[Page v]
[Greek:
TAEI PSUCHAEI
TAEI AGAPAETAEI
ASTRAPOUSAEI
KAI
ISAGGEDOI]




[Page vii]
PREFACE.

All sciences are making an advance, but Astronomy is moving at the
double-quick. Since the principles of this science were settled
by Copernicus, four hundred years ago, it has never had to beat
a retreat. It is rewritten not to correct material errors, but
to incorporate new discoveries.

Once Astronomy treated mostly of tides, seasons, and telescopic
aspects of the planets; now these are only primary matters. Once
it considered stars as mere fixed points of light; now it studies
them as suns, determines their age, size, color, movements, chemical
constitution, and the revolution of their planets. Once it considered
space as empty; now it knows that every cubic inch of it quivers with
greater intensity of force than that which is visible in Niagara.