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.
Subscribe to:
Posts (Atom)
