Eclipse of Moon on Dec. 30-31

Press, 26 November 1982, Page 26

Eclipse of Moon on Dec. 30-31

A total eclipse of the Moon will take place on the night of December 30-31. Totality begins at 11.58 p.m. on December 30 and ends at 00.59 a.m. on December §l. Before totality the Moon enters the umbra, the central part of the Earth’s shadow, at 10.50 p.m. and leaves it at 02.07 a.m. For an hour before, and after, these times the Moon will be in the penumbra, the outer, lighter portion of the Earth’s shadow. The penumbral stage is. not of much interest since all that can be noticed is a slight dimming of the light of the Moon. The times quoted are summer (daylight) time. This year there have been seven eclipses, four of the Sun and three of the Moon. Next year there will-be only four eclipses, two each of the Sun and Moon. The two lunar eclipses in 1983 will be of little interest as one is partial and the other penumbral. However, the two solar eclipses will attract a great deal of interest, although they will not be visible from New Zealand. On June 11 there will be a total eclipse of the Sun, which begins over the Indian Ocean. The path of totality then crosses Indonesia and New Guinea before ending near the New Hebrides. A party of New Zealand astronomers will be travelling to Indonesia to view this spectacular event. Indonesia will also be the host to many professional astronomers from around the world.

The second solar eclipse in 1983 happens on December 4. This will be an annular eclipse when the Moon does not entirely cover the sun, but leaves a rim of sunlight shining around the dark body of the Moon. Its path starts in the North Atlantic Ocean and then crosses Central Africa, near the Equator. The fact that the Sun is not completely hidden by the Moon demonstrates that the Moon is,not always at the same distance from us. The lunar orbit is not an exact circle, so that its distance varies from a maximum when.it is at apogee, the furthest point of its orbit, to a minimum at perigee, when it is nearest. Interested raders can easily verify this fact for themselves. Take a coin and measure its diameter carefully in millimetres. Hold the coin so that it exactly covers the moon. Measure the distance that the coin is from your eye in millimetres, Then divide this distance by the~ diameter of

the coin which you have already measured. Multiply the result by 3476. The result, if you have made your measurements accurately, will give a very close measure of the Moon’s distance in kilometres.

If a reader makes this experiment about December 2, when the Moon is at perigee, and then repeats the measurement on December 18, when it is at apogee, the difference between the two points will show how much the Moon’s distance from us varies.

For over a year astronomers have tried to locate Comet Halley in an attempt to pick it up as far away as possible. These searches have been difficult because it was known that it would be at the limit of the world’s largest telescopes. Success came on October 16 when a team of astronomers using the 5.1 m telescope at Palomar located the distant comet. Their success was confirmed three days later by observations with the 4m telescope at the Kitt Peak National Observatory in Arizona.

The comet was, as expected, very close to its predicted position at magnitude 24, at the threshold bf these instruments. At the time of recovery the comet was 11 astronomical units from the Earth, which places it well beyond the mean distance to Saturn but closer than Uranus. It will not reach perihelion until February 9, 1986, so it will be years before it is visible in a moderate-sized telescope. A careful track will be maintained of its motion during future months so that any necessary corrections can be made to its orbit. In recent weeks two other discoveries of importance have been made, both by amateur astronomers. The first was the discovery of a nova in the constellation Sagittarius, by Minoru Honha, a Japanese amateur. This observer spends, most of his time carefully examining selected areas of the Milky Way and checking off all the known stars brighter than tenth magnitude against star maps. His aim has been to find any novae and in this he has been successful on several occasions. The nova was of ninth magnitude at discovery and was then in its early postmaximum stage. It has been well observed by New Zealand astronomers who found that it steadily declined for 10 days and then brightened slightly. Although novae are

referred to as new stars, this is really misleading since such objects are generally faint stars before their outbursts.

Usually in a matter of a day or so their brightness increases many-fold, after which they decline in days, months or even in years in some cases. Such stars eject much of their outer layers into space and after declining in brightness return to their original brightness. Several are discovered each year and only occasionally does one reach naked eye visibility. Rare examples can become one of the brightest stars in the sky for a short period. The second discovery was made on November 19 by an Australian amateur astronomer, Bob Evans. He discovered an eleventh magnitude supernova in an extrenal galaxy, known as NGC 772. The discovery was made as part of.an ongoing programme in which a number of amateur .astronomers examine external 1 galaxies for supernovae. They do this by comparing the stars within such galaxies with maps that show their normal appearance. This is the third such discovery made by Evans. A supernova results from a gigantic stellar explosion when a star literally blows apart. Most of its material is ejected into space and what remains is the core of the star in which the remaining material’ is packed into a very small diameter. Such cores become neutron stars. December sees both Mercury and Venus make their appearance in the evening sky. They will be close together, very low in the western sky and difficult to see until towards, the end of the month. Then Venus, because of its greater brilliance, can be picked out in the twilight sky. They will set about an hour after the Sun. Mars this month moves from Sagittarius into Capricornus. It sets about 11 p.m. throughout the month so will be visible in .the western evening sky after twilight ends. The planet is now rather small for good telescopic views and its magnitude is 1.3.

Jupiter moves into the morning sky but will be difficult to pick out against the glare of the dawn sky until towards the end of December. -Saturn also appears in the morning sky. Rising somewhat earlier than Jupiter it can be .seen low in the east before dawn. It is in Virgo.