Day and Night (Earth's rotation)

When the earth rotates on its axis, only one portion of the earth's surface comes into the rays of the the sun and experiences daylight. The other portion which is away from the sun's rays will be in darkness. As the earth rotates from west to east, every part of the earth's surface will be brought under the sun at some time or other. A part of the earth's surface that emerges from darkness into the sun's rays experiences sunrise. Later, when it is gradually obscured from the sun's beams it experiences sunset. The sun is, in fact, stationary and it is earth which rotates. The illusion is exactly the same as when we travel in a fast-moving train. The trees and houses around us appear to move and we feel that the train is stationary.

Evidence of the Earth's Sphericity

There are many ways to prove that the earth is spherical. The following are some of them. 1. CIRCUMNAVIGATION OF THE EARTH. The first voyage around the world by Ferdinand Magellan and his crew, from 1519 to 1522, proved beyond doubt that the earth is spherical. No traveller going roumd the world by land or sea has eve encountered an abrupt edge, over which he would fall. Modern air routes and ocean navigation are based on the assumption that the earth is round. 2. THE CIRCULAR HORIZON. The distant horizon viewed from the deck of a ship at sea, or from a cliff on land is always and everywhere circular in shape. This circular horizon widens with increasing altitude and could only be seen on a spherical body. 3. SHIP'S VISIBILITY. When a ship appears over the distant horizon, the top of the mast is seen first before the hull. In the same way, when it leaves habour, its disappearance over the curved surface is equally gradual. If the earth were flat, the entire ship would be seen or obscured all at once. 4. SUNRISE AND SUNSET. The sun rises and sets at different times in different places. As the earth rotates from west to east, places in the east see the sun earlier than those in the west. If the earth were flat, the whole world would have sunrise and sunset at the same time. But we know this is not so. 5. THE LUNAR ECLIPSE. The shadow cast by the earth on the moon during a lunar eclipse is always circular. It takes the outline of an arc of a circle. Only a sphere can cast such a circular shadow. 6. PLANETARY BODIES ARE SPHERICAL. All observations from telescopes reveal that the planetary bodies, the sun, moon, satellites and stars have circular outlines from whichever angle you see them. They are strictly spheres. Earth, by analogy, cannot be the only exception. 7. DRIVING POLES ON LEVEL GROUND ON A CURVED EARTH. Engineers when driving poles of equal length at regular intervals on the ground have found they do not give a perfect horizontal level. The centre pole normally projects slightly above the poles at either end because of the curvature of the earth. Surveyors and field engineers therefore have to make certain corrections for this inevitable curvature, i.e. 12.6 cm to 1 km. 8. SPACE PHOTOGRAPHS. Pictures taken from high altitudes by rockets and satellites show clearly the curved edge of the earth. This is perhaps the most convincing and the most up-to-date proof of the earth's sphericity.

The Shape of the Earth

In the olden days, sailors feared to venture far into the distant ocean because they thought that when they reached the edge of the earth, they would slip down and perish in the bottomless ocean. This is, of course, not true. From years of accumulated knowledge, experience and observations in different parts of the world, we know that the earth is round. Its spherical shape is an established fact, proved and accepted by all. There has been so much research done on earth science that its various dimensions have been accurately found. It has an equatorial circumference of 40,084 km and its polar circumference is less by 133 km. Its equatorial diameter is 12,761 km and its polar diameter is shorter by 42 km. This simply shows that the earth is not a perfect sphere. It is a little flattened at both ends like an orange. It can, in fact, be called a geoid ('earth-shaped'). The spherical shape of the earth is also masked by the intervening highlands and oceans on its surface.

The Solar System

The solar system comprises the sun and its nine planets which are believed to have been developed from the condensation of gases and other lesser bodies. All the planets revolve round the sun in elliptical orbits. Like the earth, they shine only by the reflected light of the sun. The sun has a surface temperature of 6000'C and increases to 20 million'C in the interior. All over its surface are fiery gases that leap up in whirls of glowing flames like a volcano in eruption. In size, the sun is almost unimaginable. It is about 300,000 times as big as the earth! Amongst the nine planets, MERCURY is the smallest and closest to the sun, only 57,900,000 km away. It thus completes its orbit in a much shorter space of time than does earth. A year in Mercury is only 88 days. VENUS, twice the distance away from the sun, is the next closest planet. It is often considered as "Earth's twin" because of their close proximity in size, mass (weight) and density. But no other planet is in any way comparable to EARTH which has life and all the living things we see around us. Like many other planets, the earth has a natural satellite, the moon, 384,629 km away, that revolves eastward around the Earth once in every 27 days. The fourth planet from the sun is MARS which has dark patches on its surface and is believed by most professional astronomers to be the next planet after Earth to have the possibility of some plant life. Much attention has been focused on Mars to explore the possibilities of extending man's influence to it. Next comes JUPITER, the largest planet in the solar system. Its surface is made up of many gases like hydrogen, helium and methane. It is distinguished from other planets by its circular light and dark bands, and the twelve satellites that circle round it. As it is more than 780 million km from the sun, its surface is very cold, probably about - 128'C. Another unique planet is SATURN which has three rings and nine satellites around it. In size, it is the second largest after Jupiter. It is so far from the sun that it takes 29 and half years to complete its orbit. The seventh planet, URANUS, was not known to astronomers until the late eighteenth century when it was first seen as a faint bluish-green disc through a very powerful telescope. It is another giant planet, 50 times larger than earth and 15 times as heavy. Unlike other planets, Uranus orbits around the sun in a clockwise direction from east to west with five satellites revolving round it. The two outermost planets in the solar system, Neptune and Pluto, are just visible with telescopes. Their discoveries were the result of mathematical calculations on their irregular gravitational effects on neighbouring planetary bodies. NEPTUNE closely resembles Uranus, except that it has only two known satellites and is probably much colder. Pluto is smaller than earth. As the orbits of the planets are not circular but elliptical, the distance of Pluto from the sun during perihelion (i.e. when it is closest to the sun) is 4451 million km, and at aphelion (i.e. when it is farthest from the sun) is 7348 million km. A year in PLUTO is no less than 247 years on earth! Due to their very recent discovery and their extreme remoteness from the earth, very little is so far known about these last two planets.

Exploring the Universe

On a bright night when you look up at the sky, it seems to be studded with stars. Little do you realize that each of the stars is far bigger than the earth on which we live. Some of the larger ones have been estimated to be many millions of times the size of the earth. The stars are not scattered regularly in space; they occur in clusters, better described as galaxies or nebulas. Each galaxy may contain as many as 100,000 million stars. The stars appear small to us even through a telescope because they are so far away. The light from the nearest star traveling at the speed of light (i.e. 299,400 kilometres/186,000 miles per second) takes something like four years to reach us. A ray of light from the sun takes about eight minutes to reach the earth. Light takes only a second to reach us from the moon. In recent years much interest has been shown and vast expenditure has been made, particularly by the United States and the Soviet Union, in exploring outer space. Many problems have had to be overcome. For example, the problem of meeting man's basic needs of oxygen, water and food; temperature control and the problem of weightlessness, while he is in outer space. A number of technological advances have been made in the course of these space programmes, particularly in the fields of radio and television communications. On 4 October 1957 the Soviet Union successfully launched the first artificial satellite (Sputnik I). This was followed by a series of unmanned spacecraft that sent back to earth television pictures of the surface features of the heavenly bodies, and coded information by radio. In April 1961 the Soviet Union successfully placed the first man, Yuri Gagarin, in orbit round the earth. Since then there have been many manned flights into outer space, culminating in the first landing on the moon by American astronauts in a rocket called Apollo 11 in July 1969. The importance of space exploration to man in general, and to geographers and other earth scientists in particular, is immense. Much has been learnt about space temperature, the magnetic fields of the sun and the earth, the amount and kinds of radiation, the shape and extent of the earth's upper atmosphere. We have learnt much about the surface of the moon and about human adaptability to its environment.