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.