Astronomical Telescope.

In 1606 came a second treatise on the new star, discussing various
theories to account for its appearance, and refusing to accept the
notion that it was a "fortuitous concourse of atoms". This was followed
in 1607 by a treatise on comets, suggested by the comet appearing that
year, known as Halley's comet after its next return. He regarded comets
as "planets" moving in straight lines, never having examined sufficient
observations of any comet to convince himself that their paths are
curved. If he had not assumed that they were external to the system and
so could not be expected to return, he might have anticipated Halley's
discovery. Another suggestive remark of his was to the effect that the
planets must be self-luminous, as otherwise Mercury and Venus, at any
rate, ought to show phases. This was put to the test not long afterwards
by means of Galileo's telescope.

In 1607 Kepler rushed into print with an alleged observation of Mercury
crossing the sun, but after Galileo's discovery of sun-spots, Kepler at
once cheerfully retracted his observation of "Mercury," and so far was
he from being annoyed or bigoted in his views, that he warmly adopted
Galileo's side, in contrast to most of those whose opinions were liable
to be overthrown by the new discoveries. Maestlin and others of Kepler's
friends took the opposite view.






CHAPTER V.