ASTRONOMICAL NOTES FOR MAY.

Press, Volume L, Issue 14956, 1 May 1914, Page 6

ASTRONOMICAL NOTES FOR MAY.

(By tub Rev. P. W. FArocxoccn, F.R.A.S.)

Venus and Saturn both set early. They will bo in conjunction on the lGth when Venus will bo more than two degrees to tho north of Saturn. They are both in the neighbourhood of Aldebaran. Saturn is sinking westward, but Venus is slowly rising eastward, and will bo a little higher every night. Mars is our most conspicuous planet. He will be found above the Gemini Castor and Pollux. An interesting event will be the occultation of Mars, between 4 o'clock and 5 o'clock on th<? morning of the 31st inst. Unfortuinately Mars will set before midnight 1 , so that the event will not be seen here, but South Africa ought to havo a good view.

An occultation consists in tho moon passing over and eclipsing the star or tho planet, as the caso may be. In tho first two quarters of tho moon's ago it is the dark edgo of the moon that overtakes and extinguishes tho star. After an hour or so the star emerges on the opposite edge, or "limb"-of tho moon. When tho moon is past tho full it is the bright limb that extinguishes the star and the omexgence is on the dark side. Stars, like Regulus, who will bo occulted about 24 hours later than Mars, go out instantly when the moon touches tho line of sight. This shows that a star, like a mathematical point, has no parts. There is no such thing as the moon hiding half of it. You see all or none. This fact also proves that the moon has no sensiblo atmosphere, except perhaps at tho bottom of her deep craters. If thero wero even a very thin atmosphere on the moon it would refract tho light of the etar. bending it towards us, so that wo would see it after the moon had actually got between us and the star. It is quite different when tho moon occults a

planet, for though the planet looks like a star, it has a measurable diameter and the moon hides it gradually. One of tho most remarkable cccultations ever predicted took place a year or so ago when Jupiter's second (old) moon passed over a small star. The star was quite hidden for some moments, and the satellites's time in passing over the star gave tho first possible measure of the satellite's diameter. It proved to be much greater than astronomers had estimated. The question whether tho moon is absolutely dead, and incapable of any further change, is an interesting one. An elaborate map of the moon, showing more than 30.0C0 objects, has been prepared for tho purpose of detecting changes if any occur. It has often been alleged that a new crater had appeared —always a very small one, of course. But while no one had noticed it before, no one could be certain that it was not there all the time. Tho great map is expected, to settlo such questions. The Harvard station in Jamaica reports a lunar chango as having taken place recently. Tho crater Eimart. visible in the moon's first quarter, is 25 mites in diameter. Usually it is said to fill and overflow, each lunation, with a white appearance, such as fog or vapour, a.s if the sun's heat melted and vaporised some substance that shrinks and freezes when tho sun i.s not on it. or as if the sun's heat opened a vent for vapour to escape. Now this production of vapour has been falling off for some months, and has at last practically ceased. Formerly, when tho crater filled with fog, it was easily the brightest spot in that region, but it is no longer tho brightest. A monument to "Tho Hour" sounds fanciful, but a committee has boon sot up in Paris to secure it. It is to commemorate the international standardisation of time and of on March fith, 11)11. The sito i.s at Yiller.s-Mir-Mci', where mo Greenwich meridian cuts Franco. At the end of last month tho cable made tho following announcement: — "The scientists ot tho University of Chicago, after a series of lejijrthy experiments, designed to discover tiu. nature of tho earth's; core, announce that tho earth i.s more rigid than steel, and cannot explode or bo destroyed by hre within itself. "Professor Michelson. head of tho department of physics, states that tho iv.side oi tho earth is not a fiery mass, but resists tho tidal furco of tho moon as if it were mado of solid steel. Despite this fact, tho earth behaves as an elastic body, and is still subject to the samo influences as tho oceans. Tho earth's solid surface is subject to tho samo ebb and flow as the tides of tho ocean, but to a. lesser extent. It is sciontihcally demonstrated that tides averagui-- 12 inches aro moving the earth's surface regularly.'"' Much of this message is liko a cable announcing tho deceaso of O/ieen Anno, for it must bo forty years since Kelvin, then Sir Win. Thompson, declared that tho earth was as rigid as steel, and that it, notwithstanding, yielded to tho tideraising power of tho moon. About a year ago this column cited Mr Maxim's statement that if tho core of tho earth was all dynamite, insido a crust of 100 miles thick, it could not explode. It is simply a question of pressure. Tho rigidity of tho earth's core is possibly also a question of pressure. Tho interior of tho sun is believed to bo gas, but of tho consistency of treacle, owing to pressure If a cubic foot of this interior could bo removed into space and suddenly relioved from pressure, if' would expand, or explode with terrific violence. Tho water in thd boiler is many degrees abovo the evaporating point. It remains water simply because of pressure. Should tho boiler burst tho whole mass of water would bo instantly converted into steam. So the interior of tho earth may bo at a high temperature, and potentially liquid, and yet bo oxtromeJy rigid owing to tho tremendous pressure. Tho earth power of attraction as displayed in tho motions of tho moon, proves that she is nearly six times as heavy as if sho were all water, and nearly twice, as heavy as if sho were all granite. This indicates a very extensivo metalic core, beginning probably about 300 miles down. If the globe was ever wholly molten then the rocky shell is tho slag and scoria floating on the surface of this interior ocean of metal. Tho rocky crust can be explained, however, on other more recent r.nd possibly more probable theories. Tho manner in which the earth transmits earthquake tremors has ehed much light on the condition of tho core. One class of wave comes right round the surface of tho globe; another class strikes right through tho planet from tho point of tho shock to the point of observation. If the instrument is almost 120 degrees away tho near-cut waves trill retain tho peculiarities of speed and of vibration that characterise lesser distances. But as soon Us the distance passes 120 degrees, these waves through tho interior are interfered with in a most pronounced manner. They either cease altogether or aro greatly enfeebled and retarded. This indicates that at a certain depth the waves that set out on a short cut through tho planet meet with a very different kind of material from that which forms tho outer shell. It is supposed that tho rocky shell extends to tho depth of about 300 miles and that tho metallic core fills all tho space within that shell.

Tho breathing of tho solid earth to the^ extent of twelve inches every twelve hours, under the moon's attraction, is a very great discovery, though it is not exactly now either. Tho earth-tide has long boon suspected, and it is several years since the first measurements were made. If our memory servos us, tho results then reached wore 8 inches or 9 inches. Tho inquiry was a very delicate ono, and many surprising and upsetting thine? had to bo learned before anything could be done towards tho solution of the problem. Tho instrument devised to detect the earth-tido was of an extremely sensitive character, and, though founded on the native rock, it showed that the rock deflected as people walked about, and when the observer came near to observe, the rock dimpled to such an extent as to spoil the observation! Theiso troubles took the heart out of. the first observers in England. Later observers have had tne advantago of knowing those difficulties before they began work, and were able to provide for them. These queer little facts, however, render it easy to believe tho trite paying in the? cable message, that "the earth behaves as an elastic Ixxly."

The discovery of a sunspot 50.000 miles in diameter, announced on April 3rd, by the U.S.A. Naval Observatory, seems about tho record for that institution. Such a spot could be ween by the naked eye with the aid of a little smoked glass and it appears was actually seen at the Wnnganui observatory more than a fortnight earlier. March 15th. If that claim is correct then the Washington message must have been delayed till the spot had disappears! : for the rotation of the sun would carry the spot ont nf sight in about 13 days. It would appear that tho spot minimum is over and that a new cycle is setting

in with a vigour that bodes ill for the predictions of Mr Clement Wracr*o ln many respects tho Americansload the world in astronomy, but in astronomy of position, the" most ex'ict work, they aro nowhere. The reason is peculiar and ridiculous. The American Naval Observatory is by far the most richly endowed in tho world. To it is naturally left all nautical almanack work. Now tho law provides that tho Superintendent of the Naval Observatory must be a naval officer. H 0 'j s therefore never a professional mer; he might, however, become one if tho law did not also provide that after a short interval ashore, h 0 should uo to sea and make room for another <!U>b!er. Hence there have Won 20 superintendents in 37 years! Greenwich*'on the other hand, has had only ei"ht Astronomers Royal in 235 years'. They stick to their task and never -o to t-ea and aro trusted to guide the world's navee.

Professor Pickering proposes so'vo day to make a catalogue of'tho numerous dark regions of tho sky. There are a great many of them. Their edges are often sharply defined as thoii'-'Ti" they were vast crevasses, or rife "through luminous reirior.s of stars and' nebulous matter. A long study of these dark spots and streaks both, by means of best photographs and visually, with the 4-oin Yerkcs telescope lias" convinced the professor that they are no + rea | rifts, but dark nebulae, or clouds of opaque dust, lying between us and tho brighter baeksxound which they hide. Tho Mt. Wilson GOin reflector, with an exposure of several hours, has photographed stars down to the 20th magnitude. This i.s a gain of about four magnitudes, and as each magnitude is rather more than twice and "a half .is bright a.s tho one next below it. the 20th maeivitudo wii! bo about- 10 times fainter than tho Kith, which was the former limit of certainty.

About -1500 variables are now known and three-fourths of them have beeii found at Harvard. As many as .">OQ have been found in oi.o globular cluster of stars, all within periods of oniv a few hours! In the iittlo cluster known as ".Messier 3," one in every aoven of its thousand stars is a variable, and all of them have a ncriod of about twelve hours, which can be ascertained to a frartion of a second. How amazing it is that there should bo 140 clocks all keeping accurate, time in this one little cluster! In many cases tho chango of light is vow ra->id —in ono instance the doi7h!es*iii woven minutes. About 150 ;\!gol variables are definitely known. Th>\so aro characterised by n lengthy period of steady light with a. rapid "decline and a rapid recovery. This i.s owir_- to an eclipsing body passing between them and us.

Harvard and its Peruvian branch, Arequipa. have taken 45,000 plates, each covering 10 degrees square of tho u V 'i '• r,,oy havo therefore covered the whole sky about a hundred times. They havo also, for handier reference, 22 000 plates each of 30 degrees souard showing stars down to the 11th"magnitude. Iheso plates weigh more than 40 tons. Jho use of tins continual copying of tho sky i.s that a history of tho stars • S Thus Kros was discovered in 1893, and it was then found that ho had been much nearer to us in 18P4, and would not como so near again for 35 years. Thovo was an immediate reference to tho relates for 1594 and on them Eros was found 23 times. It was almost as good as havino- consciously observed him at the time. The best determination of tho sun's distance has been arrived at by means of these Plates. Again, on March 13th, IPI2. Nova Genii iiorum No. 2 was discovered, and it was important to know tho history of tho star. Ono plate, taken on tho Ilth, showed no traco of him, but two taken on tho 12th showed urn almost at his best. This proved that the, star had risen many hundredfold in brightness within 24' hours