the starry background was exceedingly
small--so small, indeed, that no eye could have detected it without the
aid of a telescope!

Of the two predictions of the place of Neptune in the sky, that of Le
Verrier was the nearer. Indeed, the position calculated by Adams was
more than twice as far out. But Adams was by a long way the first in the
field with his results, and only for unfortunate delays the prize would
certainly have fallen to him. For instance, there was no star-map at
Cambridge, and Professor Challis, the director of the observatory there,
was in consequence obliged to make a laborious examination of the stars
in the suspected region. On the other hand, all that Galle had to do was
to compare that part of the sky where Le Verrier told him to look with
the Berlin star-chart which he had by him. This he did on September 23,
1846, with the result that he quickly noted an eighth magnitude star
which did not figure in that chart. By the next night this star had
altered its position in the sky, thus disclosing the fact that it was
really a planet.

Six days later Professor Challis succeeded in finding the planet, but of
course he was now too late. On reviewing his labours he ascertained that
he had actually noted down its place early in August, and had he only
been able to sift his observations as he made them, the discovery would
have been made then.

Later on it was found that Neptune had only just missed being discovered
about fifty years earlier. In certain observations made during 1795, the
famous French astronomer, Lalande, found that a star, which he had
mapped in a certain position on the 8th of May of that year, was in a
different position two days later. The idea of a planet does not appear
to have entered his mind, and he merely treated the first observation as
an error!

The reader will, no doubt, recollect how the discovery of the asteroids
was due in effect to an apparent break in the seemingly regular sequence
of the planetary orbits outwards from the sun. This curious sequence of
relative distances is usually known as "Bode's Law," because it was
first brought into general notice by an astronomer of that name. It had,
however, previously been investigated mathematically by Titius in 1772.
Long before this, indeed, the unnecessarily wide space between the
orbits of Mars and Jupiter had attracted the attention of the great
Kepler to such a degree, that he predicted that a planet would some day
be