_THE PLANETOIDS._
Number.—History of their Discovery.—Dimensions and
Brightness.—Occultation of Vesta.
_Number._—These bodies, also called minor planets, and, formerly,
asteroids, comprise a very numerous class, and they are extremely
small, being quite invisible to the naked eye except in one or two
special cases. They all revolve in orbits situated between Mars and
Jupiter. The total number discovered is about 300, of which Prof. J.
Palisa of Vienna has found more than 70, and the late Dr. C. H. F.
Peters of Clinton, N.Y., 49. I have not given exact numbers in the two
former cases, because these discoveries are still rapidly progressing.
_History of their Discovery._—The first known planetoid (Ceres) was
sighted by Piazzi on Jan. 1, 1801. The following year, on March 28,
Olbers found another (Pallas). In 1804, on Sept. 1, Harding discovered
a third (Juno); and in 1807, March 29, Olbers was a second time
successful (Vesta). Then for thirty-eight years no additions were made
to the number. The host of planetoids circulating between Mars and
Jupiter preserved their incognito without disturbance from the prying
and wakeful eyes of astronomers.
But in 1845 Hencke, of Driessen, after years of watching, at length
broke the spell of tranquillity by finding another small planet; and
his example was emulated by many other observers in subsequent years.
Hind, De Gasparis, and Goldschmidt were amongst the earliest and
most successful of those who gathered new planets from amongst the
stars of the zodiacal constellations. In later years Luther, Watson,
and Borrelly further extended the list; but Palisa and Peters have
distanced all competitors, and shown a zeal in the work which has
yielded an astonishing aggregate of discoveries. Charlois, at Nice, has
latterly earned distinction in the same field.
Since 1845 new planetoids have been found at the rate of more than
six per annum, and a rich harvest yet remains to be gathered by
the planet-seekers of the future. A very large proportion of those
already detected are between the tenth and twelfth magnitudes, and are
therefore only to be discerned in good instruments. They present no
distinction from small star-like points, and are to be identified by
their motions alone. The mythological dictionary has furnished names
for them, and they are numbered in the order of their discovery as well.
_Dimensions and Brightness._—Vesta is the largest and brightest of
the group, while Ceres and Pallas rank as second and third in the
same respect. Vesta is about 214 miles in diameter; but the more
insignificant members of this family are probably not more than about
15 or 20 miles in diameter. Pallas has the most inclined orbit of all,
the inclination amounting to 30° 44′; so that its position is by no
means confined to the planet-zone of the ecliptic. Vesta is sometimes
brighter than a 6th mag. star; while Ceres, Pallas, and Juno vary
between about the 7th and 8th magnitudes, according to their distances
from the Earth. A real variation of light has been assumed to occur,
but this is not fully proved.
In March 1887 Mr. Backhouse, of Sunderland, saw an apparently new,
yellowish-white star near 103 Piscium, and it was just visible to the
naked eye. This proved to be Vesta, though the identity of the object
was not known at first, and it formed the subject of two Dun Echt
circulars.
Formerly, hazy indefinite outlines were attributed to some of the
planetoids; but the appearance probably arose from instrumental defects.
The search for these bodies is not a work likely to engage amateurs.
Professional observers are best able to grapple with the difficulties
attending this kind of observation, where large telescopes, means of
exact measurement, and ample data, such as star-charts and ephemerides
of the planetoids previously discovered, are requisite. The ‘Nautical
Almanack’ annually contains ephemerides of Ceres, Pallas, Juno, and
Vesta; and observers wishing to pick up any one of them may readily
ascertain positions by reference to this work.
_Occultation of Vesta._—An occultation of Vesta occurred on Dec. 30,
1871, and it was observed by Mr. C. G. Talmage at Leyton with a 10-inch
refractor, power 80. He says the planet was exceedingly bright right up
to the Moon’s limb.