stinct improvement over the old. The year following he sailed for the
Island of St, Helena to make observations of the heavens in the southern
hemisphere.

It was while in St. Helena that Halley made his famous observation
of the transit of Mercury over the sun's disk, this observation being
connected, indirectly at least, with his discovery of a method of
determining the parallax of the planets. By parallax is meant the
apparent change in the position of an object, due really to a change in
the position of the observer. Thus, if we imagine two astronomers making
observations of the sun from opposite sides of the earth at the same
time, it is obvious that to these observers the sun will appear to be
at two different points in the sky. Half the angle measuring this
difference would be known as the sun's parallax. This would depend,
then, upon the distance of the earth from the sun and the length of
the earth's radius. Since the actual length of this radius has been
determined, the parallax of any heavenly body enables the astronomer to
determine its exact distance.

The parallaxes can be determined equally well, however, if two observers
are separated by exactly known distances, several hundreds or thousands
of miles apart. In the case of a transit of Venus across the sun's
disk, for example, an observer at New York notes the image of the planet
moving across the sun's disk, and notes also the exact time of this
observation. In the same manner an observer at London makes similar
observations. Knowing the distance between New York and London, and
the different time of the passage, it is thus possible to calculate the
difference of the parallaxes of the sun and a planet crossing its disk.
The idea of thus determining the parallax of the planets originated, or
at least was developed, by Halley, and from this phenomenon he thought
it possible to conclude the dimensions of all the planetary orbits. As
we shall see further on, his views were found to be correct by later
astronomers.

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