ASTRONOMICAL NOTES.

Press, Volume LIV, Issue 16406, 28 December 1918, Page 7

ASTRONOMICAL NOTES.

FOE JANUARY.

(BT E. G. HOGG, M.A., F.R.A.S.)

earth will bo in perihelion, i.e., nearest to the sun, on the evening of January 3rd, when the distance between the two bodies will bo about 91,300,000 miles. Mercury will rise, on January Ist at 3.18 a.m., Jupiter at 7.41 p.m., and Saturn at 10.14 p.m.; Venus sets on that date at 8.18 p.m., and Mars at 9.24 • p.m. The following bright stars will cross the meridian of Christchurch on January Ist at the times and at the altitudes above the horizon given after each star. If the times of meridian passages are required for other dates, it will be necessary to subtract 3min 56sec for each succeeding day. Towards the north we have Beta Persei (Algol) 8.22 p.m., alt. sdeg 50min; Eta Tauri, 9.2 p.m., alt. 22deg 37min; Zeta Persei, 9.8 p.m., alt. 14deg 50min; Gamma Tauri, 9.34 p.m., alt. 31deg 2min; Alpha Tauri (Aldebaran), 9.50 p.m., alt. 30deg Bmin; Pi 3 Orionis, 10.5 p.m., alt. 39deg 9min; Beta Eridani, 10.23 p.m., 51deg 40min; Bota Orionis (Rigel), 10.30 p.m., alt. 54deg 46min; Gamma Orionis (Bellatrix), 10.40 p.m., alt. 40deg 12min; Beta Tauri, 10.40 p.mi, alt. 17deg 56min. Towards the south we have Gamma Triang Aust, 8.20 p.m., alt. 21deg 51mm j Beta Triang Aust, 9.7 p.m., alt. loceg 43min; Gamma Hydri, 9.8 p.m., alt. S9deg 2min; Alpha lteticuh, V.32 p.m., alt 70deg 51min; Alpha Itoradus. 9.0 l pm., alt. S2dog 19min; Alpha Triang Aust, 9.59 p.m., alt. 22deg 24min. The year 1919 is a bad one for eclipses" so far as New Zealand is concerned. There will be a total eclipse of the sun on May 29th-30th, and an annular eclipse on November 23rd; the moon will be partially eclipsed on May Bth. AH these phenomena are, however invisibie in this country. An interesting article on What Meteors Teach Us," in a recent issue of the "Chalda;an," opens with a remark to the effect that meteors generally seem to disappear at a height of about 50 milos above the earth, where there is apparently a surface of separation which may enter into twilight effects and the reflexion of sound. This uniformity in the height at which meteors disappear is a rather startling physical fact, tho complete explanation of which will doubtless be forthcoming at no distant date.. Of course, a large number uf meteorites rcach the oarth's surface annually, and though there are

some who hold that there is no essen- ! tial di&eronco in kind between meteors and meteorites, and that we pass by gradual stages from one to the other, yet- there are many who hold the opposite view that meteor aud meteorites are different in kind. It is certainly well established that some of the greatest outbursts of shooting stars —as for instance, that of when metoors were, during part of tho shower, glancing acrobo the iky with a frequency estimated to be about half that of flakes of snow in an average snow storm —havo not been accompanied by the fall of a single meteorite. There are many who will recall the beautiful sunsets which were visible over the Northern Hemisphere _for some years after tho explosion of Krakatoa in ISS2. At the same time there were remarkable "luminous night-clouds," as they were called, to be seen in tho skies, and during the years 1830-IS9I their height above the earth's surface was repeatedly measured and was almost invariably found to be S2 km. (51.3 miles). During recent years the higher parts of the earth's atmosphere have been the subject of much study, and the researches of more than one investigator lead to the conclusion that; outside, a radius of from 50 to CO miles, our atmosphere is practically all composed of the gas hydrogen. Hence it seems probabxe that from the closely accordant measures of the height of the "luminous night-clouds" we obtain a definite inner radius for the envelope of hydrogen which forms the outer part oT our atmosphere. In the Astronomical Notes for last May it is stated that the average height at which the luminosity of a meteor begins is about 76 miles; lienuo it would appear that the luminosity is excited while the meteor _ is within the hydrogen envelope and in general persists only so long as it is traversing such envelope, and that the matenal of the shooting-star either is dissipated very rapidly when it passes out of the hydrogen shell or loses its luminosity through the comparative absence, in tlio new medium in which it is moving ■of the ■ original exciting cause. - i, - The study of meteors is essentially an affair of the naked eye, but that patient race of astronomical observers, known as "comet seekers," has made us acquainted with the existence of 'i new type of shooting-star, with distinctly marked characteristics of its own, to which the nanie of. scopic" meteor has been given. When an ordinary shooting-star enters the field of view of a telescope its passage occupies only a fraction, of a 6econd, but* when specimens of this new type are sighted they seem like; grey, points of light which slowly float or crawl among the stars, their whole path sometimes beginning and ending within half a degree of space—sometimes they appear to move in straight lines, but often they a sinuous course before fading from view. Those slow, short meteors are far too plentiful (Mr W. F. Denning records 635 during 727. hours of comet-seeking) to be regarded as objects _ near their radiant point and travelling end-on towards the observer and their appearance, apparent velocity and length of path induce the impression that they are comparatively distant. It has been computed that if these bodies move with the same real velocities as ordinary meteors and traverse the same mean length of path, they must be 20 times as distant from us as the .ordinary shooting-star is during its luminous stage, and more than one competent authority has placed their distance from the earth as greater than 1200 miles. The actual height of a telescopic meteor has not yet been determined by simultaneous observations from two different stations, and the great distance from the earth ascribed to these bodies is based entirely on their appearance and motion. The idea of very distant luminous meteors is as yet only a strong suggestion based on analogy and inference, and appearances are sometimes very illusory. Large, fire-balls we are very apt to judge as being extraordinarily near, and casual observers frequently estimate thoir distance as no more than a few hundred yards, whereas they are generally from 50 to 200 miles, away. Similarly, in regard to these telescopic we may be deceived in the opposite way. The comparison, however, /scarcely affords a parallel, because in the latter case the deduction is based upon certain well-observed features, viz., length and velocity of flight, rather than upon very vague ideas, which are often the natural resultants of surprise observations Upon inexperienced persons. A recently issued Harvard circular gives an interesting account of some estimates made by Professor W. H. Pickoring, of the distances from the solar system of the Orion nebula, the Pleiades, and the star cluster in Coma Berenices, based on studies-of the blue stars contained in these objects. Pickering finds that the blue stars in the Orion nebula are at a distance of about 6500 light-yearsj and that the diameter of the nebula on this basis is about 1700 light-yeare. The mean distance of the Pleiades - from us is about 650 light-years; the diameter of the group is 70 light-years;. Alcyone, the most conspicuous star in the cluster, ie about 2100 times as brilliant as the sun, while the other five bright stars average '800 times the luminosity of the sun. These §ve stars are about as far from Alcyone as the sun is from Sirius. The distance of the Coma Berenices cluster, is about 300 light-years, and it is, therefore, one of our nearest, neighbours among the clusters. Pickering finds that the diameter of the cluster iB about 25 light-: years. This cluster, which is. visible to the naked eye, contains seven stars of the fifth magnitude, but as these seven lucid stare do not seem to have any common proper motion, Newcomb is inclined to ascribe their relative proximity to an accidentl