CHAPTER X.

THE MINOR PLANETS a.

Sometimes called Ultra-Zodiacal Planets.-Summary of facts.-Notes on Ceres.Pallas.-Juno.-Vesta.-Olbers's theory.-History of the search made for them.-Independent discoveries.-Progressive diminution in their size.

ETWEEN the orbits of Mars and Jupiter there is a wide

BETWEEN

interval, which, until the present century, was not known to be occupied by any planet. The researches of late years, as previously intimated in Chapter II., have led to the discovery of a numerous group of small bodies revolving round the Sun, which are known as the Minor Planets, and which have received names taken at the outset chiefly from the mythologies of ancient Greece and Rome, but in recent years from all sorts of sources, many names being most fantastic and ridiculous.

These planets differ in some respects from the other members of the system, especially in point of size, the largest being probably not more than, even if so much as, 200 miles or 300 miles in diameter. Their orbits are also, as a general rule, much more inclined to the ecliptic than the orbits of the major planets, for which reason it was once proposed to term them the Ultra

a The use of symbols has been discontinued, except for the four early ones, as follows: Ceres, Pallas, Juno, Vesta; and even these are becoming obsolete. Gould's suggestion to adopt by way of symbol the number in the order of discovery enclosed in a circle thus: has been universally adopted.

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b The old name of asteroids, proposed by Sir W. Herschel, has nearly fallen into

disuse. Such a designation was not very appropriate; planetoids would have been better. However, minor planets is preferable to either.

c The names Lumen, Bertha, and Zelia, assigned by MM. Henry, are said to commemorate members of the family of the French astronomer Flammarion, a characteristic specimen of the French way of doing things.

Zodiacal Planets: and many orbits are eccentric to a degree for which no parallel can be found amongst the major planets.

It is needless to give any detailed account of each, but a short summary may not be out of placed.

The nearest to the Sun is Medusa (149), which revolves round that luminary in 1139, or 3.1, at a mean distance of 198,134,000 miles. Next come Sita (244), and Anahita (7)

The most distant is Thule, whose period is 3220d, or 8.87, and whose mean distance is 396,454,000 miles. Next come Hilda 153), Ismene, and Andromache 75. The last-named recedes farthest from the Sun of any owing to the great eccentricity of its orbit. The least eccentric orbit is that of Philomela (196), in which e amounts to only 0.011.

The most eccentric orbit is that of Æthra 132 in which € amounts to 0.383.

The least inclined orbit is that of Massalia, in which amounts to o° 41′.

The most inclined orbit is that of Pallas

amounts to 34° 44′.

2

in which L

The brightest, and, presumably, largest planet is by the concurrent testimony of Argelander, Stone, and Pickering, Vesta The two former observers place Ceres second, and Pallas third.

The faintest cannot be specified.

2

The more recently discovered planets are all so small that it is impossible to say which is the smallest.

It has been thought that many of the minor planets (especially Vesta) are variable in their light. This may be nothing more than the result of, and a proof of their axial rotation. Prof.

d By far the most elaborate summary which has yet appeared will be found in an article by Niesten in the Annuaire de l'Observatoire Roy. de Bruxelles, 1881, p. 226; and see Prof. D. Kirkwood's very exhaustive little treatise The Asteroids, Philadelphia, 1888.

Littrow's idea that the planets which

exhibit these changes are irregular or polyhedral in form, and show sometimes one and sometimes another face, or faces (as the cases may be), seems sublime fancy. But in the more modern form that probably these planets rotate on their axes as do the major planets, his theory may be admissible.

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M. W. Harrington, on the assumption that the surfaces of all have the same reflecting power as Vesta, has estimated the volume of Vesta as of the first 230 planets; and that Ceres and Vesta together comprise about half the volume of the 230. Le Verrier calculated that the total mass of the whole number could not exceed of the mass of the Earth. Even to approach this sum total Niesten considers there would have to be several thousand minor planets in all.

Several of the minor planets have been found only to be lost again, and their positions cannot now be determined. Included in this category are Scylla 1, Sylvia, Diké, and Camilla. Others (e.g. Hilda Sirona 116 have been found again after being lost.

153

Lydia

110

Under favourable circumstances Ceres has been seen with the naked eye, having then the brightness of a star of the 7th magnitude; more usually, however, it resembles an 8th magnitude star. Its light is somewhat of a red tinge, and some observers have remarked a haziness surrounding the planet, which has been attributed to the density and extent of its atmosphere. Sir W. Herschel once fancied that he had detected 2 satellites accompanying Ceres; but its mass can scarcely be sufficient for it to retain satellites around it large enough to be visible to us. Pallas, when nearest the Earth in Opposition, shines as a full 7th magnitude star, with a decided yellowish light. Traces of an atmosphere have also been observed. Juno usually shines as an 8th magnitude star, and is of a reddish hue. Vesta appears at times as bright as a 6th magnitude star, and may then constantly be seen without optical aid, as was the case in the autumn of 1858. The light of Vesta is usually considered to be a pure white, but Hind considers it a pale yellow. Hind found Victoria to possess a bluish tinge.

The orbits most nearly alike are those of Fides and Maia, and Lespiault has remarked that when at their least distance from each other these planets are separated by a space which only

Sol. Syst., p. 85.

amounts to of the radius of the Earth's orbit, or about 4 millions of miles.

Sir J. Herschel once remarked:-" A man placed on one of the minor planets, would spring with ease 6oft, and sustain in his descent no greater shock than he does on the Earth from leaping a yard. On such planets giants might exist; and those enormous animals which, on Earth, require the buoyant powers of water to counteract their weight, might there be denizens of the land".” But to such speculations there is no end.

Respecting the past history, so to speak, of the minor planets, little can be said. Olbers, in calculating the elements of the orbit of Pallas, was forcibly struck with the close coincidence he found to exist between the mean distance of that planet and Ceres. He then suggested that they might be fragments of some large planet which had, by some catastrophe, been shivered to pieces. When this theory was started it appeared a not wholly improbable one, but the discoveries of late years have upset it1. Nevertheless, a very close connection does apparently exist between these minute bodies, and on this subject D'Arrest writes:-"One fact seems above all to confirm the idea of an intimate relation between all the minor planets; it is, that, if their orbits are figured under the form of material rings, these rings will be found so entangled, that it would be possible, by means of one among them taken at hazard, to lift up all the rest.” The circumstances which led originally to a search for planetary bodies in the space intervening between Mars and Jupiter, were these. In the year 1800, 6 astronomers, of whom Baron De

* Outlines of Ast., p. 352.

h It may be shown mathematically, that if the disruption of a large planet ever did occur, its fragments (no matter how diverse their subsequent paths might be) must, if continuing to revolve round the Sun, always pass through the point at which the explosion occurred, at one part of their orbits. Sir W. Herschel thus expressed himself on this subject to the poet Campbell according to a letter written by the latter:-"He was con

vinced that there had existed a planet between Mars and Jupiter, in our own system, of which the little asteroids, or planetkins, lately discovered, are indubitably fragments; and 'Remember,' said he, 'that though they have discovered only 4 of these parts, there will be thousands-perhaps 30,000 more yet discovered.' This planet he believed to have been lost by explosion." (Life and Letters of T. Campbell, vol. ii. p. 234.)

Zach was one, assembled at Lilienthal, and there resolved to establish a society of 24 practical observers, to examine all the telescopic stars in the zodiac, which was to be divided into 24 zones, each containing one hour of Right Ascension, for the express purpose of searching for undiscovered planets'. They elected Schröter their president, and the Baron was chosen their secretary. Such organisation was ere long rewarded by the discovery of 4 planets, but as no more seemed to be forthcoming, the search was relinquished in 1816.

It does not appear that any further labours in this field were prosecuted for some years, or till about the year 1830, when M. Hencke, an amateur of Driesen in Prussia, commenced the search for small planets, with the aid of the since celebrated Berlin Star Maps which contain all stars up to the 9th or 10th magnitudes lying within 15° of the equator. It is evident that a non-stellar body is much more likely to attract the notice of an observer possessing and using maps of this kind than of one not so provided, as a change of position virtually tells its own tale with comparatively little trouble to the astronomer. This series of maps, one for each hour of R.A., was only completed in 1859; therefore when Hencke commenced he had only a few at his command, and 15 years elapsed ere his zeal and perseverance produced any result: but when once one planet was found, the discovery of others quickly followed.

Several of these small planets were discovered independently by two or more observers, each without a knowledge of what the other had done. For example, Irene was found by Hind on May 19 1851, and by De Gasparis on May 23; Massilia by De Gasparis on Sept. 19, 1852, and by Chacornac on Sept. 20; Amphritrite by Marth on March 1, 1854, by Pogson on March 2, and Chacornac on March 3 (3 separate discoveries); Virginia by Ferguson on Oct. 4, 1857, and by Luther on Oct. 19; Eurynome by Watson on Sept. 14, 1863, and by Tempel on Oct. 3; Hecate by Watson on July 11, 1868, and by Peters on July 14; Cassandra by Peters on July 23, 1871, and by Watson on August 6; &c.

iSee p. 67, ante.