ASTRONOMICAL NOTES.

Press, Volume LV, Issue 16507, 26 April 1919, Page 12

ASTRONOMICAL NOTES.

FOB MAY.

(Bv E. G. llouf;, M.A., T.R.A.S.)

On May Ist Morcury will be a morning planet, ri.sing at -I.HO a.m.; Venus, Jupiter, and Saturn arc evening planets setting at Ij.-I-~> p.m.. S.2S p.m., and midnight respectively. Mais will be too closa to tho sun to bo seen during tho month. The brilliance of Jupiter has been declining steadily since the be-

pinning of the year: it has now tho same apparent brightness as Sirius

( — 1.0. with ■which it should be compared. There will he a total eclipse of the sun on May 29th, but it will not be visible in New Zealand. Tho following bright stars will cross tho meridian of Christchurch a t tho times and at the altitudes above- the horizon given after each star. If the 1 times of meridian passages are required for other dates it will be nnccssaiv to i subtract 3min uGsce for each succcoding day. To the north of the zenith wo have Alpha Leonis (Rcgulus) 7.30 p.m., alt. 34de<£ 7min; Gamma Leonis, 7.41 p.m., alt. 26dog. l&nin.: Delta Leonis, 8.3(> p.m., alt. 2odog. oOmin.; Theta L<?oni». 8.30 p.m., j<!t. UOtleg. 36'min: Ueta Leonis (Deuebola). 0.11 p.iw., alt. -ildeg. 27min. : Jjota Virginis, 9.12 p.m., alt. i-kle.i;. l-iniin.; Kpsilon C'orvi, 0.32 p.m.., alt. Oidcy. 3!iniin.; Gamma Com. 9.33 p.m., alt. 6udeg. 34min.; Delta Com, 9.52 p.m., ait. (52deg. l-jmin.; Beta Corvi, 9.50 p.m., alt. OSdog. oumiii.; Gamma Viryinis, 10.4 p.m.. alt. 47dt«g. 2D.-nin. Towards tho south of the zenith we have Alpha Tucanae, 7.39 p.m., alt. lideg. llmin.; Theta Argus, 8.0 p.m.. ait. G9deg. 33min.; Gamma Tucanao, 5.38 p.m., alt. 12min.; Lambda Con-, tauri, 8.58 p.m.. alt. 70dcg. 57n>in.; ' Delta Contauri, i). 30 p.m.. alt. S3deg. : luriun.; Delta Crncis, 9.37 n.m., alt. j 7odeg. IGniin.; Alpha Crucfs, 7'Odeg. j 5-tmiii.; Giininia Crncis, 9.-33 p.m., alt. 7Cdcg. o2min.; Alpha .Muscae, ?).5S p.m., all. C4deg. 52min.: Gamma Contauri, 10.3 p.m., alt. Sodeg. Imin.; Gamma Centauri, 10.3 p.m., alt. Ssdeg Imin.; lieta Crude, i.0.9 p.m., alt. 7-ideg. 17m in. In the abovo list are three double stars, -which may bo separated in a small telescope. Gcinma Virginis, Gamma Leonis, and Alpha Crncis, the last-named has a sixth magnitude vom-

njuiiou at a distance of £0 seconds of arc. The following objects avo well placed for observation' this month, niter each is given its time of culmination and altitude above tho horizon—the magnificent globular cluster 47 Tucanao (9AG p.m., alt. Gldog. 36miu S.), the greater cluster in Carina (8.23 p.m., alt. 73d«g. 2Ginin S.)~tho finest irregular cluster iu tlie heavens and possibly tho finest object. Outside "the solar system, according to Pickering, tho lesser cluster in Carina (7.2G p.m., alt. Tldee;. SGmin. S.). the irregular cluster K. Crucis (10.14 p.m., alt. 7'Sdeic. 4'imJn.), and tho Satnm Ticbula in Virgo (10.0 p.m., alt. 57dog. 34miu.

In concluding an article entitled "Tho Sixty Finest Objects iu the Sky," contributed by him to "Popular Astronomy," Professor Pickering vrrites: "The seven finest objects south of Declination 45 degrees S., the nebulaa in Dorado and Carina, tlie globular clusters in Tucana and the Centaur, the irregular cluster in Carina, and tlie white doubles in the Centaur and Cross are but poorly balanced by the nine finest objects in alj the rest of tho skv. These nino may be given as tlie nebulas in Orioji and the globular cluster in Hercules, the irregular and coarse clusters in Canis Major, Perseus and the Pleiades, the triple star in_ Monoceros, and the doubles in Gemini and Andromeda. Or these the nobulco in Orion and the doubles in Gemini and Andromeda are tho only bodies that may he said to be in the eamo class ivit'li the seven great southern objects, and even the Orion nebula itself is distinctly inferior in impressive-ness tt> tho clusters in Carina and Tucana. R is rather singular that five out of m the seven southern objects should lie iritTim an area measuring but 30 degrees by 20 degrees." An enquiry lins reached me as to the precise inenning of the torm "parsec." The word is due to Professor H. Turner, of the Oxford Observatory, and is used to express the distance from the earth of a star whoso parallax is one secoud of arc; hence if the parallax of a star is known its distance in pars'cc IB at once found by dividing unity by the parallax of tho star. Alpha Centauri— the Btar nearest to the earth—lias a parallax of .76 seconds of arc, hence its distance is about one and one-third parsees; 01 Cygni with a parallax .301 is about three and one-third parsecs away. The actual length of tho parsec is about 1.91 million million miles, and is equivalent to the distance expressed by 3.27 light years. The value or the term parsec arises largely from tho tact that for the purpose of comparing their absolute magnitudes, all stars are, !>y a simple computation, based on their relative brightness and parallax, brought to a distance of ten parsecs from tho earth. Certain conclusions of modern, scicneo have strongly suggested that there is lnoro to be learnt about the effects or gravitation than is to he found in Newton's simple, law, and there are certain outstanding difficulties in astronomydiscrepancies between theory and observation—that have not as yet been reconciled with this law. . It is now generally held by physicists that tho mass of ordinary matter is duo to tho electro-magnotic energy of tho ultimate atoms composing that matter, and that electro-magnetic energy, wherever found, must possess mass, i.e., inertia; hence tho question arises whether a" kinds of energy may not, in virtuo or their mass, exert and bo subject to grovitation. In particular, the energy projected in a ray of light may be subject to gravitation, an d have a, curved path in a sufficiently intense gravitational field, so that rays of light would bo deflected irom a rectilinear path, by gravity. If the theory be true, it would seem that a star scon close to the limb of tho sun would suffer an apparent angular displacement amounting to as much as 1.75 seconds of arc. This is in general a quantity easily measured by tho astronomer, but in this particular casethe conditions of the problem are so difficult that no observational test has as yet been made. In the case of tho plaiiet Jupiter, the deflection of a star at the moment of occultation would be only 0.017 seconds—a quantity bo small that its measurement would be masked by errors of observation. It is only at the time of a total eclipse of the sun that conditions occur admitting of an adequate test of this consequence of the new theory being made, and it so happens that the total eclipse of the sun, which takes place on siay 29th, will apparently afford the much-desired opportunity. There will be at this eclipse an unusual number of bright stars —the Hyades —in proximity to the sun, and in a recent paper on the subject, the Astronomer "Royal gave a list of 13 stars which, with good weather conditions, might be caught on a photographic nlate during too brief period or totality. The nearest of these will be only / minutes of arc from the sun's limb, and should, according to theory, suffer a displacement of 1.20 seconds; four other "stars should bo deflected moro than half a second of arc, while all the remaining stars on the list should bo shifted more than a, quarter of a second. As the stars are suitably j grouped round the sun, and are all of 1 photographic magnitude 7.0 or brigh- J would certainly; appear .tJiat.wo-

have here a.more than usually favourable combination of' circumstances for submitting the theory—or vatlmthis aspect of it—to a conclusive test, it only-the camera pets a fair chance. ]]y comparing photographs of the region surrounding the sun taken during'the period of totality with others previously secured by the same instruments, it should be easily demonstrated if tin; displacements suggested by theory have actuallv occurred.

Tho "lino of totality of the coining eclipse starts from tho west of South America in latitude "20 degrees S.. runs in a north-cast direction, and, leaving that continent near the-Equator, crosses the Atlantic: it enters Africa in Liberia and, passing in an .1i.5.15. direction, ends near Lake Tanganyika. ]fc will thus be seen that tho conditions are on the whole unfavourable, as the major part of the land tract lies in the two larpest tropical river-basins ol' the world the Amazon and tho Congo— when these regions would be experiencing their rainy seasons. Those localities nro, moreover, almost destitute of facilities for the transport of the heavy instruments of thi» eclipse parties. The stations which have been Jinally chosen bv tho Joint Permanent Eclipse Comiiiittce arc in Brazil—probably at Ceara —and on tho island of Principe, off the west eoasfc of Africa. Imt at- neither place is there more than a fair probability of good weather conditions occurrinyf towards the end of this month. It is depressing to recall that on the occasion of the total eclipse of tho sun on October 10th. 1012, no less than eight expeditions, in which England, France, Argentina. Chili, and Brazil were represented had stationed themselves on the line of totality, v..hieh passed through Ijrazii, bub all were, involved in one heaw rainstorm, which seems to have, covered this whole of thereasonably accessible, part of the line. Not a glimpse of any stage of tho eclipse v.-as obtained bv any of them. Tho proposed modification of Aowton's law of gravitation is practically of tlv* nature of a. small correction to the known law. which makes tho gravitational attraction Iwtwoen two moving depend on thoir relativo velo-cit-v and it is in the nold of -astronomy that the test of tho truth of ilia suggested change must bo made. Aβ Mr back as 18-15 Loverrier found that tho motion of the perihelion of Mercury, .as derived from observations of transit, was greater by &i sounds per century than it should be f rom tho perturbations duo to all the other planets of our system ; subsequent discussions of this, intricate point lia-ve t?hown that the oxcess of motion is actually as much as 143 .set-ouds per c&ntury, «nd all attempts to account for this discrepancy by Newtonian mechanics liavo failed. Great then was the sensation caused among astronomers when it was found recently that the modified law accounted exactly for the outstanding difference between theory and observation. The wow theory also gives a, fair result in the case of tho perihelion of Mars, but appears to break <lown in dealing with «ji outstanding difficulty in connexion with tho motion of the node, of Venus. Another result predicted from tho new law is tho displacement of all spectral lines to tho red in an intense field of gravitation—in other words, atomic vibrations are executed more slowly, Dr. St. John, of tho Mount Wilson Observatory, has beon working on this aspect of the question for sometime, but has failed to obtain any evidenco in support of tho required displacement, while two other series of observations at Mount Wilson ar© also at variance with tho new principle. With the whole matter in this critical position it is easy to understand how deep is the interest attached to the coming eclipse of tho sun.