February 1, 1892.] 



KNOWLEDGE. 



37 



Uranus, and implies that Mars is 1259 times brighter than 

 Uranus. But we have seen that Mars should be 3211-65 

 times brighter if the surfaces of the two planets had the 

 same reflecting power ; hence it follows that the albedo of 

 Uranus must be ~f|-l-|-, or 2-55 times greater than that 

 of Mars. We have, therefore, the albedo of Uranus 

 =0-2672 X 2-o5=0-68, or nearly equal to that of white 

 paper, which is 0-70. 



Let us now consider the planet Neptune, for which 

 Zollner found an albedo of 0-46. The relative distances of 

 Mars and Neptune are 1-523 and 30-051. This gives the 

 solar illumination on Mars 389-32 times that ou Neptune. 

 Taking their apparent diameters at 18 seconds and 2-9 

 seconds respectively, we have the result that Mars should 

 be 11,996-6 times brighter than Neptune. Now Pickering 

 found the stellar magnitude of Neptune to be 7-96, which 

 makes Mars 10-21 magnitudes, or 12,023 times brighter 

 than Neptune. Hence we have the albedo of Neptune 

 =-iJj>A|;_6.x 0-2672=0-333, a result in striking contrast 

 to the albedo found above for Uranus. I think there can 

 be no doubt that Uranus has the highest albedo of all the 

 planets of the solar system. Comparing it with .Jupiter 

 I find, by the same method of computation, that the 

 albedo of Uranus = albedo of .Jupiter x 1-213. Hence 

 with Zollner's value of -Jupiter's albedo, 0-62, we have the 

 albedo of Uranus 0-75, a very high value indeed, exceeding 

 that of white paper, which is 0-70, and pointing strongly 

 to the conclusion that Uranus is in a highly heated 

 condition, a conclusion which seems to be partly supported 

 by the evidence of the spectroscope. 



To further test the high albedo of Uranus, let us 

 compare the relative brightness of Uranus and Neptune. 

 According to Pro. Pickering's photometric measures, 

 Uranus is 5-56 magnitude and Neptune 7-96. Uranus is 

 therefore 2-1 magnitudes, or 9-12 times brighter than 

 Neptune. The relative distances of the two planets from 

 the Sun being 19-183 and 30-051, we have the intensity of 

 the solar light on Uranus 2-1515 times that on Neptune. 

 But the areas of the discs are as 4- to (2-9)-, or as 16 to 

 8-41. Hence, the brightness of Uranus should be g^'f ^ X 

 2-4545, or 4-67 times that of Neptune. Hence it follows 

 that the albedo of Uranus must be fif, or 1-9528 that of 

 Neptune. Assuming Zollner's value of 0-46 for the albedo 

 of Neptune, we have the albedo of Uranus = 0-46 x 1-9528 

 =0-898 (!) Even with the low value of Neptune's albedo, 

 which I have found, viz., 0-333, the albedo of Uranus 

 would be 0-333 x 1-9528=0-65, a value which still makes 

 its albedo the highest of all the planets. 



It is difficult to say what the albedo of the Earth itself 

 may be. Possibly it does not differ much from that of the 

 planet Mars. The Moon's albedo is rather low, 0-1736, 

 according to ZolLner. It is, however, greater than that of 

 Mercury, which seems to have the smallest reflecting 

 power of all the planets. 



With reference to the satellites, those of Mars are so 

 small that we have no data for computing their albedos. 

 Prof. Pickering's estimates of their diameter were made on 

 the assumption that their albedos do not differ much from 

 that of Mars itself. 



Assuming a diameter of 3400 miles for the third satellite 

 of Jupiter, the largest and brightest of the system, and the 

 mean diameter of .Jupiter itself at 87,000 miles, we have 

 the area of Jupiter's disc 655 times that of the satellite. 

 If both have the same albedo, Jupiter should therefore be 

 655 times brighter than the satellite. Now Pickering 

 finds the stellar magnitude of this satellite to be 5-24. 

 This makes Jupiter 7-76 magnitudes or 1271 times brighter 

 than the satellite. Hence the albedo of Jupiter must be 

 nearly twice that of the third satellite, 



The diameter of Saturn's largest satellite. Titan, is 

 somewhat doubtful, but assuming it at 3000 miles, and its 

 stellar magnitude to be 9-13, as measured by Pickering, 

 the diameter of Saturn being 72,000 miles, I tind that the 

 albedo of Saturn would be 22 times of Titan. This would 

 make the albedo of Titan about 0-21, but owing to the 

 uncertainty which exists as to its diameter this result 

 must be considered very doubtful. 



The satellites of I'ranus and Neptune are so faint that 

 no satisfactory results could be computed. For the satellite 

 of Neptune Pickering finds a stellar magnitude of 13-82, 

 or 5-93 magnitudes fainter than its primary. If we take 

 the diameter of Neptune at 36,000 miles, and assume that 

 its albedo is twice that of its satellite, I find that the 

 diameter of the satellite would be about 3300 miles. 

 Assuming the same albedo, the diameter would be about 

 2310 miles. 



PERIODICAL COMETS DUE IN 1892. 



By W. T. Lynx, B.A., F.E.A.S. 



DR. S. OPPENHEIM has recently published in the 

 Astronomi!<che Xacfiriclifni (No. 3064) the result 

 of an investigation of the orbit of the fourth 

 Comet of 1886, which was discovered by Mr. W. 

 R. Brooks at Phelps, N.Y., on the 22nd of May 

 in that year. He finds that the most probable length of 

 its period is 5-6 years, and as it passed its perihelion in 

 1886 on the 6th of June, another will be due in the 

 present month of January. Dr. Oppenheim thinks, how- 

 ever, that the Comet will not become visible unless the 

 perihelion passage occurs considerably later in the year 

 than this. 



Failing this, the only known Periodical Comet due to 

 return in 1892 is that of Pons-Winnecke, which was 

 also last seen in 1886, when it was detected by Mr. 

 Finlay at the Cape of Good Hope on the 9th of August, 

 and passed its perihelion on the 16th of September. 

 The first certain discovery of this Comet was made by 

 Pons at Marseilles on the 12th of June, 1819, but it 

 appears probable that it was observed by Pons himself 

 early in February, 1808, though the observations made 

 on that occasion were too few and doubtful (partly on 

 account of the close neighbourhood of several nebulse) 

 to furnish the means of determining its orbit with any 

 accuracy. It was after the return of Pons's Comet of 

 1819 in 1858, when it was re-discovered by Prof. 

 Winnecke on the 8th of March, and passed its perihelion 

 on the 2ud of May, that it was recognized as taking its 

 place amongst the Periodical Comets, with period of 

 about 5-6 years. It was not, however, seen at the next 

 return, which must have taken place about the end of 

 1863, but was observed at the returns of the summer of 

 1869 and the early spring of 1875. At the return in 1880 

 it was unfavourably placed and again escaped observation, 

 but (as already mentioned) it was observed again in the 

 latter part of the summer of 1886, and will be due once 

 more early in the present year. 



In the year 1880 the late Prof. Oppolzer, of Vienna, 

 made some calculations which appeared to indicate that 

 this Comet was imdergoing an acceleration of its period, 

 and he suggested that this might be due to the effect of 

 a resisting medium in space acting upon its motion, as 

 Encke had thought he had obtained decisive evidence in 

 the case of the Comet now always called by his name. 

 I3ut since the time of Encke the diminution of the periodic 

 time in the latter Comet has proved to be not constant, so 

 that the probability of the resisting medium explanation 

 no longer holds. And in the case of the Pons-Winnecke 



