In 1721 Halley succeeded Flamsteed as astronomer
royal at the Greenwich Observatory. Although sixty-
four years of age at that time his activity in astronomy
continued unabated for another score of years. At
Greenwich he undertook some tedious observations
of the moon, and during those observations was first
to detect the acceleration of mean motion. He was
unable to explain this, however, and it remained for
Laplace in the closing years of the century to do so,
as we shall see later.

Halley's book, the Synopsis Astronomiae Cometicae,
is one of the most valuable additions to astronomical
literature since the time of Kepler. He was first to
attempt the calculation of the orbit of a comet, having
revived the ancient opinion that comets belong to the
solar system, moving in eccentric orbits round the sun,
and his calculation of the orbit of the comet of 1682 led
him to predict correctly the return of that comet in
1758. Halley's Study of Meteors.

Like other astronomers of his time be was greatly
puzzled over the well-known phenomena of shooting-
stars, or meteors, making many observations himself,
and examining carefully the observations of other
astronomers. In 1714 he gave his views as to the
origin and composition of these mysterious visitors
in the earth's atmosphere. As this subject will be
again referred to in a later chapter, Halley's views,