ a high 'vertical wall, known as tidal bore. Bores occur at river mouths that face the direction of tidal surge and where there is a large tidal range. Rivers like the Amazon, Hooghly, Colorado, Tsientang, Elbe, Yangtze are characterised by tidal bores.
TIDAL CURRENTS The tidal changes in ocean level result in stream-like movements of water in and out of bays and tidal rivers known as tidal currents. Unusually strong tidal currents result where bays connect with the open ocean by narrow inlets.
called apogean tides, which are about 15-20 per cent les~ than average. Coincidence of spring and perigean tides results in an abnormally great tidal range, while when neap and apogean tides coincide the range is abnormally small.
RIvER TIDES Tides are experienced in the lower parts of many of the great rivers. These are known as tidal rivers, where either the coastal area has recently subsided or the ocean level has risen causing the lower part of the river to be drowned. Such water bodies are, actually, extensions of the sea itself, or estuaries. River tides are distinguishable from ocean tides by one characteristic: the interval between a low tide and the next high tide is shorter than the interval between a high tide and the next low tide.
TIDAL BORES When a tidal wave meets a tidal river, or estuary, a tidal bore is formed: where the outgoing river currents are strong and the tidal river rather shallow and funnel-shaped, the rapidly rising high water advances upstream lik~ a high 'vertical wall, known as tidal bore. Bores occur at river mouths that face the direction of tidal surge and where there is a large tidal range. Rivers like the Amazon, Hooghly, Colorado, Tsientang, Elbe, Yangtze are characterised by tidal bores.
TIDAL CURRENTS The tidal changes in ocean level result in stream-like movements of water in and out of bays and tidal rivers known as tidal currents. Unusually strong tidal currents result where bays connect with the open ocean by narrow inlets.
Neap and Spring Tides
Neap and Spring Tides The sun, because of its greater distance from the earth, though much larger in size than the moon, has a tide-producing power that is only fiveelevenths the tide-producing power of the moon. When the earth, the moon and the sun are in a straight line,' the gravitational force is at its greatest because tide-producing forces of both sun and moon complement each other and they 'pull' together. This produces tides of unusually great range, called the spring tides. These occur about twice a month: at new moon when the sun and the moon are in conjunction, and at full moon when they are in opposition.
When the earth, the moon and the sun are not in a straight line, but are at right angles to the earth, the gravitational force is less as the sun and the moon are not pulling together. This happens during phases of first and third quarter, Le., at half moon, the sun's tide-producing force tends to balance the tige-producing force of the moon, resulting in tides of unusually small range known as neap
tide? Amplitude or tidal range refers to the difference' between high tide and low tide; it is high at spring tides and low at neap tides.
When the earth, the moon and the sun are not in a straight line, but are at right angles to the earth, the gravitational force is less as the sun and the moon are not pulling together. This happens during phases of first and third quarter, Le., at half moon, the sun's tide-producing force tends to balance the tige-producing force of the moon, resulting in tides of unusually small range known as neap
tide? Amplitude or tidal range refers to the difference' between high tide and low tide; it is high at spring tides and low at neap tides.
MOON AND TIDES
MOON AND TIDES Tides are defined as slight oscillations of sea level that occur approximately twice a day and attain exaggerated proportions in marginal seas, straits and estuaries. The major cause of the tides is the gravitational pull of the moon and the sun. Though both the sun and the moon exert gravitational force on earth to produce tides, the moon, by nature of its closeness to the earth has a greater control over the timings of the" tidal rises and falls.
Lunar Tides As the moon travels in its orbit in the same direction as the earth's rotation, a period of 24 hours, 50 minutes elapses between two successive occasions when the moon is vertically above a point. The highest level the water reaches is called a high tide and the lowest level is called a low tide. High and low tides occur twice each during the period of 24 hours, 50 minutes, giving an interval of about 121h hours between successive high (or low) tides.
Under the influence of the moon, water at H2 is pulled towards the moon more than towards the earth and therefore water piles up at H2 forming a high tide. The earth is pulled towards the moon more than the water at Hl; therefore water lags behind and piles up at Hl forming a high tide. The moon's pull causes water to be drawn from Ll and L2; therefore there are low tides there.
Lunar Tides As the moon travels in its orbit in the same direction as the earth's rotation, a period of 24 hours, 50 minutes elapses between two successive occasions when the moon is vertically above a point. The highest level the water reaches is called a high tide and the lowest level is called a low tide. High and low tides occur twice each during the period of 24 hours, 50 minutes, giving an interval of about 121h hours between successive high (or low) tides.
Under the influence of the moon, water at H2 is pulled towards the moon more than towards the earth and therefore water piles up at H2 forming a high tide. The earth is pulled towards the moon more than the water at Hl; therefore water lags behind and piles up at Hl forming a high tide. The moon's pull causes water to be drawn from Ll and L2; therefore there are low tides there.
THE MOON: EARTH'S ONLY SATELLITE
Salient features of the moon, the earth's only satellite,
are as follows:
. The moon is earth's only satellite.
. The mean distance between the earth and the moon
is about 3,85,000 km.
. Moon has a diameter of about 3,480 'km and a mass
1 of about 81 that of the earth.
. The orbit of the moon is elliptical.
. The time taken by the moon to complete one revolution around the earth is 27 days, 7 hours, 43 minutes and 11 1/2 seconds, or about 273 days. (This
period is called sidereal month.)
.The period of moon's revolution of the sun is 29.53 days on an average, and is called synodic month.
. The moon's period of rotation around its axis and revolution round the earth is same.
. Moon at all times keeps the same side towards the
earth.
. The plane of the moon's orbit is inclined at an angle
of 5° 09' to the plane of the ecliptic.
. When the sun and the moon lie on the same side
of the earth, the moon is said to be in conjunction
with the sun.
. When the sun anc:l. the moon are on opposite sides
of the earth, they are sail.:! to be in opposition.
. The major cause of sea-tides is the gravitational pull of the moon. The sun, because of its greater distance from the earth, has a tide-producing power that is only five-elevenths the tide-producing power of the moon.
. When the moon is between the earth and the sun, the position is called the New Moon. On New Moon, the part of the moon facing the earth is in complete darkness. The moon takes different shapes on different days after the New Moon: waxing crescent (after 3 days), first quarter (7th day), waxing gibbuns (10th day), full moon (14th day), and waning gibbous (17th day), last quarter (21st day), and waning crescent (25th day).
are as follows:
. The moon is earth's only satellite.
. The mean distance between the earth and the moon
is about 3,85,000 km.
. Moon has a diameter of about 3,480 'km and a mass
1 of about 81 that of the earth.
. The orbit of the moon is elliptical.
. The time taken by the moon to complete one revolution around the earth is 27 days, 7 hours, 43 minutes and 11 1/2 seconds, or about 273 days. (This
period is called sidereal month.)
.The period of moon's revolution of the sun is 29.53 days on an average, and is called synodic month.
. The moon's period of rotation around its axis and revolution round the earth is same.
. Moon at all times keeps the same side towards the
earth.
. The plane of the moon's orbit is inclined at an angle
of 5° 09' to the plane of the ecliptic.
. When the sun and the moon lie on the same side
of the earth, the moon is said to be in conjunction
with the sun.
. When the sun anc:l. the moon are on opposite sides
of the earth, they are sail.:! to be in opposition.
. The major cause of sea-tides is the gravitational pull of the moon. The sun, because of its greater distance from the earth, has a tide-producing power that is only five-elevenths the tide-producing power of the moon.
. When the moon is between the earth and the sun, the position is called the New Moon. On New Moon, the part of the moon facing the earth is in complete darkness. The moon takes different shapes on different days after the New Moon: waxing crescent (after 3 days), first quarter (7th day), waxing gibbuns (10th day), full moon (14th day), and waning gibbous (17th day), last quarter (21st day), and waning crescent (25th day).
SOLAR TIME
SOLAR TIME Solar Time, or sun time, is determined in two ways. Apparent Solar Time is the system of days and hours which goes strictly by the sun itself and is thus continually changing in value from day to day. It is the time between two successive transits of the sun over the same meridian. Mean Solar Time is the system of days and hours mathematically computed in order to give the average value to every hour and day. It is 24 hours. The difference in value between apparent and mean solar time is known as equation of time.
SIDEREAL TIME OR STAR TIME Whereas the sun moves sometimes slow and sometimes fast, with a total range of half-an-hour from one extreme to the other, the stars provide a perfect time-piece. But they do not operate according to the conventional systems of days and hours that our calendars follow. A star takes 23 hours 56 minutes of mean solar time and 4.09 seconds to complete one rotation of the earth, covering 360°. This interval is called a sidereal day, which is thus about 4 minutes shorter than
the mean solar day of 24 hours.
SIDEREAL TIME OR STAR TIME Whereas the sun moves sometimes slow and sometimes fast, with a total range of half-an-hour from one extreme to the other, the stars provide a perfect time-piece. But they do not operate according to the conventional systems of days and hours that our calendars follow. A star takes 23 hours 56 minutes of mean solar time and 4.09 seconds to complete one rotation of the earth, covering 360°. This interval is called a sidereal day, which is thus about 4 minutes shorter than
the mean solar day of 24 hours.
International Date Line
INTERNATIONAL DATE LINE The 180th meridiar designated the International Date Line by the Internat Meridian Conference in 1884. It was adopted in ord avoid the confusion of the difference of one day travellers would face while travelling across the ~ Counting from Greenwich Meridian, the date immedi east of this line is one day ahead or 12 hours faster in the west.
Though the 180° meridian generally falls the ocean, the International Date Line has had to de both eastward and westward in order to permit CE landmasses and islands to have the same calendar day.
It passes through the Arctic Ocean, Chukchi sea, a, Wrangel Island and Russian landmass, passes thr, Bering Strait, veers again to avoid Aleutian Islands,
through Pacific Ocean. A few degrees south of the eql the date line has shifted 71ho eastward, avoiding Fiji Tonga island groups which have the same days as , Zealand.
CALENDAR The time taken for the earth to com] one orbit of the sun is called a year. It is measured number of ways-sidereal year, measured with resp€l fixed stars; solar year, time taken by sun to make successive appearances at the point of Aries; and the calendar year which is regulated using leap years so th is equal to that of the solar year (365.2419 mean solar d; To the usual year of 365 days, one day is added in month of February every fourth year, making it a leap year.
This correction, being too large, the leap year is omitted in the century years (1800, 1900, etc.) unless the year is divisible by 400. Thus, the year 2000 was a leap year.
The year is divided into months, originally calculated by the revolution of the moon around the earth. But the lunar month of about 29112 days has now been made slightly longer, whereby the months and seasons occur at the same time every year. Months are further divided into weeks, which are arbitrary divisions made by man, each unit having seven days.
In the ancient past, too, man had devised systems of measuring time. The earliest Roman calendar based on agricultural months is an example. This, however, had drawbacks and was corrected by Julius Ceasar by adding 90 days to 46 Be and declaring each year thereafter to be 365 days, every fourth year being a leap year. But this meant a loss of about three-quarters of a day (18 hours) in a century. It was Pope Gregory XIII who gave the final touch to the calendar, which has now been universally adopted, by decreeing that the last year of a century would be a leap year only if it were divisible by 400. Though near perfect, the Gregorian calendar is not uniformly divisible into quarters of months, neither are the months divisible into equal number of days.
The Babylonians, Sumerians and ancient Egyptians had lunar calendars.
Though the 180° meridian generally falls the ocean, the International Date Line has had to de both eastward and westward in order to permit CE landmasses and islands to have the same calendar day.
It passes through the Arctic Ocean, Chukchi sea, a, Wrangel Island and Russian landmass, passes thr, Bering Strait, veers again to avoid Aleutian Islands,
through Pacific Ocean. A few degrees south of the eql the date line has shifted 71ho eastward, avoiding Fiji Tonga island groups which have the same days as , Zealand.
CALENDAR The time taken for the earth to com] one orbit of the sun is called a year. It is measured number of ways-sidereal year, measured with resp€l fixed stars; solar year, time taken by sun to make successive appearances at the point of Aries; and the calendar year which is regulated using leap years so th is equal to that of the solar year (365.2419 mean solar d; To the usual year of 365 days, one day is added in month of February every fourth year, making it a leap year.
This correction, being too large, the leap year is omitted in the century years (1800, 1900, etc.) unless the year is divisible by 400. Thus, the year 2000 was a leap year.
The year is divided into months, originally calculated by the revolution of the moon around the earth. But the lunar month of about 29112 days has now been made slightly longer, whereby the months and seasons occur at the same time every year. Months are further divided into weeks, which are arbitrary divisions made by man, each unit having seven days.
In the ancient past, too, man had devised systems of measuring time. The earliest Roman calendar based on agricultural months is an example. This, however, had drawbacks and was corrected by Julius Ceasar by adding 90 days to 46 Be and declaring each year thereafter to be 365 days, every fourth year being a leap year. But this meant a loss of about three-quarters of a day (18 hours) in a century. It was Pope Gregory XIII who gave the final touch to the calendar, which has now been universally adopted, by decreeing that the last year of a century would be a leap year only if it were divisible by 400. Though near perfect, the Gregorian calendar is not uniformly divisible into quarters of months, neither are the months divisible into equal number of days.
The Babylonians, Sumerians and ancient Egyptians had lunar calendars.
TIME AND LONGITUDE
TIME
To avoid confusion and make the study of time relations simple, it is necessary to think of the earth as stationary and of the sun as completing one circuit about the earth every 24 hours.
TIME AND LONGITUDE Time and longitude are related. As the earth rotates, every place experiences the phenomena of sunrise, noon and sunset. It is noop when the sun is at its highest position in the sky. This position of the sun is called zenith. (At noon, shadows are at their shortest.)
One hour of time is equivalent to 15° of longitude. This can be calculated and understood from the fact that the earth completes one rotation on its axis in 24 hours, so in eaeh hour, it covers 15° of longitude to cover 360° of longitude in total. This also means that 1° of longitude is covered every 4 minutes during the daily rotation of the earth. This equality forms the basis of all calculations concerning time belts of the globe.
LOCAL TIME AND STANDARD TIME Local time is the time of the day at a place as indicated by the position of the sun, Le., it is the mean solar time based on thE; local meridian. All the places located on the same meridian, regardless of how far they are from each other, have the same local time. Places located on different meridians have, unlike local times, times differing by four minutes for every degree of longitude. If, however, each place had its local time, there would be chaos-we would have to spend our time adjusting clocks and watches. So, we have the standard time, which is the mean time of a particular meridian (generally a central one), adopted as a system of time for the entire country, and all clocks within this belt are set to a single time. Each time-zone or country differs from Greenwich Mean Time in whole or half-hour units.
The general understanding among the countries of the world is that the standard meridian selected for an areal country is in multiples of 7.50 longitude. Every 7.50 of longitude makes a difference of 30 minutes or half-an-hour. This enables simple calculation of time differences.
As the earth rotates from west to east, places
east see the sun first. Therefore, (i) For each 10 of Ion towards the east, a time of four minutes has to be , (ii) For each 10 of longitude towards the west, a ti four minutes has to be subtracted.
Most countries adopted a standard time followi International Meridian Conference of 1884, held in ington.
TIME ZONES A time zone is an area in the recording the same time. As it takes the earth 24 to make one rotation, Standard Fleming, a Can suggested dividing the earth into 24 time zones. Time of the world are described in terms of the differeJ number of hours between the standard meridian (] zone and the Greenwich meridian. In order to distir the direction of these zones, the time for all places east of Greenwich is designated fast, and time for all place~ of Greenwich is designated slow.
The Greenwich meridian at 0° longitude passes through Greenland Sea and N gian Sea, and the countries of the United Kingdom, F. Spain, Algeria, Mali, Burkina Faso and Ghana, and through the South Atlantic Ocean. In some countries a large West-East extent, more than one time zone be necessary, e.g., the USA and Canada both have fiVE zones; Australia has three; Russia has eleven.
To avoid confusion and make the study of time relations simple, it is necessary to think of the earth as stationary and of the sun as completing one circuit about the earth every 24 hours.
TIME AND LONGITUDE Time and longitude are related. As the earth rotates, every place experiences the phenomena of sunrise, noon and sunset. It is noop when the sun is at its highest position in the sky. This position of the sun is called zenith. (At noon, shadows are at their shortest.)
One hour of time is equivalent to 15° of longitude. This can be calculated and understood from the fact that the earth completes one rotation on its axis in 24 hours, so in eaeh hour, it covers 15° of longitude to cover 360° of longitude in total. This also means that 1° of longitude is covered every 4 minutes during the daily rotation of the earth. This equality forms the basis of all calculations concerning time belts of the globe.
LOCAL TIME AND STANDARD TIME Local time is the time of the day at a place as indicated by the position of the sun, Le., it is the mean solar time based on thE; local meridian. All the places located on the same meridian, regardless of how far they are from each other, have the same local time. Places located on different meridians have, unlike local times, times differing by four minutes for every degree of longitude. If, however, each place had its local time, there would be chaos-we would have to spend our time adjusting clocks and watches. So, we have the standard time, which is the mean time of a particular meridian (generally a central one), adopted as a system of time for the entire country, and all clocks within this belt are set to a single time. Each time-zone or country differs from Greenwich Mean Time in whole or half-hour units.
The general understanding among the countries of the world is that the standard meridian selected for an areal country is in multiples of 7.50 longitude. Every 7.50 of longitude makes a difference of 30 minutes or half-an-hour. This enables simple calculation of time differences.
As the earth rotates from west to east, places
east see the sun first. Therefore, (i) For each 10 of Ion towards the east, a time of four minutes has to be , (ii) For each 10 of longitude towards the west, a ti four minutes has to be subtracted.
Most countries adopted a standard time followi International Meridian Conference of 1884, held in ington.
TIME ZONES A time zone is an area in the recording the same time. As it takes the earth 24 to make one rotation, Standard Fleming, a Can suggested dividing the earth into 24 time zones. Time of the world are described in terms of the differeJ number of hours between the standard meridian (] zone and the Greenwich meridian. In order to distir the direction of these zones, the time for all places east of Greenwich is designated fast, and time for all place~ of Greenwich is designated slow.
The Greenwich meridian at 0° longitude passes through Greenland Sea and N gian Sea, and the countries of the United Kingdom, F. Spain, Algeria, Mali, Burkina Faso and Ghana, and through the South Atlantic Ocean. In some countries a large West-East extent, more than one time zone be necessary, e.g., the USA and Canada both have fiVE zones; Australia has three; Russia has eleven.
Days on Different Lengths
DAYS ON DIFFERENT LENGTHS On June 21 the sun is overhead along the Tropic of Cancer and all parallels in the northern hemisphere have their longest day of the year. At this time the length of the day increases as latitude increases north of the equator until there is continuous day north of the Arctic Circle. South of the equator the length of day decreases with increasing latitude until there is continuous night south of the Antarctic Circle. On December 22, the reverse takes place for the two hemispheres.
MIDNIGHT SUN It is a phenomenon, observable in latitudes 661,i° North and South (or the Arctic and Antarctic circles respectively) where the sun does not sink below the horizon during summer. This results due to the tilt of the earth's axis, each hemisphere being inclined towards the sun during its summer. The duration of the phenomenon increases towards the poles, where it may be observed for six months of each year.
TIME
To avoid confusion and make the study of time relations simple, it is necessary to think of the earth as stationary and of the sun as completing one circuit about the earth every 24 hours.
MIDNIGHT SUN It is a phenomenon, observable in latitudes 661,i° North and South (or the Arctic and Antarctic circles respectively) where the sun does not sink below the horizon during summer. This results due to the tilt of the earth's axis, each hemisphere being inclined towards the sun during its summer. The duration of the phenomenon increases towards the poles, where it may be observed for six months of each year.
TIME
To avoid confusion and make the study of time relations simple, it is necessary to think of the earth as stationary and of the sun as completing one circuit about the earth every 24 hours.
EQUINOXES and TWILIGHT
EQUINOXES Two days in a year when day and night are equal throughout the world are equinoxes. Falling midway between the dates of the solstices, on these dates the earth's axis lies at 900 to the line joining the centres of the earth and the sun and neither the northern nor the southern hemisphere is inclined towards the sun. The vernal equinox occurs on March 20 or 21, also called the spring equinox in the northern hemisphere, while the autumnal equinox occurs on September 22 or 23. On these two days, every place on the globe experiences 12 hours daylight and
12 hours darkness. The sun rises due east and sets due west, and is seen directly overhead on the equator.
TWIUGHT Following sunset and preceding sunrise, the diffuse illumination is called the twilight. It is caused by the refraction of the sun's rays to the earth's surface, even after the sun disappears below the horizon of the ground level. This refraction occurs due to the scattering action of air molecules and the presence ofJX\inute dust particles and moisture in the earth's atmosphere. Duration of twilight depends mainly on latitude and date, which determine the angle that the sun's path makes with the horizon. As this angle is always about 90° in the tropics, twilight is of short dUration. But in high latitudes, due to the acute angle or the low slant of the sun's path as it goes below the horizon, twilight is greatly lengthened.
12 hours darkness. The sun rises due east and sets due west, and is seen directly overhead on the equator.
TWIUGHT Following sunset and preceding sunrise, the diffuse illumination is called the twilight. It is caused by the refraction of the sun's rays to the earth's surface, even after the sun disappears below the horizon of the ground level. This refraction occurs due to the scattering action of air molecules and the presence ofJX\inute dust particles and moisture in the earth's atmosphere. Duration of twilight depends mainly on latitude and date, which determine the angle that the sun's path makes with the horizon. As this angle is always about 90° in the tropics, twilight is of short dUration. But in high latitudes, due to the acute angle or the low slant of the sun's path as it goes below the horizon, twilight is greatly lengthened.
Seasons
SEASONS Seasons are periods into which the year can be divided as a result of the climatic conditions, largely due to changes in the duration and intensity of solar radiation. Seasonal changes are caused by the inclination of earth's axis to the ecliptic plane and because the axis constantly points towards the Pole Star. Thus, in June, the North Pole of the axis is tilted towards the sun so that solar radiation
is concentrated in the northern hemisphere (summer season) and in December, the position is reversed (winter season). It is notable that all parts of the earth's surface, excepting the equatorial latitudes, experience a defiJUte rise in temperature during the rest of the year. During the four seasons-summer, autumn, winter and spring-the position of earth vis-a-vis the sun changes as it revolves. The
overhead position of the sun changes, resulting in summer season in the northern hemisphere between March 21 and September 23, while it has the winter season between September 23 and March 21, and vice-versa in the southern hemisphere. Spring and autumn are shorter seasons, marking the transition between the two main seasons. While these four seasons are the pattern in mid-latitudes, the tropical pattern is commonly a wet season (in which the sun is overhead) and a dry season. In South Asia these become summer monsoon and winter monsoon.
SOLSTICE Solstice is one of the two dates in the year on which the sun reaches its greatest altitude north or south of the equator and is directly overhead along one of the lines of the tropics.
Summer Solstice On June 21 or 22, the earth is so located in its orbit that the sun is overhead on the Tropic of Cancer (231hoN). The northern hemisphere is tipped towards the sun, having the longest day, while the southern hemisphere i$ tipped away from the sun, having the shortest day. The sun rises north of east and sets north of west.
Winter Solstice On December 21 or 22, the earth is in an equivalent position on the opposite point in its orbit; so the southern hemisphere is tipped towards the sun and the northern hemisphere away from it. The sun is overhead on the Tropic of Capricorn (231hOS), resulting in the shortest day in the northern hemisphere. The sun rises south of east and sets south of west. (However, in this context, 'winter' applies only to northern hemisphere; southern hemisphere experiences summer.)
is concentrated in the northern hemisphere (summer season) and in December, the position is reversed (winter season). It is notable that all parts of the earth's surface, excepting the equatorial latitudes, experience a defiJUte rise in temperature during the rest of the year. During the four seasons-summer, autumn, winter and spring-the position of earth vis-a-vis the sun changes as it revolves. The
overhead position of the sun changes, resulting in summer season in the northern hemisphere between March 21 and September 23, while it has the winter season between September 23 and March 21, and vice-versa in the southern hemisphere. Spring and autumn are shorter seasons, marking the transition between the two main seasons. While these four seasons are the pattern in mid-latitudes, the tropical pattern is commonly a wet season (in which the sun is overhead) and a dry season. In South Asia these become summer monsoon and winter monsoon.
SOLSTICE Solstice is one of the two dates in the year on which the sun reaches its greatest altitude north or south of the equator and is directly overhead along one of the lines of the tropics.
Summer Solstice On June 21 or 22, the earth is so located in its orbit that the sun is overhead on the Tropic of Cancer (231hoN). The northern hemisphere is tipped towards the sun, having the longest day, while the southern hemisphere i$ tipped away from the sun, having the shortest day. The sun rises north of east and sets north of west.
Winter Solstice On December 21 or 22, the earth is in an equivalent position on the opposite point in its orbit; so the southern hemisphere is tipped towards the sun and the northern hemisphere away from it. The sun is overhead on the Tropic of Capricorn (231hOS), resulting in the shortest day in the northern hemisphere. The sun rises south of east and sets south of west. (However, in this context, 'winter' applies only to northern hemisphere; southern hemisphere experiences summer.)
Motions Of The Earth And Their Effects
The earth is not stationary. It rotates on its revolves round the sun and moves, along with the system around the galaxy, even as the galaxy in its en! moves through the universe.
Here, however, we are concerned mainly with two motions-rotation and revolution.
ROTATION It is the movement of the earth on its axis from the west to east, so that the stars, the sun and the moon appear to rise in the east and set in the west. Each rotation takes 23 hours 56 minutes and 4.09 seconds. The velocity of rotation of an object on the earth's surface at the equator is about 1700 km per hour and it decreases towards the poles. Earth's rotation results in (i) causation of day and night; (il) a difference of 1 hour between two meridians which are 15" apart; (iii) deflection of ocean currents and winds-see figure below; (iv) rise and fall of tides every day.
REVOLUTION It is earth's motion in its elliptical orbit around the sun. Earth's average orbital velocity is 29.79 km/sec. One revolution is completed in 36514 days, resulting in one extra day every fourth year. This year, consisting of 366 days, for convenience is called the 'leap year' having 29 days in the month of February. Revolution of the earth results in: (i) the change of seasons; (ii) variation in the lengths of day and night at different times of the year; (iii)
differences in altitudes of the sun at noon, at different times of the year.
-It may be noted that as the earth's orbit is elliptical, the distance between the earth and sun keeps changing. The point on the orbit nearest to the sun is the perihelion which is 147 million km and occurs around January 3; the
point of maximum distance is aphelion which is 152 million km and occurs around July 4.
Inclined Axis Inclination of the earth's axis is an important feature of the earth-sun relationship. In its elliptical movement around the sun, earth's axis makes a constant angle of 661,20 with the plane of the ecliptic, and is tilted 231,20 from a line perpendicular to this plane. The two facts, Le., a fixed angle of the earth's axis to the plane of the ecliptic and the axis always pointing in the same direction, when combined with the earth's movements, result in varying lengths of day and night, seasonality and changes in the altitude of sun at different times of the year.
Here, however, we are concerned mainly with two motions-rotation and revolution.
ROTATION It is the movement of the earth on its axis from the west to east, so that the stars, the sun and the moon appear to rise in the east and set in the west. Each rotation takes 23 hours 56 minutes and 4.09 seconds. The velocity of rotation of an object on the earth's surface at the equator is about 1700 km per hour and it decreases towards the poles. Earth's rotation results in (i) causation of day and night; (il) a difference of 1 hour between two meridians which are 15" apart; (iii) deflection of ocean currents and winds-see figure below; (iv) rise and fall of tides every day.
REVOLUTION It is earth's motion in its elliptical orbit around the sun. Earth's average orbital velocity is 29.79 km/sec. One revolution is completed in 36514 days, resulting in one extra day every fourth year. This year, consisting of 366 days, for convenience is called the 'leap year' having 29 days in the month of February. Revolution of the earth results in: (i) the change of seasons; (ii) variation in the lengths of day and night at different times of the year; (iii)
differences in altitudes of the sun at noon, at different times of the year.
-It may be noted that as the earth's orbit is elliptical, the distance between the earth and sun keeps changing. The point on the orbit nearest to the sun is the perihelion which is 147 million km and occurs around January 3; the
point of maximum distance is aphelion which is 152 million km and occurs around July 4.
Inclined Axis Inclination of the earth's axis is an important feature of the earth-sun relationship. In its elliptical movement around the sun, earth's axis makes a constant angle of 661,20 with the plane of the ecliptic, and is tilted 231,20 from a line perpendicular to this plane. The two facts, Le., a fixed angle of the earth's axis to the plane of the ecliptic and the axis always pointing in the same direction, when combined with the earth's movements, result in varying lengths of day and night, seasonality and changes in the altitude of sun at different times of the year.
Latitude and Temperature Zones
. As the earth is spherical, different parts of th tend to get heated to different degrees. The sun's r concentrated more directly in the region round the e So the temperature is higher in these regions. So w from the equator, due to the curvature of the eal
sun's rays strike the earth's surface at an angle and, spread over a larger area. So these regions do fit to the same extent as do the equatorial region temperature thus decreases from the equator to the poles.
A place can be most accurately and precisely II when both its latitude and longitude are given.
DISTANCE The actual length, in kilometres, of a c of longitude will depend upon where it is measun tr.e equator this distance may be computed by dividing the circumference of the earth by 360°; i.e., 40,075 km/3 111 km (approximately). It is useful to remember th length of 1° of longitude is reduced by about one-h the 60th parallels. The length of a degree of latitu almost the same as the length of 1° of longitude, equator, j.e., about 111 km.
It is important to have universally accepted un measurement for length or distance which maybe by both air and marine navigators. The 'internal nautical mile' is defined as exactly equivalent to
international metres or 6076.1033 feet. One nautical IT equal to 1.15077 statute miles. It has been proved tha nautical mile very closely approximates the average IE of one minute of latitude.
sun's rays strike the earth's surface at an angle and, spread over a larger area. So these regions do fit to the same extent as do the equatorial region temperature thus decreases from the equator to the poles.
A place can be most accurately and precisely II when both its latitude and longitude are given.
DISTANCE The actual length, in kilometres, of a c of longitude will depend upon where it is measun tr.e equator this distance may be computed by dividing the circumference of the earth by 360°; i.e., 40,075 km/3 111 km (approximately). It is useful to remember th length of 1° of longitude is reduced by about one-h the 60th parallels. The length of a degree of latitu almost the same as the length of 1° of longitude, equator, j.e., about 111 km.
It is important to have universally accepted un measurement for length or distance which maybe by both air and marine navigators. The 'internal nautical mile' is defined as exactly equivalent to
international metres or 6076.1033 feet. One nautical IT equal to 1.15077 statute miles. It has been proved tha nautical mile very closely approximates the average IE of one minute of latitude.
Longitudes and Lattitudes of Earth
'LONGITUDES AND LATITUDES The location of points on earth's surface is done by a system of measuring the lengths of arcs along meridians and parallels, or in terms of the longitudes and latitudes. The longitude of a place can be defined as the arc, measured in degrees, of a parallel between the place and the prime meridian, or east or west of the prime meridian. The prime meridian is at 0°, passing through the Royal Observatory at Greenwich near London, England. It is often referred to as the meridian of Greenwich. This meridian is taken by many geographers to divide the earth into the eastern and western hemispheres. (Some geographers see the eastern hemisphere as Asia, Africa, Europe, Australia and New Zealand separated from the western hemisphere, represented by North and South America, by the meridians 20"W and 160"E.)
The latitude of a place is defined as the arc, measured in degrees, north or south of the meridian between that place and the equator. The equator is given the value of 0°. The latitudes thus range from 0° at the equator to 90° north or south at the poles.
Each degree of longitude and latitude is divided into 60 minutes and each minute into 60 seconds. The most important lines of latitude are, besides the equator, the Tropic of Cancer, the Tropic of Capricorn, the Arctic Circle and the Antarctic Circle.
The earth being slightly f at the poles, the linear distance of a degree of lat the pole is a little longer than that at the equato latitudes are in the region from the Arctic Circle North Pole and again from the Antarctic Circle to tl1 Pole.
The latitude of a place is defined as the arc, measured in degrees, north or south of the meridian between that place and the equator. The equator is given the value of 0°. The latitudes thus range from 0° at the equator to 90° north or south at the poles.
Each degree of longitude and latitude is divided into 60 minutes and each minute into 60 seconds. The most important lines of latitude are, besides the equator, the Tropic of Cancer, the Tropic of Capricorn, the Arctic Circle and the Antarctic Circle.
The earth being slightly f at the poles, the linear distance of a degree of lat the pole is a little longer than that at the equato latitudes are in the region from the Arctic Circle North Pole and again from the Antarctic Circle to tl1 Pole.
Points to Remember
LATITUDE
1. A latitude is the angular distance of a place north or south
of the equator.
2. There are 180 parallels of latitude.
3. Each parallel of latitude is a circle.
4. All the parallels of latitude are not of equal length. The
circles become smaller towards the pole.
5. The distance between any two parallels of latitude is always
equal.
6. The North Pole and the South Pole are fixed points and
serve as basic points of reference.
LONGITUDE
1. Longitude is thE' angular distance of a place east or west
of the prime r ,eridian.
2. There are 360 meridians of longitude.
3. The prime meridian is a 10ngitIJae of O¥<
4. Each meridian o'f' 10,ngitude is w-s.eJf1r-circle.
5. All meridians are of equal length.
E1. The distance between any two meridians is not equal. They
get ,closer (converge) fr~rn- the EqL@tor ~o> the,poles.
1. A latitude is the angular distance of a place north or south
of the equator.
2. There are 180 parallels of latitude.
3. Each parallel of latitude is a circle.
4. All the parallels of latitude are not of equal length. The
circles become smaller towards the pole.
5. The distance between any two parallels of latitude is always
equal.
6. The North Pole and the South Pole are fixed points and
serve as basic points of reference.
LONGITUDE
1. Longitude is thE' angular distance of a place east or west
of the prime r ,eridian.
2. There are 360 meridians of longitude.
3. The prime meridian is a 10ngitIJae of O¥<
4. Each meridian o'f' 10,ngitude is w-s.eJf1r-circle.
5. All meridians are of equal length.
E1. The distance between any two meridians is not equal. They
get ,closer (converge) fr~rn- the EqL@tor ~o> the,poles.
Earth Data
Age: At least 4Y2 billion years.
Motion: Rotation (spinning motion around an imaginary
line connecting the North and South Poles) once every 23 hours, 56 minutes, 4.09 seconds. Revolution (motion around the sun)-once every 365 days, 6 hours, 9 minutes, 9.54 seconds.
Size: Polar diameter (distance through the earth from North Pole to South Pole)-12,713.54 kilometres. Equatorial diameter (distance through the earth at the equator)-12,756.32 kilometres. Polar circumference (distance around the earth through the poles)-40,008.00 kilometres. Equatorial circumference (distance around the earth along the equator)-40,075.16 kilometres. .
Area: Total surface area-509,700,OOO square kilometres. Land area-approximately 148,400,000 square kilometres, about 29 per cent of total surface area. Water area-approximately 361,300,000 square kilometres, about 71 per cent of the total
surface area.
Mass: 5.882 x 1021 tonnes.
Mean density: 5.517
Surface features: Highest land-Mount Everest, 8,848
metres above sea level. Lowest land-shore of Dead Sea, about 399 metres below sea level.
Ocean depths: Deepest part of ocean-area of the Mariana Trench in Pacific Ocean southwest of Guam, 11,033 metres below surface. Average ocean depth-3,730 metres.
Temperature: Highes'-58° C at AI Aziziyah, Libya. Lowest -89.6° C at Vostok Station in Antarctica. Average surface temperature-14° C.
Atmosphere: Heig!l'-more than 99 per cent of the atmosphere is less than 80 kilometres above the earth's surface, but particles of the atmosphere are 1,600 kilometres above the surface. Regions of atmosphere-troposphere (up to 10 or 16 kilometres above surface); stratosphere (from about 16 to 48 kilometres); mesosphere (from about 48 to about 80 kilometres); thermosphere (from 80 kilometres into outer space).
"Chemical make-up of atmosphere-about 78 per cent nitrogen, 21 per cent oxygen, 1 per cent argon, and small amounts of other gases.
Chemical make-up of the earth's crust (in per cent of the crust's weight): Oxygen 46.6; silicon 27.7; aluminium 8.1; iron 5.0; calcium 3.6; sodium 2.8; potassium 2.6; magnesium 2.0; and other elements totalling 1.6.
Motion: Rotation (spinning motion around an imaginary
line connecting the North and South Poles) once every 23 hours, 56 minutes, 4.09 seconds. Revolution (motion around the sun)-once every 365 days, 6 hours, 9 minutes, 9.54 seconds.
Size: Polar diameter (distance through the earth from North Pole to South Pole)-12,713.54 kilometres. Equatorial diameter (distance through the earth at the equator)-12,756.32 kilometres. Polar circumference (distance around the earth through the poles)-40,008.00 kilometres. Equatorial circumference (distance around the earth along the equator)-40,075.16 kilometres. .
Area: Total surface area-509,700,OOO square kilometres. Land area-approximately 148,400,000 square kilometres, about 29 per cent of total surface area. Water area-approximately 361,300,000 square kilometres, about 71 per cent of the total
surface area.
Mass: 5.882 x 1021 tonnes.
Mean density: 5.517
Surface features: Highest land-Mount Everest, 8,848
metres above sea level. Lowest land-shore of Dead Sea, about 399 metres below sea level.
Ocean depths: Deepest part of ocean-area of the Mariana Trench in Pacific Ocean southwest of Guam, 11,033 metres below surface. Average ocean depth-3,730 metres.
Temperature: Highes'-58° C at AI Aziziyah, Libya. Lowest -89.6° C at Vostok Station in Antarctica. Average surface temperature-14° C.
Atmosphere: Heig!l'-more than 99 per cent of the atmosphere is less than 80 kilometres above the earth's surface, but particles of the atmosphere are 1,600 kilometres above the surface. Regions of atmosphere-troposphere (up to 10 or 16 kilometres above surface); stratosphere (from about 16 to 48 kilometres); mesosphere (from about 48 to about 80 kilometres); thermosphere (from 80 kilometres into outer space).
"Chemical make-up of atmosphere-about 78 per cent nitrogen, 21 per cent oxygen, 1 per cent argon, and small amounts of other gases.
Chemical make-up of the earth's crust (in per cent of the crust's weight): Oxygen 46.6; silicon 27.7; aluminium 8.1; iron 5.0; calcium 3.6; sodium 2.8; potassium 2.6; magnesium 2.0; and other elements totalling 1.6.
Earth Location
LOCATION
Location is the geographic situation, or a point or position in space where objects, organisms and fields of force may be found or events occur. It is also the fact or condition of occupying a particular place. The mathematical system commonly used to describe location on the earth's system is based on a'series of imaginary lines arawnon a globe according to an agreed method.
The geographic grid is a network of intersecting lines on the globe, based on two natural points-the North Pole and the South Pole.
GREAT CIRCLE The great circle is any hypothetical circle on earth's surface, the plane of which passes through earth's centre, cutting it into two equal halves, Le. hemispheres. This intersection of the plane with the globe is the largest circle that can be drawn on it. The equator is a great circle. It divides the earth into the northern hemisphere and the southern hemisphere.
An infinite number of great circles can be drawn on a sphere. However, only one great circle can be found to pass through two given points on the surface of the sphere-unless the two points are the extremities of the same diameter, in which case any number of great circles can be drawn through them. Intersecting great circles bisect each other. An arc of a great circle is the shortest distance, following the surface, between any two points on a sphere; hence the use of the great circle routes by aircraft, e.g. over the North Pole region.
SMALL CIRCLE A small circle is a hypothetical circle made by a plane passing through the globe anywhere except through the centre. It cuts the globe into unequal
parts. The Tropic of Cancer at 23"30'N and the Tropic of Capricorn at 23"30'5 are small circles. The Arctic Circle is
66"32'N (often taken as 66.1 "N) and the Antarctic Circle is
21
66"32'5 (often taken as 662" "5). All parallels of latitude
except the equator arE! small circles.
Meridians and Parallels Rotation of the earth on its axis provides two natural points-the North and the South Poles. The set of north-south lines connecting the poles are called meridians. The set of lines running east-west are called the parallels. All meridians are halves of great circles, containing 1800 of are, spaced farthest at the equator and converging to common points at the poles. Parallels are equidistant circular lines, running parallel to the equator and to each other, intersecting the meridians at right angles. A meridian exists for any point selected on the globe as an infinite number of meridians may be drawn on a globe. Every point on the globe, except the North Pole and the
POINTS TO REMEMBER
Location is the geographic situation, or a point or position in space where objects, organisms and fields of force may be found or events occur. It is also the fact or condition of occupying a particular place. The mathematical system commonly used to describe location on the earth's system is based on a'series of imaginary lines arawnon a globe according to an agreed method.
The geographic grid is a network of intersecting lines on the globe, based on two natural points-the North Pole and the South Pole.
GREAT CIRCLE The great circle is any hypothetical circle on earth's surface, the plane of which passes through earth's centre, cutting it into two equal halves, Le. hemispheres. This intersection of the plane with the globe is the largest circle that can be drawn on it. The equator is a great circle. It divides the earth into the northern hemisphere and the southern hemisphere.
An infinite number of great circles can be drawn on a sphere. However, only one great circle can be found to pass through two given points on the surface of the sphere-unless the two points are the extremities of the same diameter, in which case any number of great circles can be drawn through them. Intersecting great circles bisect each other. An arc of a great circle is the shortest distance, following the surface, between any two points on a sphere; hence the use of the great circle routes by aircraft, e.g. over the North Pole region.
SMALL CIRCLE A small circle is a hypothetical circle made by a plane passing through the globe anywhere except through the centre. It cuts the globe into unequal
parts. The Tropic of Cancer at 23"30'N and the Tropic of Capricorn at 23"30'5 are small circles. The Arctic Circle is
66"32'N (often taken as 66.1 "N) and the Antarctic Circle is
21
66"32'5 (often taken as 662" "5). All parallels of latitude
except the equator arE! small circles.
Meridians and Parallels Rotation of the earth on its axis provides two natural points-the North and the South Poles. The set of north-south lines connecting the poles are called meridians. The set of lines running east-west are called the parallels. All meridians are halves of great circles, containing 1800 of are, spaced farthest at the equator and converging to common points at the poles. Parallels are equidistant circular lines, running parallel to the equator and to each other, intersecting the meridians at right angles. A meridian exists for any point selected on the globe as an infinite number of meridians may be drawn on a globe. Every point on the globe, except the North Pole and the
POINTS TO REMEMBER
Earth and the Universe
SHAPE AND SIZE
Earth, the planet on which we live, is the third planet outward from the sun, lying with its satellite between Venus and Mars. The earth is an oblate spheroid; taking into account an IS-metre rise at the North Pole and a 26metre depression at the South Pole, it may be called pearshaped. It is fifth in order of size among the nine planets.
PROOF OF SPHERICITY How do we know that the earth is a sphere and not flat?
(i) Circumnavigation of the earth for the first time by Ferdinand Magellan in the early sixteenth century and subsequent travel by others proved that the earth had no abrupt edges. Modem air routes and ocean navigation are based on the assumption that the earth is round.
(ii) Viewed from the deck of a ship at sea or from a cliff on land, the distant horizon is always and everywhere circular in shape. Furthermore, this circular horizon widens
with increasing altitude and this could only be so on a . spherical body.
(iii) If the earth were flat, the entire world would experience sunrise and sunset at the same time. But this is not so: different places on earth observe sunrise and sunset at different times substantiating the contention of earth being a sphere.
(iv) As we observe a ship receding into the distance at sea, the ship appears to sink gradually and does not vanish abruptly. Again, when a ship appears over the horizon, it is the top of the mast that is seen before the hull. This proves that the sea surface curves downward away from us. If the earth were flat, the entire ship would be seen or hidden from view all at once.
(v) In all lunar eclipses, when the earth's shadow falls on the moon, the edge of the shadow appears as an arc of a circle, and only a spherical body can always cast a
circular shadow on another sphere.
(vi) If you stand at the equator and observe Polaris, the North Star, it appears to be on the horizon. As you travel towards the North Pole, the star appears to be located higher and higher in the sky, until at the North Pole it is directly overhead in the sky.
(vii) An object near sea level weighs very nearly the same on a spring balance at any place on the globe. The conclusion is that the pull of gravity is same all over the earth and this could happen only if the centre is equidistant from all the places on the earth-hence the earth is a sphere.
(viii) Surveying operations with telescopic instruments shew that corrections for the earth's curvature have to be made as the telescopic line of vision is tangential to the earth's surface. As the correction is approximately constant for all places on the earth, we may conclude that the earth is a sphere.
(ix) Navigation methods assume the earth to be a sphere, and the correctness of this assumption is established as the positions of vessels have been correctly determined by these methods.
(x) Photographs taken from outer space provide proof (perhaps the most convincing and up-to-date) of the earth's spherical shape.
NOT A PERFECT SPHERE The earth, however, is not a perfect sphere. It was in the seventeenth century that a French astronomer, Jean Richer, conducted some experiments using a pendulum clock. The clock lost time as he reached a place nearer the equator; he attributed this to a somewhat lesser force of gravity near the equator. This phenomenon could be explained only by supposing that the equatorial parts of the earth's surface lie farther from the earth's centre than do other places. Refined measurements later revealed that the true form of the earth is slightly compressed at the poles and bulging around the equator.
A cross-section through the poles gives an ellipse, not a circle. The equator remains a circle. The shape of the earth is thus an oblate ellipsoid or ellipsoid of revolution. The centrifugal force of the earth's rotation IS considered to cause earth's oblateness.
Still further refined measurements have led geographers to consider the earth as a geoid. The geoid "can be thought of as an undulating surface of irregular form"; a theoretical shape of the earth based on estimates of its mass, elasticity and speed of rotation, ignoring its surface irregularities.
Earth, the planet on which we live, is the third planet outward from the sun, lying with its satellite between Venus and Mars. The earth is an oblate spheroid; taking into account an IS-metre rise at the North Pole and a 26metre depression at the South Pole, it may be called pearshaped. It is fifth in order of size among the nine planets.
PROOF OF SPHERICITY How do we know that the earth is a sphere and not flat?
(i) Circumnavigation of the earth for the first time by Ferdinand Magellan in the early sixteenth century and subsequent travel by others proved that the earth had no abrupt edges. Modem air routes and ocean navigation are based on the assumption that the earth is round.
(ii) Viewed from the deck of a ship at sea or from a cliff on land, the distant horizon is always and everywhere circular in shape. Furthermore, this circular horizon widens
with increasing altitude and this could only be so on a . spherical body.
(iii) If the earth were flat, the entire world would experience sunrise and sunset at the same time. But this is not so: different places on earth observe sunrise and sunset at different times substantiating the contention of earth being a sphere.
(iv) As we observe a ship receding into the distance at sea, the ship appears to sink gradually and does not vanish abruptly. Again, when a ship appears over the horizon, it is the top of the mast that is seen before the hull. This proves that the sea surface curves downward away from us. If the earth were flat, the entire ship would be seen or hidden from view all at once.
(v) In all lunar eclipses, when the earth's shadow falls on the moon, the edge of the shadow appears as an arc of a circle, and only a spherical body can always cast a
circular shadow on another sphere.
(vi) If you stand at the equator and observe Polaris, the North Star, it appears to be on the horizon. As you travel towards the North Pole, the star appears to be located higher and higher in the sky, until at the North Pole it is directly overhead in the sky.
(vii) An object near sea level weighs very nearly the same on a spring balance at any place on the globe. The conclusion is that the pull of gravity is same all over the earth and this could happen only if the centre is equidistant from all the places on the earth-hence the earth is a sphere.
(viii) Surveying operations with telescopic instruments shew that corrections for the earth's curvature have to be made as the telescopic line of vision is tangential to the earth's surface. As the correction is approximately constant for all places on the earth, we may conclude that the earth is a sphere.
(ix) Navigation methods assume the earth to be a sphere, and the correctness of this assumption is established as the positions of vessels have been correctly determined by these methods.
(x) Photographs taken from outer space provide proof (perhaps the most convincing and up-to-date) of the earth's spherical shape.
NOT A PERFECT SPHERE The earth, however, is not a perfect sphere. It was in the seventeenth century that a French astronomer, Jean Richer, conducted some experiments using a pendulum clock. The clock lost time as he reached a place nearer the equator; he attributed this to a somewhat lesser force of gravity near the equator. This phenomenon could be explained only by supposing that the equatorial parts of the earth's surface lie farther from the earth's centre than do other places. Refined measurements later revealed that the true form of the earth is slightly compressed at the poles and bulging around the equator.
A cross-section through the poles gives an ellipse, not a circle. The equator remains a circle. The shape of the earth is thus an oblate ellipsoid or ellipsoid of revolution. The centrifugal force of the earth's rotation IS considered to cause earth's oblateness.
Still further refined measurements have led geographers to consider the earth as a geoid. The geoid "can be thought of as an undulating surface of irregular form"; a theoretical shape of the earth based on estimates of its mass, elasticity and speed of rotation, ignoring its surface irregularities.
Subfields of Human Geography
The following are the main subfields of human geog
raphy.
Cultural geography deals with tl1c location and diffusion of beliefs, customs and other cultural traits. Thus, the habitat, clothing, food habits, skills, tools and social
organisation are all aspects studied by cultural geography.
So~ial geography is close to cultural geography. It examines the relationships among groups of people, and how these social relationships affect the places where people live and work.
Economic geography deals with the location and distribution of economic activities or human activity aimed at improving material well-being through production, exchange, distribution and consumption of goods and services. This field is concerned with spatial relations and the environmental and human factors that affect the development and growth of these activities; these factors are transportation, labour supply, and resources.
Population geography is concerned with patterns of population and the reasons for a change in those patterns. It deals with birth and death rates, age and sex composition, the literacy level of populations. It also studies population movements, family size, house types and settlements.
Urban geography is concerned with cities and other urban areas, examining the importance of location in the development of cities. It could also study the distribution
of various groups within a city or why slums develop some places.
Political geography attempts to analyse the ways: which organised groups of people in different places ma} decisions or gain and use power within a political systen This branch is also concerned with relations betwee independent states, frontiers, boundaries, problems ( political instability, patterns of voting, and regional plar ning.
Historical geography tries to picturise the geograph of a region or an area as it was in the past and studie how it has evolved over time. It is concerned with the geographic forces that have caused the changes.
Anthrogeography studies the distribution of human communities on the earth in relation to their geographical environment.
Agricultural geography studies the development 01 different kinds of farrns and farming systems in particular areas and compares them with the farms and farming systems of other areas.
raphy.
Cultural geography deals with tl1c location and diffusion of beliefs, customs and other cultural traits. Thus, the habitat, clothing, food habits, skills, tools and social
organisation are all aspects studied by cultural geography.
So~ial geography is close to cultural geography. It examines the relationships among groups of people, and how these social relationships affect the places where people live and work.
Economic geography deals with the location and distribution of economic activities or human activity aimed at improving material well-being through production, exchange, distribution and consumption of goods and services. This field is concerned with spatial relations and the environmental and human factors that affect the development and growth of these activities; these factors are transportation, labour supply, and resources.
Population geography is concerned with patterns of population and the reasons for a change in those patterns. It deals with birth and death rates, age and sex composition, the literacy level of populations. It also studies population movements, family size, house types and settlements.
Urban geography is concerned with cities and other urban areas, examining the importance of location in the development of cities. It could also study the distribution
of various groups within a city or why slums develop some places.
Political geography attempts to analyse the ways: which organised groups of people in different places ma} decisions or gain and use power within a political systen This branch is also concerned with relations betwee independent states, frontiers, boundaries, problems ( political instability, patterns of voting, and regional plar ning.
Historical geography tries to picturise the geograph of a region or an area as it was in the past and studie how it has evolved over time. It is concerned with the geographic forces that have caused the changes.
Anthrogeography studies the distribution of human communities on the earth in relation to their geographical environment.
Agricultural geography studies the development 01 different kinds of farrns and farming systems in particular areas and compares them with the farms and farming systems of other areas.
Branches of Geography
BRANCHES OF GEOGRAPHY
The general practice is to divide geography into two main branches-physical and human. There are two other important branches.
MATHEMATICAL GEOGRAPHY is the study of the earth'~ size and shape, of time zones, and of the motion
of the earth.
CARTOGRAPHY is the production and study of maps and charts. This branch is responsible for geodetic and topographical surveys and the preparation of maps on certain selected scales.
PHYSICAL GEOGRAPHY is concerned with natural features such as land, water and climate. It studies these features as they are and in relationship with one another as well as with human activities. It also tries to understand what forces create and change these features.
Physical geography may be subdivided into the following:
Geomorphology is concerned with landforms, their distribution and origin and the forces that change them. It also studies the relationship between landforms and human activities.
Climatology is the study of the processes involved in
.the making of the weather and climate, the changes in climate, and how climate is affected by human activity.
Hydrology is the study of earth's water-oceans, rivers, glaciers, etc. Some geographers consider its concern to be particularly water on or under the ground before it reaches the ocean or before it evaporates into the air. It has important applications such as irrigation, flood control, water supply and hydel generation.
Oceanography is particularly concerned with the study of oceans. It covers the shape, depth, and distribution of oceans, life forms, ecology and currents, besides the legal status of oceans.
Soil geography deals with the kinds of soils, their evolution and their distribution besides their significance in land use.
HUMAN GEOGRAPHY is concerned with the earth features created by human action in the course of contriving to build and improve habitats to live in comfort and security. Agricultural fields, industrial development, settlements and roads are some examples.
Beyond these material manifestations, there are some invisible aspects that influence human action; these belong to the realm of ideas and the mind. Human geography is also concerned with how some of these aspects are expressed-in religion, the views of a population, the education and knowledge level of people.
The general practice is to divide geography into two main branches-physical and human. There are two other important branches.
MATHEMATICAL GEOGRAPHY is the study of the earth'~ size and shape, of time zones, and of the motion
of the earth.
CARTOGRAPHY is the production and study of maps and charts. This branch is responsible for geodetic and topographical surveys and the preparation of maps on certain selected scales.
PHYSICAL GEOGRAPHY is concerned with natural features such as land, water and climate. It studies these features as they are and in relationship with one another as well as with human activities. It also tries to understand what forces create and change these features.
Physical geography may be subdivided into the following:
Geomorphology is concerned with landforms, their distribution and origin and the forces that change them. It also studies the relationship between landforms and human activities.
Climatology is the study of the processes involved in
.the making of the weather and climate, the changes in climate, and how climate is affected by human activity.
Hydrology is the study of earth's water-oceans, rivers, glaciers, etc. Some geographers consider its concern to be particularly water on or under the ground before it reaches the ocean or before it evaporates into the air. It has important applications such as irrigation, flood control, water supply and hydel generation.
Oceanography is particularly concerned with the study of oceans. It covers the shape, depth, and distribution of oceans, life forms, ecology and currents, besides the legal status of oceans.
Soil geography deals with the kinds of soils, their evolution and their distribution besides their significance in land use.
HUMAN GEOGRAPHY is concerned with the earth features created by human action in the course of contriving to build and improve habitats to live in comfort and security. Agricultural fields, industrial development, settlements and roads are some examples.
Beyond these material manifestations, there are some invisible aspects that influence human action; these belong to the realm of ideas and the mind. Human geography is also concerned with how some of these aspects are expressed-in religion, the views of a population, the education and knowledge level of people.
Approaches
A Greek scholar, Erastosthenes, is considered to have been the first to use the term/geography, in the third century BC. It is derived from tWo Greek words-geo (earth) and graphe (description). Literally, therefore, geography is the description of the earth, more specifically, the surface of the earth and all that appears on it. But over the years, the subject has widened to include more than mere description; it brought into its ambit explanation for the responses of human beings to their natural environment. Geography is concerned with the study of where people, animals and plants are found and how they relate with earth features such as rivers, mountains and deserts.
Geographers also examine where earth features are located, how they came to be there, and why their location is significant. According to Hartshorne, geography is concerned with providing "accurate, orderly, and rational description and interpretation of the variable character of the earth surface". Now, the term "earth surface" generally includes the thin zone extending as far below the surface as human beings have been able to penetrate and as far above as. they have been able to go. With the advance of technology, this zone has continued to expand; thus, the "earth surface" forming the focus of a geographer's study is relative to the level of technological progress.
There are three essential characteristics of geographical work, according to Haggett. (i) There is an emphasis on location, an attempt to establish locations of phenomena on the earth. surface accurately and economically. So, cartography (making maps) is an important tool for geographical work. (ii) There is an emphasis on society-land relations. Geography has an ecological perspective-it studies environment in the context of environmental effects on humans and the changes in the environment brought about by human intervention. (iii) There is regional analysis, involving identification of regions, analysis of their internal morphology, their ecological linkages, and their relations with other regions. Two approaches are possible in this context.
In one, the focus is on areal organisation in particular places with a view to gaining in-depth knowledge of the human-environment situation prevailing there. A region at different scales-a continent, a country, a local area-is studied in all its geographical aspects. This is regional geography. In the other approach, any particular theme or element of the system is chosen, say climate, and analysed systematically over the earth surface-or a large part of it-with the idea of identifying the general law of its prevalence over the globe. This is termed as systelJ}atic geography. The two approaches are complementary.
Geographers also examine where earth features are located, how they came to be there, and why their location is significant. According to Hartshorne, geography is concerned with providing "accurate, orderly, and rational description and interpretation of the variable character of the earth surface". Now, the term "earth surface" generally includes the thin zone extending as far below the surface as human beings have been able to penetrate and as far above as. they have been able to go. With the advance of technology, this zone has continued to expand; thus, the "earth surface" forming the focus of a geographer's study is relative to the level of technological progress.
There are three essential characteristics of geographical work, according to Haggett. (i) There is an emphasis on location, an attempt to establish locations of phenomena on the earth. surface accurately and economically. So, cartography (making maps) is an important tool for geographical work. (ii) There is an emphasis on society-land relations. Geography has an ecological perspective-it studies environment in the context of environmental effects on humans and the changes in the environment brought about by human intervention. (iii) There is regional analysis, involving identification of regions, analysis of their internal morphology, their ecological linkages, and their relations with other regions. Two approaches are possible in this context.
In one, the focus is on areal organisation in particular places with a view to gaining in-depth knowledge of the human-environment situation prevailing there. A region at different scales-a continent, a country, a local area-is studied in all its geographical aspects. This is regional geography. In the other approach, any particular theme or element of the system is chosen, say climate, and analysed systematically over the earth surface-or a large part of it-with the idea of identifying the general law of its prevalence over the globe. This is termed as systelJ}atic geography. The two approaches are complementary.
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