ASTRONOMICAL NOTES FOR OCTOBER.

Press, Volume LXIV, Issue 13234, 30 September 1908, Page 6

ASTRONOMICAL NOTES FOR OCTOBER.

(By toe Rev. P. W. Fairclough, F.R.A.S.)

Mercury is an evening star in tho west, and Saturn in tho east. Mercury rises before sunset, and is due north before midnight. Ho is between Pisces and Cetus. Jupiter, Venus and Mars aro morning stars. Tho two first-named ■will bo close together at the middle of the month. Mira is now visible, but she rises late.

' During tho past month tho cable informed us of the death of Lord Rosso, son of the builder of the famous telescope, at Parsontown, in Ireland. This instrument was the Great Eastern of telescopes. It was built before its time, and was a comparative failure. It was a reflector, not a refractor. The reflector substitutes a concave mirror for lenses, and the observer has his back to' the objecfc observed, tho light, collected and focussed by the mirror, being brought to an eyepiece at the side of the tube. Lord Rosse's mirror is six feet in diameter, and is of polished metalr—copper and tin. It weighs 3i to 4 tons, and its weight deflects or buckles tho reflecting surface—very slightly, of course, but enough to blur tho definition of tho object. The telescope, however, did useful work in observing nobulse, and later, when the spectroscopo was applied to stars, it was of great service, for the spectroscopo required not definition, but a great light gatherer. Sir W. Herschel mado a reflector 4ft in diameter, and polished the mirror by hand with infinite pains. With it he discovered several moons of Saturn, xuirrors first adopted because no method was known of making lenses "achromatic," that is, of obviating rainbow tints about tho object. That difficulty has been got- over by a combination cf convex and concave lenses, and the refractor now easily holds the prido of place. Mirrors are still made, however, but they are of glass, first ground to the proper curve and then silvered. This obviates the weight and the deflections, and ako thp need of frequent and expensive re-polishing. There are two sft glass mirrors in existence.. (One of them is at the Yerkes Observatory. They are superior to Lord Rosse's. Mirrors of Bft and 10ft are being discussed.

When the lens was saved from its fringe of colour refractors grew very slowly. In 1834 the largest in England was only Hi in. Even now there are only about two dozen over 20in. Mexico, tho Cape, and Harvard have each a 2iin; Cambridge has a 25in, and Virginia. Washington and Greenwich have each a 26in: Vienna has a 27in, Greenwich a 23in, Paris a 29in, Russia and Italy ' each a 3Gin, and Germany a 31 Jin. After these come tho Lick, 36in, and the Yerkes, 40in. Mr Lick made a fortune out of organs and pianos, and then turned to the music of the spheres. Tho tube of his glass is 58ft long. The revolving dome weighs 89 toe«. Mr Lick's coffin was built into the pier that carries his splendid instrument. Mr Yerkes, another wealthy American, is said to have given ALasca Ckirke, the self-taught maker of great telescopes, a commission to "lick the Lick." The tube is C2ft long. It is of steel, and weighs six tons. The' convex lense is three inches thick at the centre and one inch at the edge. The concave is ljin at the centre and 2Jin at the edge. They weigh-2001b ar.d 3001b respectively. Tire total weight of the instrument and its mountings is 75 tons, and the whole outfit follows the heavens by clockwork.

As the diameter of the lense increases, so docs the thickness, and this involves an absorption of light. So far, however, the increased light-gathering power hosi moro than compensated for the.extra: absorption. But tho limit is

New Zealandore learned with regret that- the Carnegie Icfititutc had preferred South Amerioa to tho Dominion for tho site for a new observatory. The atmosphere of Central Otago is, no doubt, very fine. Tho writer remembers getting out of bed and going out in tho small hours to bask in the extraordinary moonlight at Alexandra. Ho had never seen anything to opproach it. Continental climates are, however, more stable than insula/ ones. Besides, in Peru, an elevation far above the Otago snowlino con be occupied. Hence, in the interests of science, the Institute has, perhaps, chosen w_elv.

Last month we speculated on tho existence of a vast multitude of dead and dark suns. There are about one hundred millions of stars, or suns, hot and luminous, in our system. If there are a thousand times,., ten thousand times, or a hundred thousand times as many spent suns, crusted, cold and dark, may.there not be, in such a raultitaide, an occasional collision? No ono can say that such an event is impossible. All must admit,, however, that the conditions under which collision is ivossibie are so narrow as to render it extremely improbable in any ono case, and very rare even among ten million millions of bodies. Collision has been seriously discussed by many distinguished scientists. Dr. Croll, in his "Stellar Evolution* (1839). adopts what ho calls tho "Impact Theory" of two bodies moving directly towards each other at high inherent velocity. Professor Kirkwood, in tho ■'Observatory" for April 1879, discusser r, nebula produced by the direct collision of two bodies. Lord Kelvin, in "Good Words," April, 1887, describes tho formation of a nebula, such as would produce tho Solar System, by iho collision of two bodies. These examples aro enough to show that collision is not outside tho realm of high scientific thought. In spito of theso and other great names, however, the impact theory has not taken deep root in theoretical astronomy—a fact which suggests that there are difficulties. Wo shall look for them as we continue these notes from time to time.

Meanwhile, it will be an interesting study in the laws of motion and the action of gravity in free space to consider some of the conditions that govern stellar collision.

Suppose two bodies, each equal to tho sun in mass, to bo stationary at tho distance of the nearest star, say, 20 million millions of miles. No other body interferes, and they are to fall together by mutual attraction alone. They will meet in about five million years. For the greater part of this time their motion will be extremely slow. When there are only about fifteen years to gq they will be as far apart as Neptune is from the sun, and each will have a velocity of about five miles per second. At tho earth's distance the velocity of each will bo 26 miles, but tho crash at meeting will be at the rate of 379 miles per second, each. The energy of the impact increases with the square of the velocity, and every atom of these bodies would have tho energy of a cannon ball: But, of course, there are no stellar bodies standing still,' and no bodies that are not perturbed and drawn aside:by tho pull of numerous stars. No such colli-, sion ao tho above is, therefore, possible, or expected by any theory. If ono of the bodies supposed had an inherent motion of one mile a year, at right angles to tho line of approach, the pull of gravity would not destroy it. That body would gain its miloof sea room every year throughout the five millions, and at their nearest approach the centres of the two bodies would bo 5,000,000 miles apart. Again, suppose two bodies, each a million miles in diameter, to be in such proximity that their inherent motion would bring them together in fifty but the lines of this native motion carry them 5,000,000 miles wide of each other. That is, they gain sea room at the rate of 100,000 miles a year.> Now, if gravity could reduce the fifty years of approach to ton years, tho bodies would just graze. This principle is very important. Gravity does not destroy th© weather gauge of approaching bodies. It only shortens tht> time in which it operates. Hence extreme rarity of impact. Gravitation helps but little. The inherent motion must be very accurately aimed, and this makes such impact as Croll supposed—a direct approach on parallel lines —very difficult to conceive.

Once more; two bodies may bo pursuing intersecting paths, at right angles to each other, with a motion such as to bring them to the point of intersection at the same time. Of course neither of them will reach that point. They will be mutually deflected towards each other. If their motions and masses aro equal they may collide, but if the masses are unequal the lesser will be deflected and expedited to a greater extent than the other, and the catastrophe may be avoided.

Impact is limited to those cases in which original motion, the pull of gravity, and the perturbations of other stars, all agree not to prevent it. In a battle two bullets may occasionally strike each other. In such a case tho impulse of tho powder, tho aim of tho rifles, the attraction of the earth, the resistance of tho atmosphere, and the drift of the wind, all co-operated to bring tho two bullets into the critical position. Any of these influences may either prevent or facilitate the collision. So it is with our theoretical, wandering, dead suns.