It is never safe, as we know, to judge by appearances, and this isperhaps more true of astronomy than of anything else.
For instance, the idea which one would most naturally form of the earthand heaven is that the solid earth on which we live and move extends toa great distance in every direction, and that the heaven is an immensedome upon the inner surface of which the stars are fixed. Such mustneeds have been the idea of the universe held by men in the earliesttimes. In their view the earth was of paramount importance. The sun andmoon were mere lamps for the day and for the night; and these, if notgods themselves, were at any rate under the charge of special deities,whose task it was to guide their motions across the vaulted sky.
Little by little, however, this simple estimate of nature began to beoverturned. Difficult problems agitated the human mind. On what, forinstance, did the solid earth rest, and what prevented the vaultedheaven from falling in upon men and crushing them out of existence?Fantastic myths sprang from the vain attempts to solve these riddles.The Hindoos, for example, imagined the earth as supported by fourelephants which stood upon the back of a gigantic tortoise, which, inits turn, floated on the surface of an elemental ocean. The earlyWestern civilisations conceived the fable of the Titan Atlas, who, as apunishment for revolt against the Olympian gods, was condemned to holdup the expanse of sky for ever and ever.
Later on glimmerings of the true light began to break in upon men. TheGreek philosophers, who busied themselves much with such matters,gradually became convinced that the earth was spherical in shape, thatis to say, round like a ball. In this opinion we now know that they wereright; but in their other important belief, viz. that the earth wasplaced at the centre of all things, they were indeed very far from thetruth.
By the second century of the Christian era, the ideas of the earlyphilosophers had become hardened into a definite theory, which, thoughit appears very incorrect to us to-day, nevertheless demands exceptionalnotice from the fact that it was everywhere accepted as the trueexplanation until so late as some four centuries ago. This theory of theuniverse is known by the name of the Ptolemaic System, because it wasfirst set forth in definite terms by one of the most famous of theastronomers of antiquity, Claudius Ptolemæus Pelusinensis
, better known as Ptolemy of Alexandria.
In his system the Earth occupied the centre; while around it circled inorder outwards the Moon, the planets Mercury and Venus, the Sun, andthen the planets Mars, Jupiter, and Saturn. Beyond these again revolvedthe background of the heaven, upon which it was believed that the starswere fixed--
"Stellis ardentibus aptum,"
as Virgil puts it
see Fig. 1
[Illustration: FIG. 1.--The Ptolemaic idea of the Universe.]
The Ptolemaic system persisted unshaken for about fourteen hundred yearsafter the death of its author. Clearly men were flattered by the notionthat their earth was the most important body in nature, that it stoodstill at the centre of the universe, and was the pivot upon which allthings revolved.