The Online Library of Knowledge



The Gran Canaria TelescopeThe Gran Canaria TelescopeAstronomy is the study of space, including planets, stars and galaxies. We can see some objects in space with just the naked eye, but many more, including those that are billions of light years away, can only be studied by using a powerful telescope. Information about the objects in space ("celestial bodies") is gained from studying visible light or other forms of electromagnetic radiation such as radio waves and X-rays given off or reflected by them. Much of what we now know about the planets comes from space probes, which travel through space sending information back to the Earth.

The ancient Egyptian god Horus The ancient Egyptian god Horus

Early astronomers

Thousands of years ago, in the time of the ancient civilizations of Egypt and China, people thought that the Sun and Moon were gods, the Earth was flat and the sky was a great dome suspended above it.

Ptolemy's view of the Solar System Ptolemy's view of the Solar System

Ancient Greece

In later years, astronomers from ancient Greece proved that the Earth was round. Many believed that the stars were fixed to a great sphere that rotated around the Earth each day. One 3rd-century BC Greek astronomer, Aristarchus, proposed that the planets, including Earth, orbited the Sun, a star, but most astronomers of this time thought that the Sun, Moon and planets all travelled in circular paths around Earth, the centre of the Universe.

The Antikythera Mechanism, built between 150 and 80 BC, was designed to calculate the positions of the Sun, Moon and planets on any particular date. Made in ancient Greece, it has been described as the world's oldest known analogue computer. It consists of at least 30 interlocked gears turned by a hand crank.
