94 ASTRONOMY 



and his satellites extending over 32 years, from which he deduces the mass and com- 

 pression of the planet, and the rates of revolution of the planes of the orbits of his 

 satellites. The results are perhaps not of general interest, but represent a notable ad- 

 vance in our accurate knowledge of the solar system. 



Among discoveries of minor planets, now made by hundreds per year, one stands out 

 as of great interest. An object designated MT was discovered by Palisa of Vienna on 

 October 3, 1911, which was apparently very close to the earth: but after observations 

 had been secured on October 4, moonlight and cloud interrupted them so that the tiny 

 object could not be recovered, and was thought to be lost. However some determined 

 computers at the Observatory of the University of California made the very most of the 

 scanty observational material, with the happy result that images were identified on 

 plates taken on October n and October 17. It now appears that the approach to 

 the Earth is not such as to offer all the advantages at first expected, but the successful 

 prosecution of the search is in itself matter for congratulation. Other minor planets of 

 special interest are Eros, and the Jupiter groups. Eros was photographed at a number 

 of observatories in 1900-1 with the object of finding the sun's distance, and the success- 

 ful completion of the campaign was marked by the award of the gold medal of the 

 Royal Astronomical Society to A. R. Hinks in February 1912 for his patient and ex- 

 haustive collation of the observations. The best measures of the sun's distance, includ- 

 ing the wholly independent measures with the spectroscope at the Cape Observatory, 

 are now in satisfactory accord at about 92,900,000 miles. The Jupiter groups represent 

 in actuality a type of motion long thought to be of merely theoretical interest. If three 

 bodies attract one another, it is impossible to predict their movements in general 

 formulae: but if they happen to be placed at the corners of an equilateral triangle it can 

 be proved that they will so remain. If the configuration be slightly disturbed, it will 

 tend to recover itself. On the line joining Jupiter to the sun as base we can erect two 

 equilateral triangles in the plane of Jupiter's orbit. The vertex of one is occupied by 

 one known minor planet (Patroclus) and the other vertex by no less than three (Hector, 

 Achilles and Nestor). The occupation is never exact; these little bodies describe com- 

 plicated paths in the neighbourhood of the theoretical point: and their recent discovery 

 has naturally led to the study of such motions, especially by E. W. Brown and G. H. 

 Darwin. They have shown that departures from the equilateral triangle may be quite 

 large, and in commenting on his results Darwin remarks: 



"These orbits throw some light on cosmogony, for we see how small planets with the 

 same mean motion as Jove (his generalised planet) in the course of their vicissitudes tend 

 to pass close to Jove, ultimately to be absorbed into its mass. We thus see something of 

 the machinery whereby a large planet generates for itself a clear space in which to circulate 

 about the sun." 



New tables of Jupiter's satellites were published by R. A. Sampson in August 1910, as 

 the outcome of twelve years' devoted work in discussing the accumulated observations 

 of the past, especially those made by modern photometric methods at Harvard Observa- 

 tory. To expedite their use in prediction Sampson issued the tables before publishing 

 the theory on which they are based, and they have accordingly been used for the predic- 

 tions of the Nautical Almanac for 1914. " In the place of antiquated tables of no value 

 except as directing an observer whjen to go to his telescope, astronomers now have 

 precise formulae with which the observations of the future will naturally be compared" 

 (R. A. S. Council Report for 1911). 



The rotation period of the planet Uranus was determined for the first time in 

 August 1911 at the Lowell Observatory. No markings on his disc have been detected 

 which would enable an observer to watch the rotation directly: the only hope was to 

 measure with the spectroscope the velocities of approach and recession of the limbs. 

 But since spectroscopes have been available the axis of Uranus has been too unfavour- 

 ably placed. Slipher and Lowell took the earliest opportunity afforded by the slow 

 revolution round the sun to better perspective, and detected a rotation in the retro- 

 grade direction in ioi{ hours. The near accordance with the 10 hour rotations of Jupiter 

 and Saturn is noteworthy. 



