GO 



KNOWLEDGE 



[Mabch, 1902. 



north of the equator, and the worst tiino for oliserviiif; 

 heliacal settings, especially for stars south of the 

 equator. 



It follows from these cousideratious that the Pleiades, 

 in sjiito of tlif frequont reference to their heliacal rising in 

 classical jioetry, were not well placed for this observation, 

 for during the 5000 years that astronomy has been in 

 active operation as a science, they were never so far from 

 the vernal equinox as at present. It must, however, be 

 borne in mind that the nearer to the equator the place 

 of observation, the more exactly could the observation of 

 heliacal rising l)e made. For the nearer the equator is 

 approached, the more nearly are the apparent paths of the 

 sun and stars perpendicular to the horizon. They, there- 

 fore, seem to rise in the sky much more sharply, and dawn 

 and twilight last for a much shorter space of time. A 

 similar remark applies in its degree to Sirius, which, 5000 

 years ago, would have been by no means a very good star 

 to observe at heliacal rising here in England. But the 

 land where Sirius was made much of for this purpose was 

 Egypt, the most southern of all the ancient civilisations, 

 and in that marvellous climate the star may often be seen 

 now to rise sheer from the waters of the Ked Sea. Under 

 such circumstances the return of the year could be fixed 

 with all desired exactness by the rising of Sirius, even 

 when it was on the vernal colure. 



There is a great interest for those who care to try to 

 enter into the thoughts and conditions of the past in 

 repeating for oneself the very observations which were so 

 all-important thousands of years ago. And there is a real 

 value in repeating them to-day. If three or four observers 

 were independently to observe the heliacal risings and 

 settings of some two dozen zodiacal stars for three or four 

 years, we should gain a great deal of needed information 

 as to the actual amount of precision of which the method 

 is capable, and some needed light would be thrown upon 

 its practical eflBciency. But more than this, the work 

 would prove very effective in teaching the observer to pick 

 up quickly minute points of light in a bright sky. This 

 training would be found afterwards of great value in more 

 ambitious fields of astronomy. The variable star observer 

 would find it of use in enabling him to discount the effect 

 of bright moonlight on his observations ; the double star 

 observer would find his eye quickened to detect a faint 

 companion in the glare which a bright star gives in the 

 field of a great refractor. 



But more than this, the work affords a chance — a rare 

 one it must be admitted, but a chance not to be despised 

 — of making a discovery of a most interesting kind. 

 Occasionally comets passing through our system so 

 approach us as not to be seen until they are close to the 

 sun, and these are usually comets of exceptional interest, 

 both from the character of their orbits and from their 

 physical behaviour. Such objects the persistent obsei-ver 

 of heliacal risings and settings is — from the nature of his 

 work, and from the special acuteness in detecting objects 

 in the twilight sky which it will have given to him — the 

 inost likely to be the first to discover. The great comet 

 of the autumn of 1882, for instance, was first seen at 

 sundown, the great southern comet of 1901 was detected 

 just before sunrise ; in both cases only a very short time 

 before the comet passed into conjunction with the sun, 

 and was temporarily lost to view. In botli cases a regular 

 " heliacal " observer would have stood a good chance of 

 being two or three days ahead of anyone else in the dis- 

 covery, to the gain of his own reputation and of astro- 

 nomical science. 



The three stai-s mentioned earlier, Sirius, Proeyon and 

 Eigel, may be observed to rise during the latter' half of 



August ; their settings fall in April and May. They are 

 thus emphatically winter stars, Kigel coming to opposition 

 on December 11th, Sirius on December Slst, and Proeyon 

 on January 12th. In the case of these stars, the opposition 

 falls nearly midway between the date of rising and that of 

 setting. Not so with stars near the equinoctial colures. 

 Thus Spica rises at the end of Octobei-, comes to ojiposition 

 on April 12, and sets in the middle of .August. Hamal, 

 on the other hand, rises near the end of May, comes to 

 opposition on October 26, and sets in the middle of April. 

 The Pleiades, too, which Hesiod reports as being invisible 

 for forty days, arc now invisible here in England for 

 over sixty, from the end of April to the beginning of July, 

 but the date of conjunction does not fall in the middle of 

 this jieriod but on May 19. 



A very interesting side question would probably arise 

 in the course of the systematic observation of heliacal 

 phenomena, and that is the influence of colour on the 

 visibility of stars of a given brightness in a bright sky. 

 Orion and the neighbourhood are particularly rich in stars 

 suitable for such a comparison. The orange tint of 

 Betelgeux, the golden colour of Aldebaran, the slight 

 suspicion of green about Bellatrix, the steel-blue of 

 Alnath and of the giant Sirius, the white of Proeyon, and 

 the contrast which is so obvious between the tints of 

 Castor and Pollux, afford ample materials for a very 

 delicate and interesting research, and one which it still 

 remains to make. 



Stabs foe Heliacal Obsbevation. 



At Setting. 



February— € Pegasi, Enif. 

 Marcb — a Pegasi, Mavkah. 

 V Pegasi, jl/»jL'nt6. 

 a Piscium, Okia. 

 April— a Arietis, Hamal. 

 /3 Orionis, Rigel. 

 Ti Tauri, ^lcyo»e. 

 May— a Oanis Maiaris, Si 

 a Tauri, Alaehavan. 

 a CaoisMinoris.Pj'i 



At Rising. 



August — )3 Orionis, RigeL 



a Canis Minoris,Procyoii. 

 a Canis Majoris, Sirius. 

 a Cancri. 

 September — a Leonis, Regulus. 

 ^ Leonis, Bentbula. 

 October— a Bootis, .Ircfurns. 

 a Virginia, Spica. 

 November — a Serpentis, Unuk. 



a Ophiuchi, Rasalague. 



THE USE OF HAND TELESCOPES IN 

 ASTRONOMY. 



By Ckcil Jackson. 



II.— THE SUN. 



Before ti-eating of the sun itself, it will be desirable 

 to consider the methods of observing it. For a small 

 pocket telescope a cap containing a piece of very dark 

 green glass will answer well. The glass should be so 

 dark that the outlines of buildings are invisible through 

 it when it is held up to the eye. A small telescope 

 requires as dark a glass as a large one. Do not on 

 any account be persuaded into having a light sun-glass 

 on any telescope, however small, as the use of such a 

 glass is veiy trying .to the eye, except in a smoky or 

 dull atmosphere. 



Another way of viewing the sun is to fix a piece of 

 paper in the sunshine, with the sim shining squarely 

 on it. The pajicr may be pinned to a board, and the 

 boai'd reared up on a chair in such a way that the 

 pins, if tnily perpendicular to its surface, cast no 

 shadow on tlic paper. Stand facing the paiier, and 

 hold the telescope — drawn out to its full length — with 

 its eye-end a few inches away from the paper, so that 

 a circulai- shadow is cast, in which case the telescope 



