It must be remembered that in these days we know of
the physical necessity which requires that a planet shall revolve in
an ellipse and not in any other curve. But Kepler had no such
knowledge. Even to the last hour of his life he remained in
ignorance of the existence of any natural cause which ordained that
planets should follow those particular curves which geometers know so
well. Kepler's assignment of the ellipse as the true form of the
planetary orbit is to be regarded as a brilliant guess, the truth of
which Tycho's observations enabled him to verify. Kepler also
succeeded in pointing out the law according to which the velocity of
a planet at different points of its path could be accurately
specified. Here, again, we have to admire the sagacity with which
this marvellously acute astronomer guessed the deep truth of nature.
In this case also he was quite unprovided with any reason for
expecting from physical principles that such a law as he discovered
must be obeyed. It is quite true that Kepler had some slight
knowledge of the existence of what we now know as gravitation. He
had even enunciated the remarkable doctrine that the ebb and flow of
the tide must be attributed to the attraction of the moon on the
waters of the earth.
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