ASTRONOMICAL NOTES.

Press, Volume LV, Issue 16434, 30 January 1919, Page 8

ASTRONOMICAL NOTES.

FOB FEBRUARY.

(Br E. 0. Hogo, M.A., F.R.A.S.)

i I Mercury is n morning planet and rises [on Fobrimry l a*- n,!TI, | ,c ' J rapidly nppmachintr the siin, and will l.c j in superior conjunction with it on ~h si)* ' rtmry 24th nt 3.30 a.m. Venue sets in thn south-west nt 8.13 p.m. on Febru- ' arv lit: it is a brilliant object being just now nearly five times as bright r.s ' Sirius. Venus and Mars will bo in conjunction v.'ith each other at 1.8 a.m. cn February 14th, when the angular distance between them will he only 3-5 minutes of arc. Mars sets on February Ist, at 8.24 p.m., and Jupiter and Saturn rise on that date ct 5.25 p.m. and 8.7 p.m. respectively. Tho rings of .Saturn aro tiltocl now relative to the earth, so that the/shorter arsis of tho apparent 1 ellipse of the outer ring is about onefifth of tho longer arisj hence if tho atmospheric conditions aro gond, ills dotails of the ring system can be well eeen in a good telescope. The bright star situated closo to Saturn is Alpha Loom's (Regains): it is interesting to traco tho paes'-'-K© of the planet, in tho eky by its gradual separation from Rogulus. Tho following bright stars will cross the meridian of Christchurch on February Ist, at the times and ut tho alti-» tu'des above the horizon given nfter eacn star. If tho times of meridian passsiitros are required for other dates, it ■will bo necessary to subtract 3min 56sec for each succeeding day. Toward tho north, we have Beta Orionis (Itigel), 8.23 p.m., alt. o4do:| 56min; Itelta Orionis, 8.45 p.m. alt. 4Cden- 50niin; Epsilon Orionis (Elnitam), 8.49 p.m., alt. 47deg 4tmin; Zeta Orionis, 8.54 p.m., alt. 48dec 27min; Kappa Orionis, 9.1 p.m., alt. 56deg lOmin f Alpha Orionis (Betelguose), 9.8 p.m., alt. 89deg smin; Theta Auriga, 9.11 p m., alt. 9deg 16min; Ifata Canis Maj. 9.86 p.m., alt. 64deg 23min; Gamma Geminorum, 9.49 p.m., alt. £0 dog; Alpha Canis Maj. (Sirius), 9.58 p.m., alt 63cleg omin; Epsilon Canis Maj. 10.12 p.m., alt. 75deg 20mjn ; Delta Canis Maj. 10.22 p.m., alt 72deg 4-tmin; Eta Canis Maj., 10.37 p.m., alt. 7odog 37min. Towards the south we have Beta Arae. 8.36 p.m., alt. lOdeg 44min: Eta Canis Maj., 10.37 p.m., alt. 70dofr 59min; Eta Pavonis, 8.55 p.m., alt. 18doc 13min; Alpha Argus (Canopus), 9.39 p.m., alt. SOdcg 53m >n ; Lnmbda Pavonis, 10.1 p.m., alt. 15dag 49min* Tau Argus, 10.5 p.m.. alt, 83deg lmin. From the above list a good knowledge of the constellations of Orion and Canis Major can be obtained. The middle star of the belt of Orion is Elnitam; to the west of this star lies X)elta, and vo the east Zeta. These three stars also give the observer a useful test of differences of apparent magnitudes, which as follows:—ltelta 2.5, Eosilon (Elnitam) 1.7, and Zeta 2.0. Above the belt of Orion lie three stars in a line, forming the sword of Orion; the middle one or. these-—a somewhat hazy, indistinct star—is the famous nebula in Orion. Beta and Delta Canis Maj. are both stars of apparent magnitude 2.0, Orioni™ ay compared with Zota The list is of further interest in containing the two most brilliant stellar objects in the sky—Sirius and Canopus whose apparent magnitudes are —1.6 and —0.9 respectively. As normal sigiit can just detect a sjar of the sixth magnitude, we have & rang© of 7.6 magnitudes visible to us. What this really means becomes clearer when we recall that any star emits about 2.512 times as j a * magnitude' h gher, and hence Sirius is sending to us about 1000 times as much light as a star or . the,- sixth magnitude. When a star of magnitude 20 is seen in apowerful telescope, we are gazing at an object whose light-intensity is only one 400 millionth part of that of Sirius. It has been computed: that sixty timek the total star-light on the clearest night would be required to equal the light of the full moon, and 33 million times the amount m order to equal sunlight. .Hence the total light received .by the earth from the stars is extremely small and, there is some justification for Lvi> tons remark, "A farthing candle is more convenient for household purposes i than all the stars." j . Much work has been done recently |

on what is palled the absolute magnitudes of stsfrs; this has been . rendered possible By the great additions which have been made of late to our knowlodge of th© distances of stars-from the earth. To measure, the absolute magnitudes we suppose all stars to be placcd at such a distance from the earth that their annual parallax is one-tenth of a second. This means that some have I ?° .b® moved away, but tho va§t majority have to be brought nearer, the earth. Haying the stars all ranged vfc the same distance,. it is not difficult do compute their luminosity, and we obtain a much clearer idea of tn e vase differ- . ences which exist between the several ■ stars. Alpha Centauri—our nearest stellar neighbour—has a parallel of 0.75 ■ seconds of arc. If it were removed to 7.5 times its actual distance from us it - would shine as a star of absolute magnitude 4.7, while Sirius, whose apparent magnitude is —1.6. would; become of absolute magnitude 1.3.' Our*sun treated in this way would bo of absolute magnitude o and therefore slightly inferior to Alpha Centauri in luminosity. When we come to* the .very distant stars the results arrived at aie very startling. Two stars now visible— Canopns and Rigel—are so far away from us that all attempts to learn their parallaxes have ended in failure, the angle to bo measured is so exceedingly small. If wo lake the parallax of Canopuß to be the minute angle—one thousandth -part of a second l—its-abso-lute magnitude will be 10.9, and as,' on this" supposition of its distance, it will bo about'twelve magnitudes- brighter than Sirius, we can form , some idea of tho transcendent magnificence of this star; and a recent study by Professor Piokorinsr of Rigel leads nim to the conclusion that it is over two million times ae bright as our sun. Tho results obtained in the case of faint stars, though; not so impressive as those cited above, , show us that there are stars Bending light to us which are almost as inferior ■ in luminosity to our sun as that body is to Canouus or Rigel. If we take Bar- 1 nard's Runaway star —Gilpin—as hav- " ing a parallax of 0.5 and apparent ' nitude 10, its absolute magnitude is < about 13.5, which makes it eight and a ' half magnitudes fainter than our orb, or 1 less than one two-thousandth ] part of its light, and the recently-dis- 1 covered companion to Alpha Centauri is ' far less brilliant even than Gilpin. j "What has been described by Madler a-s > "the Astronomy of tho Invisible'' may 1 be said'to have its origin in 1834, when { Bessel first begau to notice a lack of ] uniformity in the motion of Sirius. and 1 suspected that Sirius and some minuto 1 star formed a binary system; this sus- '

picion ho extended in 1840 to Procyon. Writing to Humboldt ho snys: "I adhere to tho conviction that Procyon and' Sirius form real binary systems, consisting of a visiblo und invisible star. There is no roason to suppose luminosity an essential quality of coemical bsdies. Tho visibility of countless stars is no argument against the invisibility of countless others." In 1856 Peters made an exhaustive examination of the orbit of Sirius based largely on transit observAtions made at Greenwich and deducedj for Sirius a period of 50.01 yoars. "When we learn that the maximum amount by wliich Sirius was early or late in passing over, the crog3 r wires of the observing .telescope• ivas less than, one-sixth of a 'second of time,"and realise that the calculation was based on intervals of timo smaller than this as a rule, we camnoti' but regard it as a wonderful piece cf | •work, and our admiration is increased when -tve find that Alvan G. Clark, while in the act of resting, on January 31st, 1862, the largest refracting tolescope then made, discovered the hypothetical companion- of Sirius in the precis© position required by theory. Since that time th-> Sirian 6ystem has been the object of constant scrutiny ! i and in a paper recently read before the j ißoyal Astronomical Society by /Mr | Jonckheere an analysis is made of the i various periods which have been calculated for Sirius and the conclusion is reached that such period is almost exactly 50 years—thus confirming, in a marked the result arrived at by Peters, liie orbit of Sirius as seen from the earth is very eccentric, the greatest distance of the companion from the star being about 10 seconds of arc, the least less than 3 seconds. Owing to the brilliant light of Sirius ■ the companion is invisible even in-, the most powerful telescope when it is nearest its primary. This was the case in 1890-2 and will again occur in 1940 when another revolution is completed. The mass of the combined Sirian- system is about 3.07 times that of the sun. Astronomers and others have long had under consideration the desirability of modifying the time-system now in vogue, and the following extract from the November number of "The Observatory" is of interest" in this connexion: —"By an Army Order recently issued, the system of twenty-four hour timereckonincr has been adopted for use in the British Army, commencing from October Ist. The timo will be denoted by four figures, denoting the hours and minutes, thus 5.8 a.m. will be written 0508 and 518 p.m. as 1708.' The system has doubtless been adopted owing to its freedom from any possibility of confusion between a.m. and p.m. The twenty-four hour time-system is already in general use at sea and on continental railways."