NATURE 



\^Nov. 25, 1886 



certain surface-markings of the planet. I think I may fairly 

 mention also our Princeton observation of the spectrum of the 

 planet's atmosphere during the transit, and our confirmation 

 of Gruithuisen's old observation of a white cap (likely enough 

 an ice-cap), at the edge of the planet's disk — probably marking 

 the planet's pole, and showing that the planet's equator has no 

 such anomalous inclination of 50° or 60°, as stated in some of 

 the current text-books. This cap has also been observed by 

 Trouvelot and Denning. But this lovely planet is most 

 refractory and unsatisfactory as a telescopic object, apparently 

 enveloped in dense clouds which mostly hide the real surface of 

 the globe, and mock us with a meaningless glare. 



We mention in passing, but without indorsement, the specula- 

 tions of Houzeau, who has attempted to account for some of 

 the older observations of a satellite to Venus, by supposing 

 another smaller sister planet, " Neith," circling around the 

 sun in an orbit a little larger than that of Venus, and from time 

 to time coming into conjunction with it. But the theory is 

 certainly untenable ; a planet large enough to show phases, as 

 the hypothetical satellite is said to have done, in the feeble tele- 

 scopes with which many of the observations were made too 

 years ago or more, would be easily visible to the naked eye even. 

 There can be little doubt that all the Venus satellites so far 

 observed are simply ghosts due to reflections between the 

 lenses of the telescope, or between the cornea of the eye and 

 the eye lens. 



Mars. — But while Venus has gained no moons during the 

 past ten years. Mars has acquired two, and they are both native 

 Americans. There is no need to recount the faithful work of 

 Prof. Hall with the then new great telescope at Washington, 

 and its brilliant result ; brilliant in a scientific sense, that is, for, 

 regarded as luminaries, it must be admitted that the Martial 

 satellites, in spite of their formidable names of Phobos and 

 Deimos, do not amount to much. Under the best of circum- 

 stances, they are too faint to be seen by any but keen eyes at 

 the end of great telescopes. Small as they are, however, the 

 little creatures punctually pursue the orbits which Hall has com- 

 puted for them, and, when the planet came to its opposition a 

 few weeks ago, they were found just in their predicted places. 

 They are interesting, too, from the light they throw upon the 

 genesis and evolution of the planetary system, almost com- 

 pelling the belief that they have come gradually into their 

 present relation to the planet. The inner one, Phobos, revolves 

 around the primary in 7h. 39m., which is less than one-third of 

 the planet's day. The theory of " tidal evolution," proposed 

 by Prof. G. H. Darwin in 1878 -80, as the result of his investi- 

 gations upon the necessary mechanical consequence of the tidal 

 reactions between the earth, sun, and moon, will account for 

 Phobos, and I know nothing else that will ; though, of course, it 

 would be rash to assert that no other account can ever be given. 



Much attention has also been paid to the study of the 

 planet's surface. In 1S76 we were already in possession of 

 three elaborate maps, by Proctor, Kaiser, and Terby, agreeing 

 in the main as to all the characteristic formations. In 1877, 

 Schiaparelli, of Milan, detected, or thought he did, on the 

 planet's surface, a numerous system of "canals" — long, straight 

 channels, some of them more than 1000 miles in length, with 

 a pretty uniform width of fifty or sixty miles ; and from his 

 observations he constructed a new map, differing from the older 

 ones somewhat seriously, though still accordant in the most 

 essential features. His nomenclature of the seas and continents, 

 derived from ancient geography, is certainly a great improvement 

 on that of his predecessors, who had aflixed to them the names of 

 their friends and acquaintances among living astronomers. 

 There has been some scepticism as to the reality of these 

 "canals"; but in 1879 and i88l they were all recovered by 

 Schiaparelli, and several other observers, notably Burton, also 

 made them out. Moreover, Terby finds, from drawings in his 

 possession, that they had before been seen, though not under- 

 stood or clearly recognised, by Dawes, Secchi, and other ob- 

 servers. At present the balance of evidence is certainly in their 

 favour, especially as the observers at Nice report seeing them 

 last spring. I do not think the same can be said in respect to 

 another observation of Schiaparelli's on the same object, made 

 in 1881. He then found nearly all of these canals — more than 

 twenty of them — to be double, i.e. in place of a single canal 

 there were two — parallel, and 200 or 300 miles apart. No one 

 else so far has confirmed this " gemination " of the canals ; but 

 the planet does not come to a really favourable opposition again 



until 1890 and 1892, when probably the question can be 

 settled. 



The time of rotation has during the past year been determined 

 with great accuracy by Bakhuyzen, who has corrected some 

 errors of Kaiser and Proctor, and finds it 24h. 37m. 22"66s, In 

 1876 there still remained some question as to the amount by 

 which the planet is flattened at the poles. The majority of 

 observers had found a difference between equatorial and polar 

 diameters amounting to between l/loo and 1/30, while, on the 

 other hand, a few of the best observers had found it insensible. 

 The writer, in 1879, made a very careful determination, and 

 found it 1/219, a quantity closely agreeing with the theoretical 

 value deduced by Adams as probable from the motion of the 

 newly-discovered satellites. 



The Asteroids. — On May i, 1876, the number of known 

 asteroids was 163. To-day it stands at 258, 95 of these little 

 bodies having been discovered within the decade, 45 of them by 

 one man, Palisa, of Vienna, while our own Peters is responsible 

 for 20. 



None of the new ones are especially remarkable, i.e. some of 

 the older ones are always more so ; the most inclined and most 

 eccentric orbits, the longest and the shortest periods, none of 

 them belong to any of the late discoveries. One point is note- 

 worthy, that the more recently discovered bodies are much 

 smaller than the earlier ones. The first 25, discovered between 

 May 1876 and October 1878, have an average opposition 

 magnitude of 11 '2, while the last 25, discovered since April 

 1S83, average only 12 '2; i.e. the first 25 average about 2 J 

 times as bright as the last. Out of the whole 95, two are of the 

 9th magnitude (one of them. No. 234, was discovered as recently 

 as August 1883), 14 are of the 10th, 33 of the nth, 33 of the 

 I2th, and 13 of the 13th. Of these last 13, 10 have been found 

 within the past two years ; and of the 12 others found in the 

 same time, 6 are of the nth magnitude, and 6 of the 12th. 



It is clear that there can remain very few to be discovered as 

 large as the loth magnitude, but there may be an indefinite 

 number of the smaller sizes. 



The Major Planets. — As regards the planet Jupiter, the one 

 interesting feature for the past ten years has been " the great 

 red spot." This is an oval spot, some 30,000 miles in length, 

 by 6000 or 7000 in width, which first attracted attention in 

 1S78. At first, and for three years, it was very conspicuous, 

 but in 1882 it became rather faint, though still remaining other- 

 wise pretty much unchanged. In 1885 it was partly covered 

 with a central whitish cloud, which threatened to obscure it 

 entirely ; but this season the veiling cloud has diminished, 

 and the marking is again as plain as it was in 1882 or 1S83. 

 How long it will continue, no one can say ; nor is there any 

 general and authoritative agreement among astronomers as to 

 its nature and cause. 



In connection with observations upon this object, several new 

 deterniinaticins have been made of the planet's rotation-period, 

 and they all show that, as in the case of the sun, the equatorial 

 markings complete the circuit more rapidly than in higher 

 latitudes ; a white spot near the equator gives 9h. 50m. 6s., 

 as against gh. 55m. 36s. for the red spot, which is approximately 

 in latitude 30°. 



We must not omit to mention Prof. Pickering's new photo- 

 metric method of observing the eclipses of this planet's satel- 

 lites. Instead of contenting himself with observing merely the 

 moments of their disappearance and reappearance — an observa- 

 tion not susceptible of much accur.acy — he makes a series of 

 rapid comparisons between the brightness of the waning or 

 waxing point of light during the two or three minutes of its 

 change, using, as the standard, one of the neighbouring un- 

 eclipicd satellites. From these comparisons he determines the 

 moment when the satellite under eclipse has just half its normal 

 brightness, and this with a probable error hardly exceeding a 

 single second, while the old-fashioned method gave results 

 doubtful by not less than a quarter of a minute. Cornu and 

 Obrecht have independently introduced the same method at 

 Paris. When we have a complete twelve years' series of such 

 observations, they will give an exceedingly precise determination 

 of the time required by light to traverse the earth's orbit, and 

 so, indirectly, of the solar parallax. 



As regards Saturn, there is nothing to report so startling as 

 Jupiter's red spot. A white spot, which appeared in 1877, 

 "enabled Hall to make a new determination of the rotation- 

 period, which came out loh. 14m. 14s. This is in substantial 



