ON MARCH 9, BEGINNING 41 minutes after midnight Universal Time, a few hardy souls willing to brave the Siberian winter will witness a total eclipse of the sun. As the lunar shadow rushes northward across the subzero landscape, intrepid observers will see, in addition to the usual spectacular solar corona, a streak of light painting the darkened sky. Comet Hale-Bopp (known to astronomers as C/1995 O1), predicted to be the brightest comet in more than two decades, will be just 22 days away from perihelion and only 13 days short of its closest approach to the earth. Its brilliantly illuminated tails should produce a dazzling display.

If a trek to subarctic Siberia doesn't fit your plans, don't worry. Hale-Bopp promises sensational views from anywhere on the planet. It also offers amateurs a chance to contribute to cometary research: the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., is coordinating a global net of observers, and anyone can participate.

Astronomers are agog over Hale-Bopp because its nucleus seems particularly active. A cometary nucleus is a fluffy ball of ice and rock whose surface evaporates as the wanderer nears the sun. The resulting streams of dust and gas make up the comet's tails. (The glowing dust traces a curved path; ionized gas travels in a straight line away from the sun.) Hale-Bopp's nucleus first began spurting out visible jets of debris as it passed the orbit of Jupiter, roughly seven astronomical units from the sun (1 AU is the average distance from the sun to the earth, or about 150 million kilometers). Experienced naked-eye observers have been watching Hale-Bopp since May 1996 (most comets are visible to the unaided eye only a few months before perihelion), and the rest of us should be able to see it starting this month.

A comet's tails (one dust, the other gas) reveal some of its most intimate secrets of composition and structure. They also give earthbound watchers a fine traveling laboratory to chart the solar wind. Tails are sometimes decorated with feathery features that flow outward under the solar wind's influence. Comet Kohoutek delighted astronomers in 1974 with at least two prominent examples of these skirting disturbances.

Any amateur can record these and other features of the comet's tails. First, you'll need a good star atlas that maps stars in terms of right ascension and declination. (Norton's 2000.0 Star Atlas and Reference Handbook, 18th edition, by Ian Ridpath, is probably best for this purpose.) You'll also need a drafting compass and a large bow-shaped angular scale. The bow, made from a flexible meter stick or yardstick and a long piece of scrap wood, will let you locate the tails' features to about 0.1 degree of arc.

Sketch the tails directly on the appropriate page of the star atlas (or on a good photocopy). Locate the comet's head by measuring the angular separation between the head and the three nearest stars in the atlas. Celestial maps mark the positions of stars in terms of declination and right ascension; to convert from angles to distance on the page, note that one hour of right ascension equals 15 degrees at the celestial equator. Elsewhere in the sky, divide the distance at the equator by the cosine of the declination.

For each measurement, set the compass to the appropriate opening and scribe a small arc through where you expect the head to be. The precise location of the head is where the arcs intersect. Follow the same procedure to mark all the other major features in the tails and then fill in the finer details. Use a telescope or binoculars for this part. By carrying out this procedure every clear night, you can document the evolution of the tails.

The comet's fuzzy head, or coma, also changes over time in size, brightness and degree of condensation. The best way to measure its size is to use an eyepiece with a calibrated scale etched into the lens. They're a bit pricey, but for accuracy they can't be beat. Check out the micro guide eyepiece from Celestron in Torrance, Calif.; call (800) 237-0600 or (310) 328-9560 to find a local dealer-mine sells the eyepiece for $189.

Those with more limited resources can use a less direct method to measure angular size. Center a telescopic sight on the coma and let the earth's rotation carry the comet across the field of view. Rotate the sight's crosshairs so that the comet drifts straight along the horizontal line, then count how many seconds it takes the coma to pass completely across the vertical crosshair. If you know the comet's declination (from position measurements with the star atlas), the width of the coma in minutes of arc is simply one quarter the cosine of the declination times the number of seconds. Repeat the measurement at least three times and average the results.

With a small telescope and a little practice, you can also estimate the comet's brightness, or visual magnitude, by comparing it with stars of known magnitude. Put the comet in sharp focus using a low-magnification eyepiece (no more than 2£ magnification per centimeter of telescope aperture) and commit its image to memory. Point your telescope to a nearby star of known magnitude and defocus the image until the star appears the same size as did the comet. Then mentally compare the brightness of the defocused star and the comet. Find one star just slightly dimmer than the comet and another just slightly brighter-recalling that smaller magnitudes mean brighter stars-and you should be able to estimate where the comet's brightness falls in the interval between them. For more information about the magnitude scale, consult any basic astronomy text.

There are a few cautions to observe when estimating magnitudes. The atmosphere absorbs much more light when a star--or comet--is close to the horizon, so if the comet is at an elevation of less than 30 degrees, compare it only with stars that are at about the same elevation. Don't use red stars for comparison, because your eyes aren't very sensitive to red. If your catalogue lists a star as type K, M, R or N, or if the listing for V-B (visible-minus-blue) magnitude exceeds 1.0, find a bluer star. You will probably find it useful to practice this technique by estimating the brightness of stars of known magnitude. Experienced observers can achieve a precision of 0.1 or 0.2 magnitude.

To find out more about observing comets or to learn how to contribute your observations, contact the Harvard-Smithsonian Center for Astrophysics at icq@cfa.harvard.edu, or visit their World Wide Web site or write to Daniel W. E. Green, Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138. I gratefully acknowledge informative conversations with Dan Green. You can purchase the center's newly published Guide to Observing Comets, the definitive resource on the subject, by sending $15, payable to International Comet Quarterly, to the same address. And do contribute your observations. Information is useless if it is not shared.

For information about other amateur scientist projects, visit the Society for Amateur Scientists's World Wide Web site at http://www.sas.org/ or call (800) 973-8767 or 1-401-823-7800.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.