[7] _Ast. Nach_., No. 1, 864.




BOOK IV. THE PHYSICAL PERIOD


We have seen how the theory of the solar system was slowly developed
by the constant efforts of the human mind to find out what are the
rules of cause and effect by which our conception of the present
universe and its development seems to be bound. In the primitive ages
a mere record of events in the heavens and on the earth gave the only
hope of detecting those uniform sequences from which to derive rules
or laws of cause and effect upon which to rely. Then came the
geometrical age, in which rules were sought by which to predict the
movements of heavenly bodies. Later, when the relation of the sun to
the courses of the planets was established, the sun came to be looked
upon as a cause; and finally, early in the seventeenth century, for
the first time in history, it began to be recognised that the laws of
dynamics, exactly as they had been established for our own terrestrial
world, hold good, with the same rigid invariability, at least as far
as the limits of the solar system.

Throughout this evolution of thought and conjecture there were two
types of astronomers--those who supplied the facts, and those who
supplied the interpretation through the logic of mathematics. So
Ptolemy was dependent upon Hipparchus, Kepler on Tycho Brahe, and
Newton in much of his work upon Flamsteed.