Earth is one of the nine planets that travel around the Sun and the largest of what are called the terrestrial planets. The terrestrial planets are the four nearest the Sun (Mercury, Venus, Earth and Mars) and are small, rocky, and dense. The outer so-called jovian planets (Jupiter, Saturn, Uranus, and Neptune) are large and composed mostly of frozen gases. Astronomers are not sure about he nature of the ninth planet, Pluto.

The Earth is special in several ways. It is the only one with large quantities of surface water whose average temperature is between 0C (32F) and 100C (212F), the freezing point and boiling point of water. This is an absolute necessity for the existence and survival of life. It is the only planet in the Solar System on which life exists, and it does so in such abundance that it clothes the surface, fills the seas and has actually changed the atmosphere, for all our oxygen has come from plant life.

Without the continents and ocean basins, we would not have the large, constant bodies of water necessary for the origin of lie, nor the persistent dry land on which man and the higher forms of animal life could have evolved.

Shape, Size, and Motion
The Earth is a ball. The photographs taken from outer space by the Apollo astronauts and from orbiting satellites prove beyond doubt what man already knew from simple observations such as the disappearance of ships below the horizon. Pythagoras, the Greek philosopher of the 6th century BC was the first man to say the Earth was round. Another Greek mathematician, Eratosthenes, was the first to measure the Earth' s circumference in the 3rd century BC. He was told that at Aswan in Egypt the Sun's rays at noon on midsummer's day came
from vertically overhead because the Sun's reflection could be seen in the bottom of a deep well. At Alexandria, which he guessed to be 800 kilometers (500 miles) away to the north, he noticed that at noon on midsummer's day the Sun's rays cam down at an angel of 7 to the vertical. A simple calculation gave his the circumference of the Earth almost exactly. In spite of this knowledge, it was not generally accepted that the Earth was round until the 16th century, when explorers actually sailed round it.

The Earth is not perfectly round, but bulges slightly at the equator. Its rotation produces a centrifugal force, which causes the bulge. The poles are slightly flattened and are about 21 kilometers (13 miles) nearer the Earth's center than is the equator.

The Earth makes one complete rotation on its axis in 23 hours 56 minutes 4.09 seconds. Thus the actual period of rotation is about four minutes less than the full 24-hour day. However, as the Earth is revolving around the Sun once in 365 days, each day the Earth travels in its orbit, or path round the Sun. The positions of points on its surface alter with respect to the Sun, so that noon at a particular place comes not after one complete rotation, but four minutes later. Thus, all places on Earth experience a day that is 24 hours long.

The rotation of the Earth on its own axis gives us night and day: the side of the Earth turned towards the Sun has daylight while the side turned away has night. However, the lengths of daytime and nighttime vary throughout the year, giving us the seasons. In summer, daytime is longer than 12 hours, in winter it is shorter. The reason for summer and winter is that the Earth's axis of rotation points in the same direction in space (towards the Pole Star) all the way round the Sun. The axis of rotation (the imaginary line between the north and south poles) is tilted at an angel of 23 27'. For half an orbit, therefore, the axis is tilted towards the Sun and for the other half tilted away. When the North Pole is tilted towards the Sun, the Northern Hemisphere is having the summer. Because of this tilt towards the Sun in summer, places in the Northern Hemisphere spend more time in sunlight than in shadow during one rotation of the Earth, so their day is longer than their night. In fact, on midsummer's day in the northern hemisphere, everywhere north of the Arctic Circle is in sunlight throughout the rotation; hence the "midnight sun" in the Arctic. The same thing happens in reverse in the Southern Hemisphere.

It is always assumed that the Earth and the other planets were formed at the same time. In one theory of the origin of the Solar System, the Sun and planets were formed at the same time; this is the "cold accretion" theory (accretion means "growing together"). Another theory (no longer widely held) suggests that the Sun already existed and the planets were formed from it: this is known as the "centrifugal ejection" theory.

In the cold accretion theory, a huge cloud of gas and dust in our galaxy began to contract under its own gravity. Most of the cloud concentrated to form the Sun, which reached a size large enough for nuclear reactions to start in its highly compressed interior. Small concentrations of matter within the surrounding cloud became the early planets. These early planets or "proto-planets" consisted of gas, but were too small for nuclear reactions to get under way, as they had done in the Sun. As the Sun heated up, the gaseous envelopes of the inner, terrestrial planets were stripped off, leaving rocky, probably molten cores. The outer Jovian planets have remained more or less unchanged.

In the centrifugal ejection theory, a disk of matter is thought to have been thrown out from the Sun in the early stages of its development, when it was 50 to 60 times its present size and spinning rapidly. The inner planets composed of less volatile rocky and metallic materials condensed first, followed by the more gaseous outer planets.

1988 Encyclopaedia Britannica, Inc.

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