The most fertile region in astronomical discovery during the last
quarter of a century has been the planetary members of the solar
system. In 1833, Sir John Herschel enumerated ten planets as visible
from the earth, either by the unaided eye or by the telescope; the
number is now increased more than fivefold. With the exception of
Neptune, the discovery of new planets is confined to the class called
Asteroids. These all revolve in elliptic orbits between those of
Jupiter and Mars. Zitius of Wittemberg discovered an empirical law,
which seemed to govern the distances of the planets from the sun; but
there was a remarkable interruption in the law, according to which a
planet ought to have been placed between Mars and Jupiter. Professor
Bode of Berlin directed the attention of astronomers to the possibility
of such a planet existing; and in seven years’ observations from the
commencement of the present century, not one but four planets were
found, differing widely from one another in the elements of their
orbits, but agreeing very nearly at their mean distances from the sun
with that of the supposed planet. This curious coincidence of the mean
distances of these four asteroids with the planet according to Bode’s
law, as it is generally called, led to the conjecture that these four
planets were but fragments of the missing planet, blown to atoms by
some internal explosion, and that many more fragments might exist, and
be possibly discovered by diligent search.
Concerning this apparently wild hypothesis, Sir John Herschel offered
the following remarkable apology: “This may serve as a specimen of the
dreams in which astronomers, like other speculators, occasionally and
harmlessly indulge.”
The dream, wild as it appeared, has been realised now. Sir John, in the
fifth edition of his _Outlines of Astronomy_, published in 1858, tells
us:
Whatever may be thought of such a speculation as a physical
hypothesis, this conclusion has been verified to a considerable
extent as a matter of fact by subsequent discovery, the result
of a careful and minute examination and mapping down of the
smaller stars in and near the zodiac, undertaken with that express
object. Zodiacal charts of this kind, the product of the zeal and
industry of many astronomers, have been constructed, in which
every star down to the ninth, tenth, or even lower magnitudes, is
inserted; and these stars being compared with the actual stars of
the heavens, the intrusion of any stranger within their limits
cannot fail to be noticed when the comparison is systematically
conducted. The discovery of Astræa and Hebe by Professor Hencke,
in 1845 and 1847, revived the flagging spirit of inquiry in this
direction; with what success, the list of fifty-two asteroids,
with their names and the dates of their discovery, will best show.
The labours of our indefatigable countryman, Mr. Hind, have been
rewarded by the discovery of no less than eight of them.