March, April best time to see Comet Halley

Press, 30 January 1986, Page 9

March, April best time to see Comet Halley

Comet Halley reaches perihelion on February 9. It will then be at its brightest, but as it will be on the far side of the sun it will be invisible to us.

The comet will emerge from the glare of the sun in the last week of February on its outward journey to the far reaches of the solar system. Unfortunately the full moon on February 25 will make naked eye observations difficult. The comet will rise at 4.35 a.m. on February 27 at a point slightly south of east. This is Summer Time and the time stated will differ by a minute or two according to the position of the viewer.

The comet will be only 10 degrees above the horizon as dawn ends on February 27. Ten degrees is twice the distance between the pointers to the Southern Cross. Even to pick up the comet with binoculars or a telescope at this time a very clear, low eastern horizon is essential, as well as freedom from any city lights.

The best estimate of the comet’s brightness is that it will be slightly brighter than fourth magnitude. That is about the limit of visibility with the unaided eye from the normal suburban back yard on a clear dark night. The tail of the comet will be inclined slightly to the east. It will probably be several degrees long and will rise slightly ahead of the head.

The best views of Comet Halley will come in March and April. Throughout March the comet will rise earlier each morning, attaining a much higher altitude before dawn. It will be closest to the earth on April 11 at a distance of about 63 million kilometres. It will then be visible all night, especially from sites away from city lights.

This time it does not come anything like as close to the earth as it did in 1910. Many -readers have very vivid impressions of its marvellous appearance in 1910 and may be disappointed this time. It will, however, still be a sight worth seeing.

The comet was first detected on its inward

journey on October 16, 1982,' and has been followed by astronomers since then. This has enabled its positions from night to night to be very accurately determined. It became visible to the unaided eye in December and many people in New Zealand have reported observations of the comet as a faint, fuzzy patch of light. ~ Considerable activity was seen in the tail in early January before it passed behind the sun. A major outburst was seen on January 6, when two short, sunward-pointing jets were observed with a third short jet in an antisolar position. This activity was reported as occurring in a period of just over two days and it is surmised that it is connected with the period of rotation of the head. Similar activity is shown on photos of the comet in 1910. A photograph I have received shows the head of the comet as an ovalshaped patch of light without any indication of a star-like point of light of the nucleus. A short tail is visible, clearly divided into three divisions with the central one somewhat longer than the other two. There appears to be a widespread belief that our wet summer is caused by the comet. I have received many letters asking if this is true, as well as many telephone inquiries, some from as far away as Australia. The answer is, of course, that there is no connection, as the comet has nothing to do with the prevailing weather.

It was predicted last September that this summer would be much wetter than the previous two seasons. This prediction was based entirely on known meteorological factors.

A number of spacecraft are en route to fly by Comet Halley. The first that will reach it will be the two Russian Craft, Vega 1 and Vega 2. Their cameras will be used to obtain photos which will assist the following European and Japanese craft to adjust their trajectories if necessary. The European Space

Agency’s Giotto is probably the most sophisticated of all the craft launched. It carries a very advanced camera and 10 experimental packages. It is aimed to pass very close to the nucleus of the comet at a distance of 1000 km or possibly less. This may cause problems since if a ’ large amount of dust particles are ejected from the head the spacecraft could easily be struck by one of these and damaged. That problem will be answered only when the craft gets there. The Japanese have launched two craft, Sagikake and Planet-A. Their objective is to investigate the outer envelope of the comet and, for that reason, they will fly by at a considerable distance from the. head. Additional observations are planned from space by a probe to be launched from the orbiting Shuttle and also from balloon flights into the earth’s upper atmosphere. The results from space will most certainly provide much better information on comets in general and on Comet Halley in particular. The results from the intensive worldwide Halley Watch network will also result in so much data that it will take years to analyse it. Mercury reaches superioi conjunction on February 1 and then passes into the evening sky, reaching its greatest elongation east of the sun on February 28. This will not be a favourable one from our latitudes and this planet will be setting shortly after the sun. Venus also will be only briefly visible in the bright twilit evening sky as it too sets very shortly after the sun. Jupiter also becomes lost in evening twilight as it reaches conjunction on February 18. This leaves only two naked eye planets for casual viewers to follow. Mars, at first magnitude, passes Antares, the bright red star in Scorpius on February 17 as the planet passes from Libra, through Scorpius and into Ophiuchus this month. Saturn passes by Antares on February 10 and the rapid

movement of Mars enables it to catch up on Saturn so that on February 18 the two planets will be close together.

Mars and Saturn can only be seen in the morning sky as they do not rise till after midnight throughout the month. Mars will be worth watching over the next few months as it steadily brightens to become a prominent object as it reaches opposition on July 10. Its rapid motion against the starry background can be noticed within a few nights of watching.