The Asteroids.

by Daniel Kirkwood.

PREFACE.

The rapid progress of discovery in the zone of minor planets, the anomalous forms and positions of their orbits, the small size as well as the great number of these telescopic bodies, and their peculiar relations to Jupiter, the ma.s.sive planet next exterior,--all ent.i.tle this part of the system to more particular consideration than it has. .h.i.therto received. The following essay is designed, therefore, to supply an obvious want. Its results are given in some detail up to the date of publication. Part I. presents in a popular form the leading historical facts as to the discovery of Ceres, Pallas, Juno, Vesta, and Astraea; a tabular statement of the dates and places of discovery for the entire group; a list of the names of discoverers, with the number of minor planets detected by each; and a table of the princ.i.p.al elements so far as computed.

In Part II. this descriptive summary is followed by questions relating to the origin of the cl.u.s.ter; the elimination of members from particular parts; the eccentricities and inclinations of the orbits; and the relation of the zone to comets of short period. The elements are those given in the Paris _Annuaire_ for 1887, or in recent numbers of the _Circular zum Berliner Astronomischen Jahrbuch_.

DANIEL KIRKWOOD.

BLOOMINGTON, INDIANA, November, 1887.

PART I.

THE ASTEROIDS, OR MINOR PLANETS BETWEEN MARS AND JUPITER.

1. Introductory.

PLANETARY DISCOVERIES BEFORE THE ASTEROIDS WERE KNOWN.

The first observer who watched the skies with any degree of care could not fail to notice that while the greater number of stars maintained the same relative places, a few from night to night were ever changing their positions. The planetary character of Mercury, Venus, Mars, Jupiter, and Saturn was thus known before the dawn of history. The names, however, of those who first distinguished them as "wanderers" are hopelessly lost.

Venus, the morning and evening star, was long regarded as two distinct bodies. The discovery that the change of aspect was due to a single planet's change of position is ascribed to Pythagoras.

At the beginning of the seventeenth century but six primary planets and one satellite were known as members of the solar system. Very few, even of the learned, had then accepted the theory of Copernicus; in fact, before the invention of the telescope the evidence in its favor was not absolutely conclusive. On the 7th of January, 1610, Galileo first saw the satellites of Jupiter. The bearing of this discovery on the theory of the universe was sufficiently obvious. Such was the prejudice, however, against the Copernican system that some of its opponents denied even the reality of Galileo's discovery. "Those satellites," said a Tuscan astronomer, "are invisible to the naked eye, and therefore can exercise no influence on the earth, and therefore would be useless, and therefore do not exist. Besides, the Jews and other ancient nations, as well as modern Europeans, have adopted the division of the week into _seven_ days, and have named them from the seven planets; now, if we increase the number of planets this whole system falls to the ground."

No other secondary planet was discovered till March 25, 1655, when t.i.tan, the largest satellite of Saturn, was detected by Huyghens. About two years later (December 7, 1657) the same astronomer discovered the true form of Saturn's ring; and before the close of the century (1671-1684) four more satellites, j.a.petus, Rhea, Tethys, and Dione, were added to the Saturnian system by the elder Ca.s.sini. Our planetary system, therefore, as known at the close of the seventeenth century, consisted of six primary and ten secondary planets.

Nearly a century had elapsed from the date of Ca.s.sini's discovery of Dione, when, on the 13th of March, 1781, Sir William Herschel enlarged the dimensions of our system by the detection of a planet--Ura.n.u.s--exterior to Saturn. A few years later (1787-1794) the same distinguished observer discovered the first and second satellites of Saturn, and also the four Uranian satellites. He was the only planet discoverer of the eighteenth century.

2. Discovery of the First Asteroids.

As long ago as the commencement of the seventeenth century the celebrated Kepler observed that the respective distances of the planets from the sun formed nearly a regular progression. The series, however, by which those distances were expressed required the interpolation of a term between Mars and Jupiter,--a fact which led the ill.u.s.trious German to predict the discovery of a planet in that interval. This conjecture attracted but little attention till after the discovery of Ura.n.u.s, whose distance was found to harmonize in a remarkable manner with Kepler's order of progression. Such a coincidence was of course regarded with considerable interest. Towards the close of the last century Professor Bode, who had given the subject much attention, published the law of distances which bears his name, but which, as he acknowledged, is due to Professor t.i.tius. According to this formula the distances of the planets from Mercury's...o...b..t form a geometrical series of which the ratio is two. In other words, if we reckon the distances of Venus, the earth, etc., from the orbit of Mercury, instead of from the sun, we find that--interpolating a term between Mars and Jupiter--the distance of any member of the system is very nearly half that of the next exterior.

Baron De Zach, an enthusiastic astronomer, was greatly interested in Bode's empirical scheme, and undertook to determine the elements of the hypothetical planet. In 1800 a number of astronomers met at Lilienthal, organized an astronomical society, and a.s.signed one twenty-fourth part of the zodiac to each of twenty-four observers, in order to detect, if possible, the unseen planet. When it is remembered that at this time no primary planet had been discovered within the ancient limits of the solar system, that the object to be looked for was comparatively near us, and that the so-called law of distances was purely empirical, the prospect of success, it is evident, was extremely uncertain. How long the watch, if unsuccessful, might have been continued is doubtful. The object of research, however, was fortunately brought to light before the members of the astronomical a.s.sociation had fairly commenced their labors.[1]

On the 1st of January, 1801, Professor Giuseppe Piazzi, of Palermo, noticed a star of the eighth magnitude, not indicated in Wollaston's catalogue. Subsequent observations soon revealed its planetary character, its mean distance corresponding very nearly with the calculations of De Zach. The discoverer called it Ceres Ferdinandea, in honor of his sovereign, the King of Naples. In this, however, he was not followed by astronomers, and the planet is now known by the name of Ceres alone. The discovery of this body was hailed by astronomers with the liveliest gratification as completing the harmony of the system.

What, then, was their surprise when in the course of a few months this remarkable order was again interrupted! On the 28th of March, 1802, Dr.

William Olbers, of Bremen, while examining the relative positions of the small stars along the path of Ceres, in order to find that planet with the greater facility, noticed a star of the seventh or eighth magnitude, forming with two others an equilateral triangle where he was certain no such configuration existed a few months before. In the course of a few hours its motion was perceptible, and on the following night it had very sensibly changed its position with respect to the neighboring stars.

Another planet was therefore detected, and Dr. Olbers immediately communicated his discovery to Professor Bode and Baron De Zach. In his letter to the former he suggested Pallas as the name of the new member of the system,--a name which was at once adopted. Its...o...b..t, which was soon computed by Gauss, was found to present several striking anomalies.

The inclination of its plane to that of the ecliptic was nearly thirty-five degrees,--an amount of deviation altogether extraordinary.

The eccentricity also was greater than in the case of any of the old planets. These peculiarities, together with the fact that the mean distances of Ceres and Pallas were nearly the same, and that their orbits approached very near each other at the intersection of their planes, suggested the hypothesis that they are fragments of a single original planet, which, at a very remote epoch, was disrupted by some mysterious convulsion. This theory will be considered when we come to discuss the tabulated elements of the minor planets now known.

For the convenience of astronomers, Professor Harding, of Lilienthal, undertook the construction of charts of all the small stars near the orbits of Ceres and Pallas. On the evening of September 1, 1804, while engaged in observations for this purpose, he noticed a star of the eighth magnitude not mentioned in the great catalogue of Lalande. This proved to be a third member of the group of asteroids. The discovery was first announced to Dr. Olbers, who observed the planet at Bremen on the evening of September 7.

Before Ceres had been generally adopted by astronomers as the name of the first asteroid, Laplace had expressed a preference for Juno. This, however, the discoverer was unwilling to accept. Mr. Harding, like Laplace, deeming it appropriate to place Juno near Jupiter, selected the name for the third minor planet, which is accordingly known by this designation.

Juno is distinguished among the first asteroids by the great eccentricity of its...o...b..t, amounting to more than 0.25. Its least and its greatest distances from the sun are therefore to each other very nearly in the ratio of three to five. The planet consequently receives nearly three times as much light and heat in perihelion as in aphelion.

It follows, also, that the half of the orbit nearest the sun is described in about eighteen months, while the remainder, or more distant half, is not pa.s.sed over in much less than three years. Schroeter noticed a variation in the light of Juno, which he supposed to be produced by an axial rotation in about twenty-seven hours.

The fact that Juno was discovered not far from the point at which the orbit of Pallas approaches very near that of Ceres, was considered a strong confirmation of the hypothesis that the asteroids were produced by the explosion of a large planet; for in case this hypothesis be founded in truth, it is evident that whatever may have been the forms of the various...o...b..ts a.s.sumed by the fragments, they must all return to the point of separation. In order, therefore, to detect other members of the group, Dr. Olbers undertook a systematic examination of the two opposite regions of the heavens through which they must pa.s.s. This search was prosecuted with great industry and perseverance till ultimately crowned with success. On the 29th of March, 1807, while sweeping over one of those regions through which the orbits of the known asteroids pa.s.sed, a star of the sixth magnitude was observed where none had been seen at previous examinations. Its planetary character, which was immediately suspected, was confirmed by observation, its motion being detected on the very evening of its discovery. This fortunate result afforded the first instance of the discovery of two primary planets by the same observer.

The astronomer Gauss having been requested to name the new planet, fixed upon Vesta, a name universally accepted. Though the brightest of the asteroids, its apparent diameter is too small to be accurately determined, and hence its real magnitude is not well ascertained.

Professor Harrington, of Ann Arbor, has estimated the diameter at five hundred and twenty miles. According to others, however, it does not exceed three hundred. If the latter be correct, the volume is about 1/20000 that of the earth. It is remarkable that notwithstanding its diminutive size it may be seen under favorable circ.u.mstances by the naked eye.

Encouraged by the discovery of Vesta (which he regarded as almost a demonstration of his favorite theory), Dr. Olbers continued his systematic search for other planetary fragments. Not meeting, however, with further success, he relinquished his observations in 1816. His failure, it may here be remarked, was doubtless owing to the fact that his examination was limited to stars of the seventh and eighth magnitudes.

The search for new planets was next resumed about 1831, by Herr Hencke, of Driessen. With a zeal and perseverance worthy of all praise, this amateur astronomer employed himself in a strict examination of the heavens represented by the Maps of the Berlin Academy. These maps extend fifteen degrees on each side of the equator, and contain all stars down to the ninth magnitude and many of the tenth. Dr. Hencke rendered some of these charts still more complete by the insertion of smaller stars; or rather, "made for himself special charts of particular districts." On the evening of December 8, 1845, he observed a star of the ninth magnitude where none had been previously seen, as he knew from the fact that it was neither found on his own chart nor given on that of the Academy. On the next morning he wrote to Professors Encke and Schumacher informing them of his supposed discovery. "It is very improbable," he remarked in his letter to the latter, "that this should prove to be merely a variable star, since in my former observations of this region, which have been continued for many years, I have never detected the slightest trace of it." The new star was soon seen at the princ.i.p.al observatories of Europe, and its planetary character satisfactorily established. The selection of a name was left by the discoverer to Professor Encke, who chose that of Astraea.

The facts in regard to the very numerous subsequent discoveries may best be presented in a tabular form.

TABLE I.

_The Asteroids in the Order of their Discovery._

-----------------+----------------+---------------+------------ Asteroids.

Date of

Name of

Place of

Discovery.

Discoverer.

Discovery.

-----------------+----------------+---------------+------------ 1. Ceres

1801, Jan. 1

Piazzi

Palermo 2. Pallas

1802, Mar. 28

Olbers

Bremen 3. Juno

1804, Sept. 1

Harding

Lilienthal 4. Vesta

1807, Mar. 29

Olbers

Bremen 5. Astraea

1845, Dec. 8

Hencke

Driessen 6. Hebe

1847, July 1

Hencke

Driessen 7. Iris

1847, Aug. 14

Hind

London 8. Flora

1847, Oct. 18

Hind

London 9. Metis

1848, Apr. 26

Graham

Markree 10. Hygeia

1849, Apr. 12

De Gasparis

Naples 11. Parthenope

1850, May 11

De Gasparis

Naples 12. Victoria

1850, Sept. 13

Hind

London 13. Egeria

1850, Nov. 2

De Gasparis

Naples 14. Irene

1851, May 19

Hind

London 15. Eunomia

1851, July 29

De Gasparis

Naples 16. Psyche

1852, Mar. 17

De Gasparis

Naples 17. Thetis

1852, Apr. 17

Luther

Bilk 18. Melpomene

1852, June 24

Hind

London 19. Fortuna

1852, Aug. 22

Hind

London 20. Ma.s.salia

1852, Sept. 19

De Gasparis

Naples 21. Lutetia

1852, Nov. 15

Goldschmidt

Paris 22. Calliope

1852, Nov. 16

Hind

London 23. Thalia

1852, Dec. 15

Hind

London 24. Themis

1853, Apr. 5

De Gasparis

Naples 25. Phocea

1853, Apr. 6

Chacornac

Ma.r.s.eilles 26. Proserpine

1853, May 5

Luther

Bilk 27. Euterpe

1853, Nov. 8

Hind

London 28. Bellona

1854, Mar. 1

Luther

Bilk 29. Amphitrite

1854, Mar. 1

Marth

London 30. Urania

1854, July 22

Hind

London 31. Euphrosyne

1854, Sept. 1

Ferguson

Washington 32. Pomona

1854, Oct. 26

Goldschmidt

Paris 33. Polyhymnia

1854, Oct. 28

Chacornac

Paris 34. Circe

1855, Apr. 6

Chacornac

Paris 35. Leucothea

1855, Apr. 19

Luther

Bilk 36. Atalanta

1855, Oct. 5

Goldschmidt

Paris 37. Fides

1855, Oct. 5

Luther

Bilk 38. Leda

1856, Jan. 12

Chacornac

Paris 39. Laet.i.tia

1856, Feb. 8

Chacornac

Paris 40. Harmonia

1856, Mar. 31

Goldschmidt

Paris 41. Daphne

1856, May 22

Goldschmidt

Paris 42. Isis

1856, May 23

Pogson

Oxford 43. Ariadne

1857, Apr. 15

Pogson

Oxford 44. Nysa

1857, May 27

Goldschmidt

Paris 45. Eugenia

1857, June 27

Goldschmidt

Paris 46. Hestia

1857, Aug. 16

Pogson

Oxford 47. Aglaia

1857, Sept. 15

Luther

Bilk 48. Doris

1857, Sept. 19

Goldschmidt

Paris 49. Pales

1857, Sept. 19

Goldschmidt

Paris 50. Virginia

1857, Oct. 4

Ferguson

Washington 51. Nemausa

1858, Jan. 22

Laurent

Nismes 52. Europa

1858, Feb. 4

Goldschmidt

Paris 53. Calypso

1858, Apr. 4

Luther

Bilk 54. Alexandra

1858, Sept. 10

Goldschmidt

Paris 55. Pandora

1858, Sept. 10

Searle

Albany 56. Melete

1857, Sept. 9

Goldschmidt

Paris 57. Mnemosyne

1859, Sept. 22

Luther

Bilk 58. Concordia

1860, Mar. 24

Luther

Bilk 59. Olympia

1860, Sept. 12

Chacornac

Paris 60. Echo

1860, Sept. 16

Ferguson

Washington 61. Danae

1860, Sept. 9

Goldschmidt

Paris 62. Erato

1860, Sept. 14

Foerster and

Berlin

Lesser

63. Ausonia

1861, Feb. 10

De Gasparis

Naples 64. Angelina

1861, Mar. 4

Tempel

Ma.r.s.eilles 65. Maximiliana

1861, Mar. 8

Tempel

Ma.r.s.eilles 66. Maia

1861, Apr. 9

Tuttle

Cambridge, U.S.

67. Asia

1861, Apr. 17

Pogson

Madras 68. Leto

1861, Apr. 29

Luther

Bilk 69. Hesperia

1861, Apr. 29

Schiaparelli

Milan 70. Panopea

1861, May 5

Goldschmidt

Paris 71. Niobe

1861, Aug. 13

Luther

Bilk 72. Feronia

1862, May 29

Peters and

Clinton

Safford

73. Clytie

1862, Apr. 7

Tuttle

Cambridge 74. Galatea

1862, Aug. 29

Tempel

Ma.r.s.eilles 75. Eurydice

1862, Sept. 22

Peters

Clinton 76. Freia

1862, Oct. 21

D'Arrest

Copenhagen 77. Frigga

1862, Nov. 12

Peters

Clinton 78. Diana

1863, Mar. 15

Luther

Bilk 79. Eurynome

1863, Sept. 14

Watson

Ann Arbor 80. Sappho

1864, May 2

Pogson

Madras 81. Terpsich.o.r.e

1864, Sept. 30

Tempel

Ma.r.s.eilles 82. Alcmene

1864, Nov. 27

Luther

Bilk 83. Beatrix

1865, Apr. 26

De Gasparis

Naples 84. Clio

1865, Aug. 25

Luther

Bilk 85. Io

1865, Sept. 19

Peters

Clinton 86. Semele

1866, Jan. 14

Tietjen

Berlin 87. Sylvia

1866, May 16

Pogson

Madras 88. Thisbe

1866, June 15

Peters

Clinton 89. Julia

1866, Aug. 6

Stephan

Ma.r.s.eilles 90. Antiope

1866, Oct. 1

Luther

Bilk 91. aegina

1866, Nov. 4

Borelly

Ma.r.s.eilles 92. Undina

1867, July 7

Peters

Clinton 93. Minerva

1867, Aug. 24

Watson

Ann Arbor 94. Aurora

1867, Sept. 6

Watson

Ann Arbor 95. Arethusa

1867, Nov. 24

Luther

Bilk 96. aegle

1868, Feb. 17

Coggia

Ma.r.s.eilles 97. Clotho

1868, Feb. 17

Coggia

Ma.r.s.eilles 98. Ianthe

1868, Apr. 18

Peters

Clinton 99. Dike

1868, May 28

Borelly

Ma.r.s.eilles 100. Hecate

1868, July 11

Watson

Ann Arbor 101. Helena

1868, Aug. 15

Watson

Ann Arbor 102. Miriam

1868, Aug. 22

Peters

Clinton 103. Hera

1868, Sept. 7

Watson

Ann Arbor 104. Clymene

1868, Sept. 13

Watson

Ann Arbor 105. Artemis

1868, Sept. 16

Watson

Ann Arbor 106. Dione

1868, Oct. 10

Watson

Ann Arbor 107. Camilla

1868, Nov. 17

Pogson

Madras 108. Hecuba

1869, Apr. 2

Luther

Bilk 109. Felicitas

1869, Oct. 9

Peters

Clinton 110. Lydia

1870, Apr. 19

Borelly

Ma.r.s.eilles 111. Ate

1870, Aug. 14

Peters

Clinton 112. Iphigenia

1870, Sept. 19

Peters

Clinton 113. Amalthea

1871, Mar. 12

Luther

Bilk 114. Ca.s.sandra

1871, July 23

Peters

Clinton 115. Thyra

1871, Aug. 6

Watson

Ann Arbor 116. Sirona

1871, Sept. 8

Peters

Clinton 117. Lomia

1871, Sept. 12

Borelly

Ma.r.s.eilles 118. Peitho

1872, Mar. 15

Luther

Bilk 119. Althea

1872, Apr. 3

Watson

Ann Arbor 120. Lachesis

1872, Apr. 10

Borelly

Ma.r.s.eilles 121. Hermione

1872, May 12

Watson

Ann Arbor 122. Gerda

1872, July 31

Peters

Clinton 123. Brunhilda

1872, July 31

Peters

Clinton 124. Alceste

1872, Aug. 23

Peters

Clinton 125. Liberatrix

1872, Sept. 11

Prosper Henry

Paris 126. Velleda

1872, Nov. 5

Paul Henry

Paris 127. Johanna

1872, Nov. 5

Prosper Henry

Paris 128. Nemesis

1872, Nov. 25

Watson

Ann Arbor 129. Antigone

1873, Feb. 5

Peters

Clinton 130. Electra

1873, Feb. 17

Peters

Clinton 131. Vala

1873, May 24

Peters

Clinton 132. aethra

1873, June 13

Watson

Ann Arbor 133. Cyrene

1873, Aug. 16

Watson

Ann Arbor 134. Sophrosyne

1873, Sept. 27

Luther

Bilk 135. Hertha

1874, Feb. 18

Peters

Clinton 136. Austria

1874, Mar. 18

Palisa

Pola 137. Meliba

1874, Apr. 21

Palisa

Pola 138. Tolosa

1874, May 19

Perrotin

Toulouse 139. Juewa

1874, Oct. 10

Watson

Pekin 140. Siwa

1874, Oct. 13

Palisa

Pola 141. Lumen

1875, Jan. 13

Paul Henry

Paris 142. Polana

1875, Jan. 28

Palisa

Pola 143. Adria

1875, Feb. 23

Palisa

Pola 144. Vibilia

1875, June 3

Peters

Clinton 145. Adeona

1875, June 3

Peters

Clinton 146. Lucina

1875, June 8

Borelly

Ma.r.s.eilles 147. Protogenea

1875, July 10

Schulhof

Vienna 148. Gallia

1875, Aug. 7

Prosper Henry

Paris 149. Medusa

1875, Sept. 21

Perrotin

Toulouse 150. Nuwa

1875, Oct. 18

Watson

Ann Arbor 151. Abundantia

1875, Nov. 1

Palisa

Pola 152. Atala

1875, Nov. 2

Paul Henry

Paris 153. Hilda

1875, Nov. 2

Palisa

Pola 154. Bertha

1875, Nov. 4

Prosper Henry

Paris 155. Scylla

1875, Nov. 8

Palisa

Pola 156. Xantippe

1875, Nov. 22

Palisa

Pola 157. Dejanira

1875, Dec. 1

Borelly

Ma.r.s.eilles 158. Coronis

1876, Jan. 4

Knorre

Berlin 159. aemilia

1876, Jan. 26

Paul Henry

Paris 160. Una

1876, Feb. 20

Peters

Clinton 161. Athor

1876, Apr. 19

Watson

Ann Arbor 162. Laurentia

1876, Apr. 21

Prosper Henry

Paris 163. Erigone

1876, Apr. 26

Perrotin

Toulouse 164. Eva

1876, July 12

Paul Henry

Paris 165. Loreley

1876, Aug. 9

Peters

Clinton 166. Rhodope

1876, Aug. 15

Peters

Clinton 167. Urda

1876, Aug. 28

Peters

Clinton 168. Sibylla

1876, Sept. 27

Watson

Ann Arbor 169. Zelia

1876, Sept. 28

Prosper Henry

Paris 170. Maria

1877, Jan. 10

Perrotin

Toulouse 171. Ophelia

1877, Jan. 13

Borelly

Ma.r.s.eilles 172. Baucis

1877, Feb. 5

Borelly

Ma.r.s.eilles 173. Ino

1877, Aug. 1

Borelly

Ma.r.s.eilles 174. Phaedra

1877, Sept. 2

Watson

Ann Arbor 175. Andromache

1877, Oct. 1

Watson

Ann Arbor 176. Idunna

1877, Oct. 14

Peters

Clinton 177. Irma

1877, Nov. 5

Paul Henry

Paris 178. Belisana

1877, Nov. 6

Palisa

Pola 179. Clytemnestra

1877, Nov. 11

Watson

Ann Arbor 180. Garumna

1878, Jan. 29

Perrotin

Toulouse 181. Eucharis

1878, Feb. 2

Cottenot

Ma.r.s.eilles 182. Elsa

1878, Feb. 7

Palisa

Pola 183. Istria

1878, Feb. 8

Palisa

Pola 184. Deiopea

1878, Feb. 28

Palisa

Pola 185. Eunice

1878, Mar. 1

Peters

Clinton 186. Celuta

1878, Apr. 6

Prosper Henry

Paris 187. Lamberta

1878, Apr. 11

Coggia

Ma.r.s.eilles 188. Menippe

1878, June 18

Peters

Clinton 189. Phthia

1878, Sept. 9

Peters

Clinton 190. Ismene

1878, Sept. 22

Peters

Clinton 191. Kolga

1878, Sept. 30

Peters

Clinton 192. Nausicaa

1879, Feb. 17

Palisa

Pola 193. Ambrosia

1879, Feb. 28

Coggia

Ma.r.s.eilles 194. Procne

1879, Mar. 21

Peters

Clinton 195. Euryclea

1879, Apr. 22

Palisa

Pola 196. Philomela

1879, May 14

Peters

Clinton 197. Arete

1879, May 21

Palisa

Pola 198. Ampella

1879, June 13

Borelly

Ma.r.s.eilles 199. Byblis

1879, July 9

Peters

Clinton 200. Dynamene

1879, July 27

Peters

Clinton 201. Penelope

1879, Aug. 7

Palisa

Pola 202. Chryseis

1879, Sept. 11

Peters

Clinton 203. Pompeia

1879, Sept. 25

Peters

Clinton 204. Callisto

1879, Oct. 8

Palisa

Pola 205. Martha

1879, Oct. 13

Palisa

Pola 206. Hersilia

1879, Oct. 13

Peters

Clinton 207. Hedda

1879, Oct. 17

Palisa

Pola 208. Lachrymosa

1879, Oct. 21

Palisa

Pola 209. Dido

1879, Oct. 22

Peters

Clinton 210. Isabella

1879, Nov. 12

Palisa

Pola 211. Isolda

1879, Dec. 10

Palisa

Pola 212. Medea

1880, Feb. 6

Palisa

Pola 213. Lilaea

1880, Feb. 16

Peters

Clinton 214. Aschera

1880, Feb. 26

Palisa

Pola 215. none

1880, Apr. 7

Knorre

Berlin 216. Cleopatra

1880, Apr. 10

Palisa

Pola 217. Eudora

1880, Aug. 30

Coggia

Ma.r.s.eilles 218. Bianca

1880, Sept. 4

Palisa

Pola 219. Thusnelda

1880, Sept. 20

Palisa

Pola 220. Stephania

1881, May 19

Palisa

Vienna 221. Eos

1882, Jan. 18

Palisa

Vienna 222. Lucia

1882, Feb. 9

Palisa

Vienna 223. Rosa

1882, Mar. 9

Palisa

Vienna 224. Oceana

1882, Mar. 30

Palisa

Vienna 225. Henrietta

1882, Apr. 19

Palisa

Vienna 226. Weringia

1882, July 19

Palisa

Vienna 227. Philosophia

1882, Aug. 12

Paul Henry

Paris 228. Agathe

1882, Aug. 19

Palisa

Vienna 229. Adelinda

1882, Aug. 22

Palisa

Vienna 230. Athamantis

1882, Sept. 3

De Ball

Bothcamp 231. Vindobona

1882, Sept. 10

Palisa

Vienna 232. Russia

1883, Jan. 31

Palisa

Vienna 233. Asterope

1883, May 11

Borelly

Ma.r.s.eilles 234. Barbara

1883, Aug. 13

Peters

Clinton 235. Caroline

1883, Nov. 29

Palisa

Vienna 236. Honoria

1884, Apr. 26

Palisa

Vienna 237. Clestina

1884, June 27

Palisa

Vienna 238. Hypatia

1884, July 1

Knorre

Berlin 239. Adrastea

1884, Aug. 18

Palisa

Vienna 240. Vanadis

1884, Aug. 27

Borelly

Ma.r.s.eilles 241. Germania

1884, Sept. 12

Luther

Dusseldorf 242. Kriemhild

1884, Sept. 22

Palisa

Vienna 243. Ida

1884, Sept. 29

Palisa

Vienna 244. Sita

1884, Oct. 14

Palisa

Vienna 245. Vera

1885, Feb. 6

Pogson

Madras 246. Asporina

1885, Mar. 6

Borelly

Ma.r.s.eilles 247. Eukrate

1885, Mar. 14

Luther

Dusseldorf 248. Lameia

1885, June 5

Palisa

Vienna 249. Ilse

1885, Aug. 17

Peters

Clinton 250. Bettina

1885, Sept. 3

Palisa

Vienna 251. Sophia

1885, Oct. 4

Palisa

Vienna 252. Clementina

1885, Oct. 27

Perrotin

Nice 253. Mathilde

1885, Nov. 12

Palisa

Vienna 254. Augusta

1886, Mar. 31

Palisa

Vienna 255. Oppavia

1886, Mar. 31

Palisa

Vienna 256. Walpurga

1886, Apr. 3

Palisa

Vienna 257. Silesia

1886, Apr. 5

Palisa

Vienna 258. Tyche

1886, May 4

Luther

Dusseldorf 259. Aletheia

1886, June 28

Peters

Clinton 260. Huberta

1886, Oct. 3

Palisa

Vienna 261. Prymno

1886, Oct. 31

Peters

Clinton 262. Valda

1886, Nov. 3

Palisa

Vienna 263. Dresda

1886, Nov. 3

Palisa

Vienna 264. Libussa

1886, Dec. 17

Peters

Clinton 265. Anna

1887, Feb. 25

Palisa

Vienna 266. Aline

1887, May 17

Palisa

Vienna 267. Tirza

1887, May 27

Charlois

Nice 268.

1887, June 9

Borelly

Ma.r.s.eilles 269.

1887, Sept. 21

Palisa

Vienna 270.

1887, Oct. 8

Peters

Clinton 271.

1887, Oct. 16

Knorre

Berlin -----------------+----------------+---------------+------------

3. Remarks on Table I.

The numbers discovered by the thirty-five observers are respectively as follows:

Palisa 60 Peters 47 Luther 23 Watson 22 Borelly 15 Goldschmidt 14 Hind 10 De Gasparis 9 Pogson 8 Paul Henry 7 Prosper Henry 7 Chacornac 6 Perrotin 6 Coggia 5 Knorre 4 Tempel 4 Ferguson 3 Olbers 2 Hencke 2 Tuttle 2 Foerster (with Lesser) 1 Safford (with Peters) 1 and Messrs. Charlois, Cottenot, D'Arrest, De Ball, Graham, Harding, Laurent, Piazzi, Schiaparelli, Schulhof, Stephan, Searle, and Tietjen, each 1

Before arrangements had been made for the telegraphic transmission of discoveries between Europe and America, or even between the observatories of Europe, the same planet was sometimes independently discovered by different observers. For example, Virginia was found by Ferguson, at Washington, on October 4, 1857, and by Luther, at Bilk, fifteen days later. In all cases, however, credit has been given to the first observer.

Hersilia, the two hundred and sixth of the group, was lost before sufficient observations were obtained for determining its elements. It was not rediscovered till December 14, 1884. Menippe, the one hundred and eighty-eighth, was also lost soon after its discovery in 1878. It has not been seen for more than nine years, and considerable uncertainty attaches to its estimated elements.

Of the two hundred and seventy-one members now known (1887), one hundred and ninety-one have been discovered in Europe, seventy-four in America, and six in Asia. The years of most successful search, together with the number discovered in each, were:

Asteroids.

1879 20 1875 17 1868 12 1878 12

And six has been the average yearly number since the commencement of renewed effort in 1845. All the larger members of the group have, doubtless, been discovered. It seems not improbable, however, that an indefinite number of very small bodies belonging to the zone remain to be found. The process of discovery is becoming more difficult as the known number increases. The astronomer, for instance, who may discover number two hundred and seventy-two must know the simultaneous positions of the two hundred and seventy-one previously detected before he can decide whether he has picked up a new planet or merely rediscovered an old one. The numbers discovered in the several months are as follows: