POPULAR ASTRONOMY

Otago Daily Times, Issue 13996, 31 August 1907, Page 6

POPULAR ASTRONOMY

NOTES FOR SEPTEMBER." „

Bv Rev. D. Button, F.R.A.S., F.G.S.

Tlie Sun'enters the sign of Libra on the 24-th, 4hrs 3 p.m., New Zealand mean time. Tho Moon mil he new on the Bth September, first quarter tho 15th, full tho 22nd, last quarter tho 29th.

Tho evening plancls aro Mars and Satan}. Tho position, of Mars .is wollknown by this time. It is nearly overhead between 7 and 8 o'olock. Saturn is in -Pisces (the .Fishes). This constellation may ho noticed in tho eastern sky. . The principal' evening con6tollations to bo noticed this month aro Scorpio," towards tho west; Sagittarius, to the right of Soorpio and higher lip; followed by Oapricornus, •.Aquarius, anil Pisces, • Theso later constellations are not very conspicuous owing to, the a'beence of brilliant stare. Cetus (tho-Whale) is ahovo tho eastern horizon. In tho samo direction, but' much higher, will bo found 'Pisces (the Fish), with its bright star Fomalhaut, and near by is Gius (tho Crane), looking northcast, Pegasus (tho Flyin'g Horse) may he scon not faj;. above tho horizon. Tho arrangement of several of its stars into what is called the " Great Square," 'makes it easy to recognise it. Aquila is nearly duo north. The bright star Achernar, in Eridanus (tho River), may be seen well ahovo tlie eastern horizon. Canopns is near tho horizon a little east of south. Our early morning sky is maide jjlorious by a grand array of brilliant stars in tho constellations of Orion, Canis Major, and Taurus. ITEMS OF INTEREST.

It appeals to bo desirable in -order to avoid ' confusion in the minds of • some roador3 of notes that'an explanation should be given of tho difference existing between tho signs of tho Zodiac and the constellations bearing tho samo names. Hipparchus, an ancient. astronomer who flourished 150 years B.c, divided tlio Zodiao into 12. signs, each about 30 degrees in length,: oallcd the signs of the Zodiac. At the time this was dono tho signs aaul tho corresponding constellations coincided. This is not tho caso now. Owing to tho procession of equinoxes, tiie po'tnln where tho Sun orosses tho Equator in March and September, called the Vernal and Autumnal Equinoxes respectively, move slowly westward, travelling at tho rate of n Utile over SQdcg per year,, tho, whole processional cyelo being completed in. a little under 26,000 years. One result of this movement is that the Sun is now in ; the "sign" much earlier-'than it is in, the constellation of 'tho same name. When the Sun is said to enter the first point: of Aries, its real location is in the preceding constellation, l'isccs; when the Sun is said to enter Libra, its location is in Virgo. 'Hie old signs are still used because it is found convenient to do so. Nearly all tile almanacs follow this usage, including Whitaker's and the Now, Zealand Nautical Almanao, . published by. tho Government. It is needless to say that this,,method of locating tho heavenly bodies' is not followed in the observatories. Tlicro strict account must bo taken of the effects of precession, and the required corrections made to tho smallest fraction of a second. Wo aro thankful tlmt sjieh precision is not required for tho purposes of thc6o notes. There aro many oxcollont text books of astronomy available for those who are thirsting for a deeper khowlcdgo of the subject than' " Notes on Popular Astronomy" undertake to supply. Wo cannot say much at present about Flammarion's oomct. It .developed rapidly from a hazy, nobulous objcot into a. comet of well-defined nucleus and an increasing longth of tail as it drew nearer to tho Sun, Wo shall hear moro about it shortly, and liopo to sco it in the evenings in tho j western sky. Comets arc very numerous, and no year passes without several being discovered. Flammarion's is the fifth for this year. of tho comets rocordetl aro small tolcseopio bodies, and not- a tenth of them ever become visible to tho n«kqd e,yo. Though formerly comets wcrb objects of dread and regarded as cclcstial horalds' of manifold evils and disasters, they have, through our, increasing knowledge of thoir nature and ways, ceased to play thoir anoiont sinister rolo. It is not impossible for a comet to collide with the' Earth, though tho chances against it aro so great that such an eyent is very improbable. Tho Earth has passed through a comct's tail and probably camo in contact with tho scattered debris of tho head of Beila's comet in 1872, and on several occasions since that date. There is a very close connection between the wellknown meteoric streams which tho earth encounter's from time to timo and certain w£ll-known comets. When a comet entere the Bo!ar system it is not seeking for trouble with the planets. It is making for the Run, and if it is.to get anywhriro near its objective it will cross the orbits of the earth and othei f planets, arid in doing so does • not stop to 600 whether tho planets arc dno at tile crossing points. When a planet is anywhere near wlicro tho comet is crossing it is the comet and not the planet that suffers tho penalty. Jupiter especially, has set his mark upon quito a numbor of them, and mainly through tho. influence of tho great planets j a considerable number of comets aro now members of tho solar systom. Tho-Earth's orbit has been compared to a grand race track of about 576,600,000 miles, along which it raccs at an average of 18 miles a second. When a comet is crossing this traok the earth may Iks anywhere—close at hamd or millions of miles away. So there aro very many cbanccs of the comet crossing long before or after the earth reaches tho crossing point.

Tlio minor planet. Eros is not only inforesting to astronomers, but is also exceedingly useful in determining tile distances of the Sun and the planets. On favourable occasions it approaches the 'Tilth to within 15,000,000 miles. The importance of this fact will bo seen when it is remembered. that Mars, at its nearest .approach to the. Earth, is about •35,000,000, miles distant, and this only occurs' in periods of IS or 17 years. The planet Eros, therefore, with a more favourable position, affords a very valuable means' for the study of parallax, of which full .advantage is being taken. Tho parallax .of. Venus or Mars, when nearest to us, is only about 40 seconds of ; arc, wliioh gives a small angle of measurement, while the parallax of Eros is about 60 seconds of arc, thus giving a much larger angle.

I For a popular illustration of what is ; meant by parallax, fcbe following from Profasor Pavnn ms* be .given;— I "Parallas

is simply tho apparent displacement of a body looked at in apace, caused by the chango of the position of the observer, Anyono can illustrate this fact in a very ■simple way. Hold a pencil before the face and towards a window, and then cJojs one eye after tho other, and notico tho apparent change of the, Jlceition of the pcneil as it is projected against tho window. Tho chango of tho lino of Gight from each eye respectively causes tho poncil to appear to ohango its place in the window. If wo think of an anglo formed by drawing linos from the eyea of the observer to tho poncil we have what is called, in this instance, tho parallax of Iho pencil, and it is plain tliat thio fingie gives tjto amount of toe diaplnoemfvnfc .M' tho pcncil in the window. Another fifing 1 may bo as easily seen, and that is that t-jie angle of the parallax will vary inversely as tho object is far or near to the o'lrsorvor." (Hold tho pencil near tho eyes, and again hold it at arm's length from tho eyes, and noto \he wido difference of displacement of the pencil against tho window.) "Now, if tho parallax of any heavenly body varies inversely and exactly as tho distance, than the nearer the body is to the observer tho greater will bo- tho angle of parallax. Hcuco tho vahio of Ems, which at times 'is 60 much nearer -tho Earth than any othor planet, and therefore gives a much greater angle of parailax. Astronomers :in America and in Europo compare tho places of Eros as seen related to ocrtain stare. Tho apparont plaecs of tho planet with reference to these same stars will differ as seen by tho observers in America, or Europe respectively, boeauso they aro not observing from tho samo place on the earth';' just as is tho apparent place of' the pencil 111 the window when 6cen alternately with the right eyo and the left. From' this apparent difference' of pWpo in tho ca6o of Eros tho angle of its parallax can be obtained, and when this is pecurcd ita own distance can bp calqulatcii, and from that data, the distances of tho other planets and of tho Sun can bo found. .

Tho .astronomical journals dro beginning to supply us with some of the results of tho observations of Maw during!! the recent favourable opposition. In tho Lowell Observatory Bulletin, Professor Lowell records, that Solis Iccub showed double on May 18, this {wing the,first timo that it has appeared divided sincc 1894; Tho 6outh polar cap .Jim. retreated sino© tho last presentation, i leaving; dark ground behind, it;-and it is noticeablo. that the oanals connected with Sob's Lr.<m? on tho south' are darker and moro easily seen than those proceeding from itHhroiigb the' north, and Professor Lowoll argues that i runniiiK water is plainly'suggested by'this appearance. , As a oonsequenoe, it" follows that tho temperature wiiich some contend novor rises ftbore tho freezing point, was; certainly above the freezing point, on May 18, that is in the Martian year on a. date corresponding to March 13 in our calendar. Wo shall wait with interest-further reports. Solis Lacus is one of. tho' best , known regions .?( the; pkvpot, andis about 53(1 miles long. Light is said to travel at tho enormous speed of 186,333 milea in a. 6Co(>nd, and there are upwards of thirty-ono iralliopa and l a-lialf of seconds in a year. Tho distance light would travel in - ail those millions of Bcconds constitutes what is called- a light-year, . Uspg. this as a standard of measurement, "Air Hoath, in his'hook "Our StoUar Universe," , sajo that Canopus, our lirillaant. oircunipolar star, which is sorond in brightness to Siriua, cannot be less than 296. light-years,distant from our "sola,? .system, arid that it would take 21,000 orbs, cao'n as largo as the' sun, to equal Canopus, Ho asks wtat-.brto of beings can tlioy bo; "who inhabit planets ruled over by such a sun as this , mighty star must be, At'' tho ' distance from Conopus- tli# Jupiter is from tho 'Stin, , beings constituted *as 'wo are would. bo utterly oohs'uincd. 1 Our Sim removed to a .distance, of. 60 ligjit-yi^rs,would .to. reduced ,to less."than'"a, srtav. of the'sixth magnitude, and- wo'uid be invisible, to the naked eye oxoept,jn the caso of a few gifted withexoeedinglyigood sight. Yet Canopus, nearly five tipios as far away, is the second brightest star in tho wholo heavens.

For a summary of celestial phenomena for September, as it applies to tho' Sun, Moon, and planets,'our readers aro referred to tho notes supplied by Mr H. M.Lovingc.