Astronomical Notes

Press, Volume CXIV, Issue 33654, 2 October 1974, Page 22

Astronomical Notes

by

F. M. BATESON

OCTOBER METEORS LEAVE TRAINS

October gives a good opportunity of seeing meteors from one of the best known annual showers.

By watching the eastern sky for half an hour, after midnight, during the period October 17 to 26, an observer will see several meteors. If, however, he observes after 2 a.m. the numbers seen will be much greater. The point from which these meteors appear to radiate is close to the wellknown red star, Betelgeuse, in Orion. Thus the shower is known as the Orionid shojwer. It is one of two annual showers that are due to Halley’s Comet. In May meteors 'radiate from the constellation Aquarius. Both the (May and October meteors (move in nearly the same orbit as this famous comet. The reason that one comet provides two annual showers is simply that we cross the! orbit of the stream twice — ■ once as the stream of met-1 eors is approaching per-1 jihelion and again after the stream has passed per- ! ihelion. This situation can! result only when the origi-j nal orbit is such that the meteors are at the Earth’s distance from the Sun each itime the stream crosses the i plane of the Earth’s orbit. In addition the particles that form the meteors are ‘widely scattered along the path of Halley’s Comet. Orionid meteors The Orionid meteors are generally whitish in colour, with long, swift paths and are often quite bright. Late on the morning of October ! 22, when the radiant has ; risen higher in the sky it is I usual to see 15 to 20 metjeors in an hour. On favourable occasions I have ob- '

served 30 in the hour before dawn. Many of these meteors i leave a phosphorescent : streak in their wake. This is j particularly so with the ■brightest ones. These trains usually last only for a second or two, except for the (very bright meteors. Then (the train may endure fori ' several minutes or longer. I Occasionally one is seen i whose train persists for half an hour. These trains expand after the meteor fades away andi ■ within a few seconds form a hollow cylinder a kilometre across. Before the days of rockets and spacecraft much was learnt about the winds of the upper atmosphere by studying meteor trains. The long enduring trains .are formed by very bright meteors moving with above average velocity. The trains tend to form at I heights about 82 kilometres, ilf the meteor itself pene--1 trades the atmosphere well , below this height, the train (forms only along the upper Ipart of its path.

I Solid particles The solid particles that give rise to meteors are very small. They enter the Earth's atmosphere at speeds of many kilometres per second. Friction with the air causes them to vaporize. The streak of light that appears to dart across the sky is the result. It is from this appearance | that meteors are popularly called “shooting” or “falling” stars, although they have nothing to do with the stars. Those readers who see some of these meteors will in effect be watching the end of parts of Halley's Comet. For that is • where these particles originated. On each return to the neigh-

e bourhood of the Sun a ' comet loses some of its matsi erial, which, as the comet t ages, can eventually form a s i swarm of meteors right 5 around the path of the s i comet. In the case of comets that e have not made many returns t to the Sun the swarm of r particles tends to be bun- ■ ched up in one part of the > orbit. Should the Earth pass f through that swarm then a brilliant meteor shower rer I suits. Halley’s Comet has long; l ; passed that stage but does 1 ; ! provide us annually with‘ (two good displays. ’ Only three of the bright! 1 planets will be visible in I ' October as Venus and Mans! , are too close to the Sun to! ; be seen. The favourable east-j : ern elongation of Mercury! ; continues for the first fort- ! ! night this month. On Octo-i ber 1 it is of magnitude 0.2; - fading to 0.9 by October 15,i ■ but still a bright object in* ’ the north western sky after! ! sunset. It seta about 8.40 p.m. oni October 1 and at 8.10 p.m.: on October 15. After that it! 'draws rapidly close to the; 'Sun to reach inferior conjunction on October 27. i J' . ' Midnight sky Jupiter, in the con-1 stellation Aquarius, domi-l nates the midnight sky. Its • magnitude of -2.3 makes it; the brightest object visible in the night sky,' except when the Moon is present. It seta around 5.0 a.m. on October 1 and at 3.0 a.m. on October 31. Any reader can make a simple naked eye observa- ! tion using Jupiter as a ; mark. Line the planet up i with a telephone pole, or , chimney, so that line of ■ sight puts Jupiter at the top I of the pole. Mark the spot ; from which you did this. I Return an hour later and you will notice that Jupiter 1 has shifted west by an r appreciable amount. ( The stars around the planet t have also moved west- < ward. This is due to the t rotation of the Earth and t your observation will show y you how much the stars, move in an hour as a result. Saturn, in Gemini, rises in 1 the north east at 2.20 a.m. c On October 1 and at 0.35 £ a.m. on October 31. Being ’ well north of the celestial 1 equator Saturn is not so favourably placed as Jupiter, t Ita magnitude of 0.3 will ( make it quite conspicuous f and it is always worth view- t ing even with a small tele-ip scope. i