The Minor Planets
Up to 1908 the number of minor planets (or asteroids) certainly known
amounted to over 650.
From an examination of the distribution of the first 512 of these small
bodies, Dr. P. Stroobant finds that a decided maximum in number occurs
between the limits of distance of 2·55 and 2·85 (earth's mean distance
from sun = 1), "199 of the asteroids considered revolving in this
annulus." He finds that nearly all the asteroidal matter is concentrated
near to the middle of the ring in the neighbourhood of the mean distance
of 2·7, and the smallest asteroids are relatively less numerous in the
richest zones.[115]
There are some "striking similarities" in the orbits of some of the
asteroids. Thus, in the small planets Sophia (No. 251 in order of
discovery) and Magdalena (No. 318) we have the mean distance of Sophia
3·10, and that of Magdalena 3·19 (earth's mean distance = 1). The
eccentricities of the orbits are 0·09 and 0·07; and the inclinations of
the orbits to the plane of the ecliptic 10° 29' and 10° 33'
respectively.[116] This similarity may be--and probably is--merely
accidental, but it is none the less curious and interesting.
Some very interesting discoveries have recently been made among the minor
planets. The orbit of Eros intersects the orbit of Mars; and the following
have nearly the same mean distance from the sun as Jupiter:--
Achilles (1906 TG), No. 588,
Patrocles (1906 XY), No. 617,
Hector (1907 XM), No. 624,
and another (No. 659) has been recently found. Each of these small planets
"moves approximately in a vertex of an equilateral triangle that it forms
with Jupiter and the sun."[117] The minor planet known provisionally as HN
is remarkable for the large eccentricity of its orbit (0·38), and its
small perihelion distance (1·6). When discovered it had a very high South
Declination (61-1/2°), showing that the inclination of the plane of its
orbit to the plane of the ecliptic is considerable.[118]
Dr. Bauschinger has made a study of the minor planets discovered up to the
end of 1900. He finds that the ascending nodes of the orbits show a
marked tendency to cluster near the ascending node of Jupiter's orbit, a
fact which agrees well with Prof. Newcomb's theoretical results. There
seems to be a slight tendency for large inclinations and great
eccentricities to go together; but there appears to be no connection
between the eccentricity and the mean distance from the sun. The
longitudes of the perihelia of these small planets "show a well-marked
maximum near the longitude of _Jupiter's_ perihelion, and equally
well-marked minimum near the longitude of his aphelion," which is again in
good agreement with Newcomb's calculations.[119] Dr. Bauschinger's
diameter for Eros is 20 miles. He finds that the whole group, including
those remaining to be discovered, would probably form a sphere of about
830 miles in diameter.
The total mass of the minor planets has been frequently estimated, but
generally much too high. Mr. B. M. Roszel of the John Hopkins University
(U.S.A.) has made a calculation of the probable mass from the known
diameter of Vesta (319 miles, Pickering), and finds the volume of the
first 216 asteroids discovered. From this calculation it appears that it
would take 310 asteroids of the 6th magnitude, or 1200 of the 7th to equal
the moon in volume. Mr. Roszel concludes that the probable mass of the
whole asteroidal belt is between 1/50th and 1/100th of that of the
moon.[120] Subsequently Mr. Roszel extended his study to the mass of 311
asteroids,[121] and found a combined mass of about 1/40th of the moon's
mass.
Dr. Palisa finds that the recently discovered minor planet (1905 QY)
varies in light to a considerable extent.[122] This planet was discovered
by Dr. Max Wolf on August 23, 1905; but it was subsequently found that it
is identical with one previously known, (167) Urda.[123] The light
variation is said to be from the 11th to the 13th magnitude.[124]
Variation in some of the other minor planets has also been suspected.
Prof. Wendell found a variation of about half a magnitude in the planet
Eunomia (No. 15). He also found that Iris (No. 7) varies about a quarter
of a magnitude in a period of about 6{h} 12{m}.[125] But these variations
are small, and perhaps doubtful. The variability of Eros is well known.
The planet Eros is a very interesting one. The perihelion portion of its
orbit lies between the orbits of Mars and the earth, and the aphelion part
is outside the orbit of Mars. Owing to the great variation in its distance
from the earth the brightness of Eros varies from the 6th to the 12th
magnitude. That is, when brightest, it is 250 times brighter than when it
is faintest.[126] This variation of light, is of course, merely due to the
variation of distance; but some actual variation in the brightness of the
planet has been observed.
It has been shown by Oeltzen and Valz that Cacciatore's supposed distant
comet, mentioned by Admiral Smyth in his _Bedford Catalogue_, must have
been a minor planet.[127]
Dr. Max Wolf discovered 36 new minor planets by photography in the years
1892-95. Up to the latter year he had never seen one of these through a
telescope! His words are, "Ich selsbt habe noch nie einen meinen kleinen
Planeten am Himmel gesehen."[128]
These small bodies have now become so numerous that it is a matter of much
difficulty to follow them. At the meeting of the Royal Astronomical
Society on January 8, 1909, Mr. G. F. Chambers made the following
facetious remarks--
"I would like to make a suggestion that has been in my mind for
several years past--that it should be made an offence punishable by
fine or imprisonment to discover any more minor planets. They seem to
be an intolerable nuisance, and are a great burden upon the literary
gentlemen who have to keep pace with them and record them. I have
never seen, during the last few years at any rate, any good come from
them, or likely to come, and I should like to see the supply stopped,
and the energies of the German gentlemen who find so many turned into
more promising channels."
Among the minor planets numbered 1 to 500, about 40 "have not been seen
since the year of their discovery, and must be regarded as lost."[129]