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convenient to relate the motion of heavenly bodies to the solar system."
The history of the above said mathemetical convenience in the words of the famous scientist Sir James is "The history of science provides many intances of situations such as we have been discussing. To begin with the most obvious, Ptolemy and his Arabian successors built up the famous system of cycles and epicycles which enabled them to predict the future positions of the planets.
Many, indeed felt that it was too complex to correspond to the ultimate facts. In the thirteenth century, Alphonso X of Castille is reported to have said that if the heavens were really like that, I could have given the Deity good advice, had He consulted me for His creation'. At a later date Copernicus also thought the Ptolemic system too complex to be true and, after years of thought and labour, showed that the planetary motions could be described much more simply if the background of the motions were changed Ptolemy has assumed a fixed earth, Copernicus substituted a fixed Sun We now know that the sun can no more be said to be at rest, in any absolute sense, than the earth; it is one of the thousands of millions of stars which together from the galactic system, and it moves round the centre of this system just as the earth moves round the centre of the solar system. And even this centre of the galactic system cannot be said to be at rest, for millions of galactic systems can be seen in the sky, all very much like
