A History of Science, vol 3 | Page 8

state of the meteorological learning at that time. That this
ball of fire, rushing "at a greater velocity than the swiftest cannon-ball,"
was simply a mass of heated rock passing through our atmosphere, did
not occur to him, or at least was not credited. Nor is this surprising
when we reflect that at that time universal gravitation had been but
recently discovered; heat had not as yet been recognized as simply a
form of motion; and thunder and lightning were unexplained mysteries,
not to be explained for another three-quarters of a century. In the
chapter on meteorology we shall see how the solution of this mystery
that puzzled Halley and his associates all their lives was finally
attained.
BRADLEY AND THE ABERRATION OF LIGHT
Halley was succeeded as astronomer royal by a man whose useful
additions to the science were not to be recognized or appreciated fully
until brought to light by the Prussian astronomer Bessel early in the
nineteenth century. This was Dr. James Bradley, an ecclesiastic, who
ranks as one of the most eminent astronomers of the eighteenth century.
His most remarkable discovery was the explanation of a peculiar
motion of the pole-star, first observed, but not explained, by Picard a
century before. For many years a satisfactory explanation was sought
unsuccessfully by Bradley and his fellow-astronomers, but at last he
was able to demonstrate that the stary Draconis, on which he was
making his observations, described, or appeared to describe, a small
ellipse. If this observation was correct, it afforded a means of
computing the aberration of any star at all times. The explanation of the
physical cause of this aberration, as Bradley thought, and afterwards
demonstrated, was the result of the combination of the motion of light
with the annual motion of the earth. Bradley first formulated this theory
in 1728, but it was not until 1748--twenty years of continuous struggle
and observation by him--that he was prepared to communicate the

results of his efforts to the Royal Society. This remarkable paper is
thought by the Frenchman, Delambre, to entitle its author to a place in
science beside such astronomers as Hipparcbus and Kepler.
Bradley's studies led him to discover also the libratory motion of the
earth's axis. "As this appearance of g Draconis. indicated a diminution
of the inclination of the earth's axis to the plane of the ecliptic," he says;
"and as several astronomers have supposed THAT inclination to
diminish regularly; if this phenomenon depended upon such a cause,
and amounted to 18" in nine years, the obliquity of the ecliptic would,
at that rate, alter a whole minute in thirty years; which is much faster
than any observations, before made, would allow. I had reason,
therefore, to think that some part of this motion at the least, if not the
whole, was owing to the moon's action upon the equatorial parts of the
earth; which, I conceived, might cause a libratory motion of the earth's
axis. But as I was unable to judge, from only nine years observations,
whether the axis would entirely recover the same position that it had in
the year 1727, I found it necessary to continue my observations through
a whole period of the moon's nodes; at the end of which I had the
satisfaction to see, that the stars, returned into the same position again;
as if there had been no alteration at all in the inclination of the earth's
axis; which fully convinced me that I had guessed rightly as to the
cause of the phenomena. This circumstance proves likewise, that if
there be a gradual diminution of the obliquity of the ecliptic, it does not
arise only from an alteration in the position of the earth's axis, but
rather from some change in the plane of the ecliptic itself; because the
stars, at the end of the period of the moon's nodes, appeared in the same
places, with respect to the equator, as they ought to have done, if the
earth's axis had retained the same inclination to an invariable plane."[2]
FRENCH ASTRONOMERS
Meanwhile, astronomers across the channel were by no means idle. In
France several successful observers were making many additions to the
already long list of observations of the first astronomer of the Royal
Observatory of Paris, Dominic Cassini (1625-1712), whose reputation
among his contemporaries was much greater than among succeeding
generations of astronomers. Perhaps the most deserving of these
successors was Nicolas Louis de Lacaille (1713-1762), a theologian
who had been educated at the expense of the Duke of Bourbon, and

who, soon after completing his clerical studies, came under the
patronage of Cassini, whose attention had been called to the young
man's interest in the sciences. One of Lacaille's first under-takings was
the remeasuring of the French are of the meridian, which had been