A History of Science, vol 3 | Page 6

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 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,
representing the most advanced views of his age, are of interest.
"The theory of the air seemeth at present," he says, "to be perfectly well
understood, and the differing densities thereof at all altitudes; for
supposing the same air to occupy spaces reciprocally proportional to

the quantity of the superior or incumbent air, I have elsewhere proved
that at forty miles high the air is rarer than at the surface of the earth at
three thousand times; and that the utmost height of the atmosphere,
which reflects light in the Crepusculum, is not fully forty-five miles,
notwithstanding which 'tis still manifest that some sort of vapors, and
those in no small quantity, arise nearly to that height. An instance of
this may be given in the great light the society had an account of (vide
Transact. Sep., 1676) from Dr. Wallis, which was seen in very distant
counties almost over all the south part of England. Of which though the
doctor could not get so particular a relation as was requisite to
determine the height thereof, yet from the distant places it was seen in,
it could not but be very many miles high.
"So likewise that meteor which was seen in 1708, on the 31st of July,
between nine and ten o'clock at night, was evidently between forty and
fifty miles perpendicularly high, and as near as I can gather, over
Shereness and the buoy on the Nore. For