![[Illustration: Kepler]](https://oldbook.b-cdn.net/kitaplar/2/pg12406-h/images/kepler.jpg "Kepler")
Contents.
Chapter I.
Astronomy Before Kepler.
In order to emphasise the importance of the reforms introduced intoastronomy by Kepler, it will be well to sketch briefly the history ofthe theories which he had to overthrow. In very early times it must havebeen realised that the sun and moon were continually changing theirplaces among the stars. The day, the month, and the year were obviousdivisions of time, and longer periods were suggested by the tabulationof eclipses. We can imagine the respect accorded to the Chaldaean sageswho first discovered that eclipses could be predicted, and how thephilosophers of Mesopotamia must have sought eagerly for evidence offresh periodic laws. Certain of the stars, which appeared to wander, andwere hence called planets, provided an extended field for thesespeculations. Among the Chaldaeans and Babylonians the knowledgegradually acquired was probably confined to the priests and utilisedmainly for astrological prediction or the fixing of religiousobservances. Such speculations as were current among them, and alsoamong the Egyptians and others who came to share their knowledge, werealmost entirely devoted to mythology, assigning fanciful terrestrialorigins to constellations, with occasional controversies as to how theearth is supported in space. The Greeks, too, had an elaborate mythologylargely adapted from their neighbours, but they were not satisfied withthis, and made persistent attempts to reduce the apparent motions ofcelestial objects to geometrical laws. Some of the Pythagoreans, if notPythagoras himself, held that the earth is a sphere, and that theapparent daily revolution of the sun and stars is really due to a motionof the earth, though at first this motion of the earth was not supposedto be one of rotation about an axis. These notions, and also that theplanets on the whole move round from west to east with reference to thestars, were made known to a larger circle through the writings of Plato.To Plato moreover is attributed the challenge to astronomers torepresent all the motions of the heavenly bodies by uniformly describedcircles, a challenge generally held responsible for a vast amount ofwasted effort, and the postponement, for many centuries, of realprogress. Eudoxus of Cnidus, endeavouring to account for the fact thatthe planets, during every apparent revolution round the earth, come torest twice, and in the shorter interval between these “s