THE ASTEROIDS
In the year 1772 Bode of Berlin published the statement of a curiously
symmetrical relation existing among the planets of our system. The
gist of this relation, known as Bode's law, though it was really
discovered by Titius of Wittenberg, may be summed up briefly thus:
'The interval between the orbits of any two planets is about twice as
great as the inferior interval, and only half the superior one.'
Thus the distance between the orbits of the earth and Venus should,
according to Bode's law, be half of that between the earth and Mars,
which again should be half of that which separates Mars from the
planet next beyond him. Since the discovery of Neptune, this so-called
law has broken down, for Neptune is very far within the distance which
it requires; but at the time of its promulgation it represented with
considerable accuracy the actual relative positions of the planets,
with one exception. Between Mars and Jupiter there was a blank which
should, according to the law, have been filled by a planet, but to all
appearance was not. Noticing this blank in the sequence, Bode ventured
to predict that a planet would be found to fill it; and his foresight
was not long in being vindicated.
Several continental astronomers formed a kind of planet-hunting
society to look out for the missing orb; but their operations were
anticipated by the discovery on January 1, 1801, of a small planet
which occupied a place closely approximating to that indicated for the
missing body by Bode's law. The news of this discovery, made by Piazzi
of Palermo in the course of observations for his well-known catalogue
of stars, did not reach Bode till March 20, and 'the delay just
afforded time for the publication, by a young philosopher of Jena
named Hegel, of a "Dissertation" showing, by the clearest light of
reason, that the number of the planets could not exceed seven, and
exposing the folly of certain devotees of induction who sought a new
celestial body merely to fill a gap in a numerical series.'
The remarkable agreement of prediction and discovery roused a
considerable amount of interest, though the planet actually found, and
named Ceres after the patron-goddess of Sicily, seemed disappointingly
small. But before very long Olbers, one of the members of the original
planet-hunting society, surprised the astronomical world by the
discovery of a second planet which also fulfilled the condition of
Bode's law; and by the end of March, 1807, two other planets equally
obedient to the required numerical standard were found, the first by
Harding, the second by Olbers. Thus a system of four small planets,
Ceres, Pallas, Juno, and Vesta, was found to fill that gap in the
series which had originally suggested the search. To account for their
existence Olbers proposed the theory that they were the fragments of
a large planet which had been blown to pieces either by the disruptive
action of internal forces or by collision with a comet; and this
theory remained in favour for a number of years, though accumulating
evidence against it has forced its abandonment.
It was not till 1845 that there was any addition to the number of the
asteroids, as they had come to be named. In that year, however, Hencke
of Driessen in Prussia, discovered a fifth, which has been named
Astræa, and in 1847 repeated his success by the discovery of a sixth,
Hebe. Since that time there has been a steady flow of discoveries,
until at the present time the number known to exist is close upon 700,
of which 569 have received permanent numbers as undoubtedly distinct
members of the solar system; and this total is being steadily added to
year by year, the average annual number of discoveries for the years
1902 to 1905 inclusive, being fifty-two. For a time the search for
minor planets was a most laborious business. The planet-hunter had
to construct careful maps of all the stars visible in a certain small
zone of the ecliptic, and to compare these methodically with the
actual face of the sky in the same zone, as revealed by his telescope.
Any star seen in the telescope, and not found to be marked upon the
chart, became forthwith an object of grave suspicion, and was watched
until its motion, or lack of motion, relatively to the other stars
either proved or disproved its planetary nature. At present this
lengthy and wearisome process has been entirely superseded by the
photographic method, in which a minor planet is detected by the fact
that, being in motion relatively to the fixed stars, its image will
appear upon the plate in the shape of a short line or trail, the
images of the fixed stars being round dots. Of course the trail may be
due to a planet which has already been discovered; but should there
be no known minor planet in the position occupied by the trail, then
a new member has been added to the system. Minor-planet hunting
has always been a highly specialized branch of astronomy, and a few
observers, such as Peters, Watson, Charlois and Palisa, and at present
Wolf, have accounted for the great majority of the discoveries.
It was, however, becoming more and more a matter of question what
advantage was to be gained by the continuance of the hunt, when a
fresh fillip was given to interest by the discovery in 1898 of the
anomalous asteroid named Eros. Hitherto no minor planet had been known
to have the greater portion of its orbit within that of Mars, though
several do cross the red planet's borders; but the mean distance of
Eros from the sun proves to be about 135,000,000, while that of Mars
is 141,000,000 miles. In addition, the orbit of the new planet is
such that at intervals of sixty-seven years it comes within 15,000,000
miles of the earth, or in other words nearer to us than any other
celestial body except the moon or a chance comet. It may thus come
to afford a means of revising estimates of celestial distances. Eros
presents another peculiarity. It has been found by E. von Oppolzer
to be variable in a period of two hours thirty-eight minutes; and the
theory has been put forward that the planet is double, consisting of
two bodies which revolve almost in contact and mutually eclipse one
another--in short, that Eros as a planet presents the same phenomenon
which we shall find as a characteristic of that type of variable stars
known as the Algol type. An explanation, in some respects more simple
and satisfactory, is that the variation in light is caused by the
different reflective power of various parts of its surface; but the
question is still open.
The best results for the sizes of the four asteroids first discovered
are those of Barnard, from direct measurements with the Lick telescope
in 1894. He found the diameter of Ceres to be 485 miles, that of
Pallas 304, those of Vesta and Juno 243 and 118 miles respectively.
There appears to be as great diversity in the reflective power of
these original members of the group as in their diameters. Ceres is
large and dull, and, in Miss Clerke's words, 'must be composed of
rugged and sombre rock, unclothed probably by any vestige of air,'
while Vesta has a surface which reflects light with four times the
intensity of that of Ceres, and is, in fact, almost as brilliantly
white as newly fallen snow.
In the place of Olbers' discredited hypothesis of an exploded planet,
has now been set the theory first suggested by Kirkwood, that instead
of having in the asteroids the remnants of a world which has become
defunct, we have the materials of one which was never allowed to
form, the overwhelming power of Jupiter's attraction having exerted
a disruptive influence over them while their formation was still only
beginning.
So far as I am aware, they share with Mars the distinction of being
the only celestial bodies which have been made the subjects of a
testamentary disposition. In the case of Mars, readers may remember
that some years ago a French lady left a large sum of money to be
given to the individual who should first succeed in establishing
communication with the Planet of War; in that of the asteroids, the
late Professor Watson, a mighty hunter of minor planets in his day,
made provision for the supervision of the twenty-two planets captured
by him, lest any of them should get lost, stolen, or strayed.
Small telescopes are, of course, quite impotent to deal with such
diminutive bodies as the asteroids; nor, perhaps, is it desirable
that the ranks of the minor-planet hunters should be reinforced to any
extent.