FIRST the good news: Halley's comet is back from the dark, distant recesses of the solar system. Over the next few months, it will be visible in our skies as it pays its ``once in a lifetime'' visit to the vicinity of the sun. But don't expect too much of this rare celestial visitor. The bad news is that on this appearance the comet will be far from spectacular. In fact, it will appear fainter than on any of its previous recorded visits, stretching back over 2,000 years. As a result, astronomers are playing down their expectations this time around.
Halley's comet will, however, be bright enough to be visible to the unaided eye -- away from bright city lights. As seen from the deserts of Australia, it will be a memorable sight, if not an awesome one.
Despite its name, the comet wasn't actually discovered by Edmond Halley. He did, however, see its appearance in 1682. At that time most people still regarded comets as portents of doom. As Shakespeare had written earlier in the century: ``When beggars die, there are no comets seen; the heavens themselves blaze forth the death of princes.''
Astronomers knew that comets were celestial bodies moving between the planets, but they had no idea of the shape of comets' paths, and thought that each comet appeared only once.
Halley was a brilliant scientist, making original discoveries in physics, meteorology, and the earth's magnetic field. His expertise also went well outside the realms of pure science. He drew up the first tables of life expectancy, so providing the mathematical basis for life assurance; he built a practicable diving bell; and he undertook diplomatic work, including a little spying, for the British government.
But Halley's first love was astronomy, and he was eventually made Astronomer Royal -- at the grand age of 65.
Before this, he had discussed the problem of comets with his friend, the great physicist Isaac Newton. Newton had just formulated his law of gravitation, and showed that comets must go around the sun in long paths, either ellipses or open-ended parabolas. Halley calculated the paths of 24 comets, and he found that three of them had very similar orbits.
The comet that he had seen in 1682 seemed to be the same as one seen in 1607, which was possibly the reappearance of a comet observed in 1531. Halley predicted that it would reappear in another 76 years' time. On Christmas night 1758 -- well after Halley's death -- an astronomer in Germany did indeed pick up the comet as it returned to the sun -- and Halley's immortality was assured.
Halley's comet was first recorded by the Chinese, in 240 B.C., and we have records of every appearance since then. In A.D. 837, the comet was an awe-inspiring sight as it swept past the earth in the closest approach in history. Its most famous appearance, however, was in the early summer of 1066, just before King Harold of England was defeated by the Normans at the Battle of Hastings. In 1301, the Italian painter Giotto de Bondone saw Halley's comet, and portrayed it in a fresco as the Star of Bethlehem .
The comet's last appearance was in 1910. It was a brilliant sight, with a tail stretching a quarter of the way across the sky. The earth almost passed through the comet's tail, in fact, and doomsters predicted the end of the world -- although one entrepreneur benefited immensely by selling ``comet pills'' that would supposedly ward off any evil effects.
The comet is now heading in toward the sun, for its closest approach on Feb. 9, 1986. It is now visible from countries in the Northern Hemisphere. The best views from higher northern latitudes will be in December and January. In December, the comet appears high in the southern part of the sky, under the four stars that make up the great Square of Pegasus. The comet looks like a faint patch of light, visible to the naked eye (away from street lights) and easily seen through binoculars.
Later this month, the moon will be near the comet in the sky, and its brilliance will drown out the comet's feeble light.
By the time that the moon moves away again, at the end of the year, the comet will have moved across to the southwestern sky. It will be starting to grow a short, stubby tail, which will point upwards in the sky as the comet hangs in the glow of twilight after sunset.
This is the best view that people in Europe or Canada will get; after the comet passes the sun, it won't rise above the horizon again until late in April when it will be fading rapidly from sight.
People in mid-northern latitudes, in the southern United States or Japan, will be more fortunate. They will see the comet again in March, suspended with its elongated tail low above the southwestern horizon before dawn.
But the best views are reserved for inhabitants of the Southern Hemisphere. They will see little of the comet before it passes the sun in February, but the comet will rise high in the sky in March and April. It will now have a prominent tail. It will move away from the morning twilight at the beginning of March, to appear higher in the sky as it approaches the earth.
On April 11, Halley's comet will make its closest pass to the earth on this visit -- 63 million kilometersaway -- and it will be at its most spectacular.
As the comet passes our planet, its great tail will appear to swing round anticlockwise in the sky. Then the comet will gradually fade as it moves away.
But there is one final, unforgettable sight in store for people observing from the South Pacific. On the night of April 24, the comet will at first be difficult to see, in the glare of a full moon. But the moon then moves into eclipse; as the moon's light is blotted out by the earth's shadow, the sky will go dark, and the comet will reappear in its full glory, next to the dim red disc of the eclipsed moon.
Many astronomers from the Northern Hemisphere, determined to get a good view of Halley, are making plans to travel south to the equator in April. There are over 100 organized expeditions.
South America will receive an influx of astronomers from the United States and Canada; South Africa expects visitors from Europe. But the most popular destination is Australia: literally thousands of astronomers from North America, Europe, and especially Japan are converging on the desert towns of the Australian outback where skies are dark and crystal clear.
The expeditions are not just land-based. Dozens of cruises will ply the Indian and South Pacific Oceans -- a romantic ideal for naked-eye viewing, but the ship's rolling won't help those trying to keep binoculars or a telescope steady.
The best views -- although fleeting -- may come from above the earth's surface. Aircraft operators are planning a variety of flights to see Halley's comet. For people in the worst-possible places for comet-observing, such as Europe, aircraft companies will provide short trips before Christmas that will at least give a chance to see the comet -- and from above the clouds.
At the other end of the scale is a 19-hour flight from London to New Zealand in the supersonic Concorde, flying up in the stratosphere at a height of 20,000 meters.
Even farther off the earth, a bevy of unmanned spacecraft will provide the best views of all. A fleet of five spacecraft from Japan, the SOviet Union, and the European Space Agency are carrying cameras to take closeup pictures. The European craft, Giotto, will plunge right into the fuzzy ball of gas that makes up a comet's head, to try to photograph what's at the center.
Scientists generally believe that all of a comet's glory springs from a small ``dirty snowball,'' only a few kilometers across, whose ices evaporate in the sun's heat to make up the large head and tremendous tail.
As it plunges into Halley's comet on March 13, 1986, Giotto should reveal whether this theory is true -- before it is destroyed by high-speed impact with dust grains in the comet's head.
Whatever we learn this time, astronomers will undoubtedly have more questions to answer when Halley returns in 2061.
And the view from earth -- at least from the Northern Hemisphere -- will be no better than in 1985-1986.
But by then the ultimate in comet trips probably won't be a supersonic flight around the earth, but a ride in Giotto's successor -- to see the comet firsthand.