Here's your ticket to see Halley's. The show is free; all you need is a clear night and a map
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.''Skip to next paragraph
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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.