Many of the earliest discoveries are indeed prehistoric. The great
diurnal movement of the heavens, and the annual revolution of the
sun, seem to have been known in times far more ancient than those to
which any human monuments can be referred. The acuteness of the
early observers enabled them to single out the more important of the
wanderers which we now call planets. They saw that the star-like
objects, Jupiter, Saturn, and Mars, with the more conspicuous Venus,
constituted a class of bodies wholly distinct from the fixed stars
among which their movements lay, and to which they bear such a
superficial resemblance. But the penetration of the early
astronomers went even further, for they recognized that Mercury also
belongs to the same group, though this particular object is seen so
rarely. It would seem that eclipses and other phenomena were
observed at Babylon from a very remote period, while the most ancient
records of celestial observations that we possess are to be found in
the Chinese annals.

The study of astronomy, in the sense in which we understand the word,
may be said to have commenced under the reign of the Ptolemies at
Alexandria. The most famous name in the science of this period is
that of Hipparchus who lived and worked at Rhodes about the year
160BC. It was his splendid investigations that first wrought the
observed facts into a coherent branch of knowledge. He recognized
the primary obligation which lies on the student of the heavens to
compile as complete an inventory as possible of the objects which are
there to be found. Hipparchus accordingly commenced by undertaking,
on a small scale, a task exactly similar to that on which modern
astronomers, with all available appliances of meridian circles, and
photographic telescopes, are constantly engaged at the present day.
He compiled a catalogue of the principal fixed stars, which is of