It is said that about 300 years ago an apple fell from a tree on to the head of Isaac Newton, the great English scientist. Newton knew that the apple fell because of the pull of the Earth's gravity
Newton then began to wonder just how far away from the Earth the pull of gravity could be felt.
He soon realised that it went on and on into space, to the Moon, the Sun and the stars, but that the farther away the weaker the pull.
Newton was then able to explain the way the planets moved round the Sun. They are attracted by the pull of the Sun's gravity.
Tie a small object to a piece of string and swing it round above your head.
The Earth and the other planets move round the Sun in much the same way. The pull of the Sun's gravity takes the place of the string.
But the planets do not move round the Sun in circles like the weight at the end of the string. Each planet moves round in an orbit called an ellipse. The Earth takes a year to move right round the Sun. It speeds round the Sun in an ellipse at about 19 miles a second.
Think again of the object swinging round at the end of the string. If the string were suddenly cut, the object would fly away from you. This is what would happen to the Earth if the Sun were to suddenly disappear; but there is no chance of this happening. Astronomers have seen stars explode, but they have learnt enough about our Sun to know that this cannot happen.
The Earth is like a huge ball. Because of this, the pull of its gravity is always towards its centre. Builders make use of the pull of gravity towards the centre of the Earth to help them to get an upright line.
They make sure that buildings are not leaning by using a plumb-line. This is a long piece of string with a weight ( the plumb ) at one end. It is held close to the building which is being constructed, with the weight hanging free.
The string points towards the centre of the Earth, and so dives a correct upright, or vertical, line.
If a heavy and a light object fall from the same height they both hit the ground at the same time. Gravity make all things fall equally fast.
This was discovered by a scientist named Galileo. He is said to have dropped objects of different weights from the top of a leaning tower. This famous tower may still be seen in the town of Pisa, in Italy.
Men have been studying gravity for many hundred of years. Today, scientists are still learning more about it. Men are now beginning to travel in spaceships. Powerful rockets are needed to overcome the pull of the Earth's gravity.
Man has also landed on the Moon. The pull of the Moon's gravity is much weaker than that of the earth's. This is because the Moon is much smaller and lighter than the earth. The Moon's gravity is so weak that it cannot hold the atoms and molecules of the air near it and so without a special air supply astronauts will not be able to breathe on Moon.
On Earth, a good athlete can jump upto six feet up into the air. On the Moon he would be able to jump about ten times as high.