Planets in history
Before the curses of light pollution and smog, people were more familiar with the night sky than they tend to be today. Planets in the sky were recognized as special by ancient cultures, because they are ‘wandering stars’ that migrate against the background of the ‘fi xed’ stars. Five planets have been known since antiquity:Mercury, Venus, Mars, Jupiter, and Saturn – which are the only ones bright enough to come to the attention of the unaided eye.Of course, the Sun and Moon were obvious too, but the ‘planets’appear as wandering points of light, whereas the Sun and Moon show disks and tended to be regarded differently. Throughout most of humankind’s existence, the Earth was imagined to be the centre of creation, unrelated to objects in the sky, so it was not thought of as a planet.
The intellectual leaps that recognized that the Earth is a ball of rock going round the Sun, that the planets do likewise, and that the Earth is just one of their number were a long time coming. The process was slow, and there were many false dawns. During the 5th century BC, the ancient Greek philosopher Anaxagoras correctly surmised that the Moon is a spherical body refl ecting the light of the Sun, and he was sent into exile on account of his beliefs. In the succeeding centuries, various Chinese astronomers developed similar ideas, but the idea of the Moon as a globe probably did not embed itself into popular consciousness until its appearance through a telescope became known during the 17th century.
As for the planets, they were generally regarded as points of light going round the Earth, until the counterintuitive ‘heliocentric’ view with the Sun as the centre of motion became accepted. The earliest written suggestions that the Earth goes round the Sun occur in Indian texts dating from the 9th century BC, but despite this and subsequent independent suggestions, notably by Hellenic and Islamic sages and eventually by Nikolas Copernicus in 1543, the concept did not achieve ascendancy until the 18th century. Partly on account of his advocacy of the heliocentric theory, Galileo Galilei (who through his telescope had seen mountains on the Moon, the phases of Venus, and four tiny moons orbiting Jupiter)was held under house arrest from 1633 until his death in 1642.