Rare series of lunar eclipses in 1982

Press, 7 January 1982, Page 15

Rare series of lunar eclipses in 1982

Three total eclipses of the Moon rarely happen in a year: the last occasions were in 1898 and 1917. Three lunar eclipses will occur in 1982 but only two of them will be visible from New Zealand.

The maximum possible number of eclipses in any year is seven, consisting of four or five solar eclipses and three or two lunar eclipses respectively. In 1982 there will be four solar eclipses, all partial. These will be of little interest to us because only a small part of one, in January, will be able to be seen from our latitudes. However, all the lunar eclipses will be total and those on July 6 and December 30 will provide us with interesting views. The eclipse on January 10 will not be visible from New Zealand.

A total eclipse of the Moon occurs when the Moon passes completely through Earth’s shadow. The axis, or central line, of Earth’s shadow is always directed to a point directly opposite the Sun. If the Moon passes through only part of the shadow cone then the eclipse is partial. Naturally the Moon can be eclipsed only when it is at the full-moon phase as it is then opposite the Sun and on the shadow side of Earth. Readers have often asked why the Moon is not eclipsed at every full moon. For such an event to occur the Moon must be near one of the nodes of its orbit. Usually this does not happen so there is no eclipse. The Moon moves in a great circle that is inclined to the ecliptic. At the same time Earth moves around the Sun in an almost circular orbit. The path of this orbit is the ecliptic, which, therefore, is the path travelled by the Sun against the stellar background.

The Moon crosses the ecliptic at two points, called the nodes. The point at which it crosses the ecliptic from south to north is termed the ascending node and the point at which it crosses in the opposite direction is called the descending node. These terms originated with ancient astronomers, who all lived in the Northern Hemisphere. For a lunar eclipse to occur the Moon must not only be at the full phase but it must also be close to one

of the nodes. Usually at the times of full moon if is north or south of a node, so no eclipse results. In some years there can be no lunar eclipse at all. It is unusual for there to be more than two lunar eclipses in any year. The Moon has an eclipse year of just over 346 days long. During that interval the Moon crosses the ecliptic twice, so in the lunar-eclipse year there can be no more than two lunar eclipses. However, in the longer calendar year it is possible for the Moon to be at the right place three times, provided that the first occurs in early January. This will happen in 1982 and this makes it possible for the Moon to pass the same node again in December. The other node is passed in late June or early July. This separation of the nodes shows that lunar eclipses must occur at different seasons.

A total lunar eclipse can have a duration of about Ihr 40min, although the partial phase may last about 2hr longer. Lunar eclipses are visible over a much larger area than solar eclipses. This accounts for the belief that the former are more common. Few people have not seen a total eclipse of the Moon but a total eclipse of the Sun, even when its path sweeps across a thickly populated country, has not been seen by most people.

Another fallacy concerning total lunar eclipses is that the Moon sometimes becomes invisible during the total phase. It is quite clear that reports of the Moon’s being invisible during an eclipse have been due to atmospheric effects: mist and fog have obscured the eclipsed Moon, especially when the eclipse took place with the Moon at a low altitude in the sky. One effect that has been noticed from early times when an eclipse occurs when the Moon is near the horizon is that it is possible to see both the Sun and the eclipsed Moon at the same time. This effect can last for only a few minutes.

The Sun, Earth, and the Moon are approximately in a straight line at the time of an eclipse. Refraction can raise the Sun and Moon each

by o.sdeg. when they are close to the horizon. At the same time, if the Moon is situated above the centre of Earth’s shadow, then it is possible to see the setting Sun and the rising eclipsd Moon (or vice versa) at the same fleeting moment. The reason why the Moon is never invisible when it is eclipsed is due to the refraction of sunlight reflected from our atmosphere on to the Moon. Should the atmosphere be particularly dusky or cloudy at the time then these reflected rays are much weaker than usual. That results in making the eclipsed Moon appear darker than usual during an eclipse. On the other hand, should the atmosphere be especially clear the eclipse is a bright one.

For several months Venus has been a brilliant object in the evening sky, attracting much interest. In January it will dip down sharply towards the Sun to reach inferior conjunction on January 22. It will then move very rapidly into the morning sky, where it will be glimpsed at sunrise by the end of the month. In a telescope, Venus shrinks from a hairline crescent at inferior conjunction to a much smaller three-quarters phase in June. It will remain visible in the morning sky until September. Mercury will make a fleeting appearance in the evening sky in January, when it will reach its greatest elongation east of the Sun on January 17. This elongation will not be a favourable one for us.

Mars in the constellation Virgo, Jupiter in Libra, and Saturn in Virgo are all now coming in to better viewing positions. However, they are all best seen well after midnight. Mars will come in to opposition at the end of March and is now brightening rapidly. It will have a magnitude of 0.6 and is now slightly brighter than Saturn but two magnituudes fainter than Jupiter. The distant planet, Uranus, will be well placed for observation with telescopes during the first half of the year. It can be located in Scorpius and, at the beginning of January, it will be just south of the bright star, Beta Scorpii.