July 8, 1922] 



NA TURE 



53 



Our Astronomical Column. 



Epiiemeris of Skjellerup's Comet, 1922 b. — 

 This ephemeris is for Greenwich midnight from the 

 elliptical elements given in Nature of July 1, p. 20, 

 which are approximately true. 



R.A. 



N. Deri. 



R.A. 



N.Ded. 



July 6 15 58 43 37 53' July 14 16 37 46 3I°47' 



8 16 10 1 36 19 16 16 45 45 30 16 



10 16 20 14 34 45 18 16 53 4 28 48 



12 16 29 30 33 13 20 16 59 32 27 29 



During the interval, June 30-July 20, log r increases 

 from 0-0486 to 0-1131 ; log A from 9-5124 to 9-6830. 

 Owing to its short period, it is important to follow 

 it as long as possible in order to facilitate its recovery 

 on its return. 



Prof. Plaskett's Massive Star. — Some further 

 particulars about this star (see Nature, June 17, 

 p. 791) may be of interest. It is in Monoceros, in 

 the middle of the Galaxy, its place for 1900 being 

 R.A. 6 h 32-o m , N. Decl. 6° 13', visual magnitude 

 6-06. The spectral type in the Henry Draper 

 Catalogue is Bop, but Plaskett prefers Oe5 ; the 

 orbital velocities of the two components are 206-38, 

 and 246-7 km. /sec, the period 14-414 days, the 

 eccentricity 0-0349, the minimum masses of the 

 components 75-6 sun and 63-3 sun. From the non- 

 occurrence of eclipses it is inferred that the orbit 

 is at least 17 from the edgeways position, and the 

 masses 14 per cent, greater than the minimum values. 

 The centre of gravity is receding at 23-94 km. /sec. 

 The H and K lines of calcium show no orbital motion, 

 but a steady recession of 15-9 km. /sec, which is 

 exactly the amount of the sun's resolved motion, 

 so that the calcium is at rest with respect to the 

 star-system, a result obtained in other spectroscopic 

 binaries. The following estimates are given of the 

 star's size and distance : density o-oi of sun's, surface 

 brightness 4 magnitudes in excess of sun's, diameters 

 20 and 18 times sun's, distance between centres 65 

 sun-diameters, distance from the earth 10,000 light- 

 years, absolute magnitude of brighter component 

 -5-65. It is noted that the recession of the centre 

 of gravity, corrected for solar motion, is 8 km. /sec. ; 

 with the estimated dimensions and masses, the 

 Einstein spectral shift would account for 2-8 km. /sec. 

 of this quantity. 



Since this star, the most massive known, lies so 

 near the mean galactic circle, it may be suggested 

 as a suitable zero of galactic longitude ; it seems 

 wrong to use the terrestrial equator as the zero 

 point, for it reintroduces precession, which it is the 

 object of galactic co-ordinates to avoid. 



Origin' of the Asteroids. — Dr. K. Hirayama 

 discusses this old problem once more in the June 

 number of Scientia. He recalls the early suggestion 

 of an exploded planet, and its abandonment when 

 the wide range of the orbits became known. He 

 then mentions the rival hypotheses, one by one, 

 showing that they too have difficulties. Thus many 

 have suggested that it was the disturbing action of 

 Jupiter that prevented the nebulous ring, assumed 

 to have existed in this region, from forming into a 

 single planet ; but he notes that the four great 

 satellites of Jupiter are quite near it, and yet much 

 larger than any of the asteroids. He also notes 

 objections to the theory that the asteroids came from 

 a distance, and had been captured by Jupiter, like 



the short-period comets. The orbits of many of 

 them do not approach near enough to Jupiter for 

 this, and their major axes are almost free from 

 perturbation. 



Dr. Hirayama himself favours a theory, put 

 forward by Young, which invokes not one but 

 several explosions. Each " family " of asteroids, of 

 which many have been traced, is explained as the 

 result of an explosion of a single body. In support 

 of this view he refers to the rapid and irregular light- 

 variation of many asteroids, notably Eros. He 

 supposes that they are irregular, angular fragments, 

 their own gravitation being too weak to compel 

 them to take a spherical form ; if they were rotating 

 about an axis that was not a principal axis, both the 

 position of the axis in the body and the period of 

 rotation would vary ; this agrees with observed 

 facts. It would be possible, by assuming a sufficient 

 number of explosions, to trace the whole system of 

 asteroids to a single primitive planet. As the whole 

 mass of the known asteroids is only some 1 /2000 of that 

 of the earth, he thinks it possible that many fragments 

 may have been absorbed by the sun and Jupiter, 

 and in conclusion suggests a similar origin for the 

 ring of Saturn, noting the many resemblances between 

 it and the asteroid system. 



Norman Lockyer Observatory (1921-1922). — In 

 his report for the year 1921, April 1, to 1922, March 31, 

 Dr. W. J. S. Lockyer, the director of the Observatory, 

 directs attention to several advances which will be of 

 interest to observers who have followed the progress 

 of this new institution. 



At present the greater part of the work is confined 

 to stellar investigations, and observations were made 

 on 137 of the 149 nights which were sufficiently clear. 

 The McClean telescope, with the 12-inch prismatic 

 camera, has been used for obtaining stellar spectra 

 for classification and parallax determinations. During 

 the year 654 negatives have been secured. 



With the 9-inch Kensington prismatic camera 79 

 negatives have been obtained in the progress of a 

 scheme to photograph the spectra of all stars down 

 to about the fourth magnitude. Special attention is 

 being paid to large-scale spectra of standard giant 

 and dwarf stars of types F to M. These are being 

 examined by Adams' method for the determination 

 of stellar parallax. At the present time 1200 nega- 

 tives are available, and 370 have been measured, 

 giving preliminary curves showing correlations be- 

 tween absolute magnitude and line-intensity differ- 

 ences. A wedge method of determining the line 

 intensities has been devised, and details of the pro- 

 cedure have been published. 



The routine classification of stellar spectra by means 

 of the Kensington nomenclature of generic class 

 names has been discontinued, and the Harvard sys- 

 tem, based on numerical measures of line-intensities 

 in the spectra, combined with the separation of stars 

 into groups of increasing (giant) and decreasing 

 (dwarf) temperatures, has been adopted in its place. 

 For laboratory investigations a 10-feet Littrow 

 spectrograph, by Hilger, has been presented to the 

 Observatory by Lady Lockyer. 



It is evident that an observatory of this character, 

 with extensive instrumental equipment, is well suited 

 to further the prosecution of investigations beyond 

 those covered by the immediate routine, and it is 

 pleasant to note that during the past year two 

 student observers have been encouraged to spend 

 short periods at the Observatory. 



NO. 2749, VOL. I IO] 



