Comet Halley unlikely to be spectacular

Press, 16 November 1981, Page 18

Comet Halley unlikely to be spectacular

F. M. BATESON

BY

Astronomers have been searching for Comet Halley since November 13, 1977. Their failure to find it is understandable because the comet is still very far away and its estimated magnitude is 26.

By the end of this year the comet’s magnitude will be 24, placing it at the photographic limit of the world’s largest instruments. But it will be hard to photograph because it is situated against the rich stellar background of the Milky Way. It will not be clear of this region until early 1984. Astronomers will persist in their search because they wish to follow the comet from as far away as possible, although it is not due to reach perigelion (closest point to Earth) until February 9, 1986. Comet Halley is probably the best known of all comets. Many people can still retain vivid memories of its spectacular appearance in 1910. At that apparition it was first detected as a very small fuzzy object in August and September 1909. After reaching perihelion on April 2, 1910, it became a marvellous sight, especially to observers in the Southern Hemisphere. At its greatest brilliancy its head was of second magnitude, while its tail reached a maximum length of 140 deg, stretching right across the sky. The comet was very active and many changes were seen in its head. The Earth actually passed through part of the curved tail without ill effects. The Chinese made useful observations of Comet Halley

from 240 B.C. on with the exception of the return in 164 8.C., of which they left no records. Some authorities assert that the earliest record of a comet that can be assigned as a return of Comet Halley was made in 467 8.C., although the next two returns were not recorded. If this is accepted as the earliest record then the comet has made at least 32 perihelion passages. Until the true nature of comets was understood, their appearances were being taken as foretelling the death of kings or of war, famine and pestilence. Comet Halley was no exception and many of its early apparitions were linked with some disaster, the best known being its appearance in 1066 when it was taken as a bad omen for King Harold. The comet appears on the Bayeux Tapestry that commemorates the Battle of Hastings at which Harold was slain. The famous Danish astronomer, Tycho Brahe, proved that the bright comet of 1577 was more distant than the Moon. This removed comets from the realm of atmospheric phenomena where they had been considered to belong. In 1687 Isaac Newton published his “Principia,” in which he provided the theory that accounted for cometary motions under his law of gravitation. Edmund Halley, an English astronomer, applied Newton’s theory to determine the orbits of 24 well observed comets. He noticed that the orbital elements of the comets of 1531, 1607, and 1682 were similar. He concluded correctly that they were different apparitions of

the same comet. He then predicted that this comet would return in 1758. That prediction was greeted with much amusement at the time since it was obvious that Halley could not live to see how wrong he was. But Halley was correct and this was the first accurate prediction of a comet’s return. Normally a comet is named after its discoverer, but Comet Halley is so named in recognition of the work of Edmund Halley. Comet Halley is due at perihelion again on February 9, 1986. Before perihelion it will be mainly a northern object, but southern observers will see it best after perihelion. The comet unfortunately is unlikely to be as spectacular as that of 1910 because it will not be so well placed. It is not expected to be brighter than twenty-first magnitude until early 1984. A year later estimates suggest that it will still be a faint object of eighteenth magnitude. It should then increasae rapidly in brightness to be just below naked eye visibility in late November, 1985. Then it will make a close approach to the Earth at a distance of 0.62 astronomical units. One astronomical unit is the average distance between the Sun and Earth, which is about 150,000,000 kilometres. ■

At perihelion on February 9, 1986, Comet Halley will not be seen visually against the brightness of the Sun. When it passes from conjunction with the Sun late that month, or in • early March, estimates suggest that it will be only, fourth magnitude. On April 20, 1986, as the comet recedes from the Sun it will

again cross the Earth’s orbit. At that time the minimum distance from the Earth will be 0.42 astronomical units. It is expected that then it will be no brighter than fifth magnitude, and thus just visible to the naked eye. Unfortunately, if these calculations prove correct Comet Halley will be a disappointing object in 1986. There will not be the spectacular display of 1910 mainly because the comet will not be so well placed. In spite of this, several nations plan to fly space missions to Comet Halley in 1985-86. The objectives of these missions will be discussed in, next month’s article.

Watchers of the bright planets will focus their attention in November on Venus, high in the western evening sky after sunset. Daylight saving time will mean that Venus will set shortly after midnight throughout the month. This contradicts the often repeated statement that Venus can never be seen at midnight. Venus reaches its greatest elongation east of the Sun on November 11 when it is 47 degrees from the Sun. Its brightness continues to increase so that its apparent magnitude is minus 4.1 in mid-month, making it g a very brilliant object. In the morning sky Mercury, Jupiter and Saturn will all be too close to the Sun to be clear of the dawn sky. Mars, alone, will be seen in the morning sky rising about 3.45 a.m. on November 1 and one hour earlier at the end of November. It can be found moving rapidly through the constellation Leo to reach Virgo by the end of the month. Its magnitude is 1.5.