The 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.
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.
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.
Ptolemy, who lived in the 2nd century AD, observed that, while the stars moved across the night sky along regular paths, the planets appeared to “wander” from theirs. He proposed that they each moved in their own small circles, called epicycles, as they orbited Earth.
Astronomers in BabylonBabylon ruled a powerful empire in the 6th-7th centuries BC. The Babylonians were keen astronomers. They studied the stars and planets and tried to work out their positions in relation to the Earth. They believed that the Earth was a flat disc suspended in space on a cushion of air. Some scientists of ancient Greece also adopted this theory.
The Maya, who built a great civilization in what is now Mexico and Central America between AD 250 and 900, became very advanced in branches of astronomy and mathematics. Maya priests used this knowledge to draw up a calendar. Maya astronomers had no telescopes, but they may have used observatories to plot the positions of the planet Venus, Sun and Moon.
While there were few advances in astronomy in Europe during the Middle Ages, astronomy flourished in the Islamic World during its Golden Age, between the 8th and 15th centuries. The Persian astronomer Azophi made the first recorded observations of the Andromeda Galaxy in 964. The brightest stellar event in history, a supernova, was observed by the Arabic astronomer Ali ibn Ridwan in 1006. Muslim astronomers gave names to many stars during this period, including Aldebaran and Altair.
The Polish priest and astronomer, Nicolaus Copernicus (1473–1543), challenged Ptolemy’s view of the Solar System, declaring—correctly—that the Sun lay at the centre of a system of orbiting planets. Only the Moon orbited the Earth. Copernicus did, however, wrongly believe that the planets’ orbits were perfect circles and that they moved in epicycles.
No supporter of the Copernican view, Danish astronomer Tycho Brahe (1546–1601) realized that the Ptolemaic system did not work either. His careful measurements of the planets’ movements suggested some other pattern. Brahe’s proposal that the planets orbited the Sun while the Sun moved around the Earth gained few supporters, however.
It was left to the German astronomer Johannes Kepler (1571–1630), who showed, using Brahe’s detailed observations, that the planets moved in elliptical, rather than perfectly circular, orbits. The shapes of their orbits also explained the “wandering” that so perplexed earlier observers, thus disproving the idea that the planets moved in epicycles. Kepler was the first person to arrive at the completely correct view of the Solar System.
Italian scientist Galileo Galilei (1564–1642) was the first person to use a telescope, newly invented in 1608, to study the heavens. Using a telescope of his own design, he found, to his amazement, great mountain ranges and craters on the Moon.
The phases of the Moon sketched by Galileo He witnessed the phases of Venus—its changing shape from crescent to full disc as it circles the Sun—and discovered many new stars never seen before. Galileo also discovered four large Moons circling Jupiter. That they clearly orbited their "parent" planet was, to him, firm evidence that not all objects in the Universe orbited the Earth. From this, he concluded that Copernicus had been correct: the planets do orbit the Sun.
Consultant: Dave Hawksett