HALLEY'S comet has become a feature of Earth's nighttime sky in the Northern Hemisphere, and you don't need a fancy telescope to see it. It's dimly visible through binoculars and getting brighter every night. By mid-December many people should be able to see it with the naked eye. In fact, says British amateur astronomer Patrick Moore, that's one of the best ways to admire the comet. He explains that, unless you are a professional astronomer or a dedicated amateur, ``you're going to see Halley's comet best if you're looking at it just casually and without doing any serious scientific observation. You're going to see it best either with the naked eye or with binoculars.''
Moore -- who says he has been studying the sky since he was six years old, some 51/2 decades ago -- believes you can see a lot in the sky with the unaided eye, plus the occasional help of binoculars, if you have them. And Halley's comet is a good object with which to try your skill.
As seen from Earth, stars appear to group themselves into distinctive patterns called constellations. These provide guideposts for finding the comet as it moves from one star group or constellation to another. This month, for example, it is tracking westward across the constellations Taurus (the Bull), Aries (the Ram), and Pisces (the Fish).
Many newspapers publish constellation maps, which help you find your way about the sky (The Christian Science Monitor's monthly Sky Chart appears on the fourth Wednesday of each month). Shortly, these maps should begin to indicate the comet's position. Also, Patrick Moore has just published a very helpful guide entitled ``Astronomy: Stargazing Without a Telescope'' (Woodbury, N.Y.: Barron's Educational Series Inc., $19.95). Working with this, you can quickly become familiar with the night sky.
The comet is not a spectacular sight. Those who located it last week near the Pleiades -- a small star group in Taurus -- saw a hazy blob with little hint of a comet tail.
Halley is heading more or less straight toward us right now. This makes it hard to see any incipient tail. However, by using the 60-inch telescope at Palomar Observatory, James Gibson of the NASA Jet Propulsion Laboratory in Pasadena, Calif., has detected what may be the first sign of a tail.
Halley has become visible for casual observers just as it is transforming itself into what most people think a comet should be -- a glowing object with a long, sweeping tail. John C. Brandt, chief of the Laboratory for Astronomy and Solar Physics at the NASA Goddard Space Flight Center in Greenbelt, Md., explains the process vividly:
To make a comet, he says, roll some water, snow, and ices of other materials into a ball a mile or so across. Add a bit of dirt and a few rocks. Pack lightly and you have a comet. This is what Halley has looked like during most of its journey through the solar system after its last swing by the sun in 1910. Now, expose that dirty snowball -- the comet nucleus -- to increasingly strong solar radiation and streams of solar particles, and it undergoes a spectacular transformation.
Brandt explains: ``When the sun heats the surface of the comet -- ices and snows -- to the temperature that is approximately the temperature of the Antarctic Plateau, in the vacuum of space you get what has been called an inverse snowstorm.'' Particles of ice and snow are blasted off the comet surface. These particles interact with the solar radiation and particle streams to form a vast cometary atmosphere, called a coma, which is blown off to form the tail.
This is what now is happening to Halley's comet. Susan Wyckoff and her co-workers at Arizona State University, together with Michel C. Festou of the Institute for Astrophysics in Paris, were the first to see Halley turn on earlier this year in observations made with the Multiple Mirror Telescope at the Fred L. Whipple Observatory on Mt. Hopkins in Arizona. The astronomers said that, to their knowledge, this was ``the first observation of the onset of sublimination in any comet'' ever made.
By now, the gas-and-dust coma around Halley's nucleus is about 300,000 miles across. Seen through binoculars, this should be a grayish ``cottonball'' about half the apparent size of the full moon.
For those who don't already have binoculars and are thinking of buying a pair, Moore has the following advice. First, note that binoculars are classed by a pair of numbers -- 7 x 50, for example. That means a magnification of 7 and a 50-millimeter diameter for each object lens. Then, he says, go for ``magnification between 7 and 12, aperture between 30 and 70, and you'll get your best view.'' Anything smaller is too wimpy to show the comet well. Anything larger will be unwieldy for hand-held viewing. A lso, Moore adds, forget about buying a telescope, unless you mean to become an amateur astronomer. Telescopes have poor light-gathering power, a small field of view, and are clumsy to handle. Binoculars, on the other hand, have a wider field, are handier to use, and are generally more useful.
Right now, Halley is well up in the evening sky as one looks toward the east. If you can get away from city-light pollution, you should be able to find the comet. However, the moon is waxing and will be full on Nov. 27. Its light will make seeing the comet increasingly difficult. Next month, though, as the moon wanes, Halley should be visible again. By mid-December, it will be moving into the Southern Hemisphere, then shifting toward the west. It will be low down in the west late in December and into th e new year. By late January, Halley will disappear in the sun's glare until it reappears in the pre-dawn southwestern sky in late February.
Don't expect a spectacular show. But if you would like a glimpse of the famous comet, now is the time to start looking for it.
Best conditions for viewing the comet: on a clear night with no full moon, at a distance from city lights (out in the country is best). The first two weeks of January should give Northern Hemisphere viewers the best look; that's when the comet will be the brightest. The comet's closest brush with Earth will be on April 11. At that time, however, it will be visible only in the Southern Hemisphere.
November: The comet will make its second-closest pass of Earth this year on the 27th.
December: Early in the month is best; as the moon waxes, the comet will be harder to see.
January: It should be bright enough to see with unaided eye under ideal conditions at the be- ginning of the month; by mid-month it will be too low at sunset and will set before dark.
February: Halley disappears from view as it rounds the sun.
March: It's good viewing, but only from very southerly United States and farther south. By now the comet will have developed a more distinctive tail.
April: There's good viewing from the 1st to the 15th, in Southern Hemisphere; later that month it will reappear to northern viewers, though it won't be as bright.
May: You may still get a chance to view the comet as it recedes.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.