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Hitching a Ride on a Comet  

Mercury, November/December 1996 Table of Contents

by James C. White II, Middle Tennessee State University

(c) 1996 Astronomical Society of the Pacific

Though no longer considered to portend disasters, comets remain romantic, ethereal mysteries. Some simple observations make their appearances all the more meaningful.

This month's project: observing comets

Late in the evening of Jan. 30, 1996 in Japan's Kyushu region, Yuji Hyakutake left his home in Kagoshima and drove to an observing site far from city lights. There, in the sky above him, was a new comet soon to bear his name. Hyakutake, an amateur astronomer, was bursting onto the scene as a comet finder. In December he had discovered another, much fainter comet, and on that chilly January evening he discovered what was to become the brightest comet to visit our evening skies since comets Arend- Roland and Mrkos in 1957.

For all the joy and excitement they bring, comets are simple creatures. They have changed comparatively little since they formed, along with the rest of the solar system, about 5 billion years ago. In a seminal 1950 paper, astronomer Fred Whipple likened comets to giant, dirty snowballs. Composed of water ice, dry ice, and a mush of hydrocarbon compounds, the cometary nucleus is a low-density, porous object 5 to 10 miles in size -- like an enormous loofah sponge.

As this loofah sponge approaches the Sun from its home in the murk of the outer solar system, it gets hot; its outer frozen material evaporates. The vapor released during this process forms a large cloud around the nucleus. Called the coma, this cloud can grow to hundreds of thousands of miles in diameter. The gassy, dusty material in the coma is subject to the Sun's stiff breeze. It streams away from the nucleus, forming long, diaphanous tails millions of miles long.

Ancient skywatchers thought the wispy tails looked like hair; hence the Greek word kome, "hair." For a long time, Western culture considered comets to be harbingers of doom. English poet John Milton described Satan as a comet that "from its horrid hair/Shakes pestilence and war."

Comet Hyakutake was made spectacular by its close approach to Earth. But it may have been only the appetizer for a cometary feast next spring: Comet Hale-Bopp. I'm sure comet hunter Yuji Hyakutake will still be spending hours each evening just looking up, searching.

Observing Guidelines

This past spring I received a telephone call from an upset parent. "We bought our daughter a telescope to see the comet," she said, "and we can't see anything!" The exasperated mother said they found binocular and naked-eye views to be better than those through the telescope. "What are we doing wrong?" she asked.

Absolutely nothing! For novice telescope users, comets through the eyepiece disappear. These are pretty faint objects. They may be bright overall, but all that light is coming from a big area of space. Even out in the suburbs, the contrast between the diffuse cometary glow and the not-so-dark night sky can be lost. A telescope, by gathering light from only a small patch of the sky, often just makes it worse.

Naked-eye comet-watching is the easiest and most satisfying way to begin, particularly in a rural area. Find yourself the darkest location possible, set up the chaise lounge, break out the hot chocolate, and look up. Binoculars are a nice addition; they allow you to gather a bit more light, while still providing a wide field of view. I recommend binoculars with at least a 7-degree field of view. In suburban and urban areas, the binoculars are crucial. You need that extra light-gathering ability.

OK, you say, I found Comet Hale-Bopp this evening. What next? Comets are not big fireballs that sprint across the heavens; theirs are leisurely strolls. Because they constantly change as they ramble about our skies, comets provide us with an opportunity to observe astrophysical phenomena on short time scales. For your observations of Hale-Bopp or any other comet, consider the following:

  • Does the comet have a tail? If so, has its appearance -- color, length, width -- changed since your last observation?
  • Can you see two tails (dust and plasma)? If so, are there color or shape differences between the tails?
  • Has the shape or color of the coma changed?
  • How does the brightness of the comet compare to nearby deep-sky objects? This is a way to see whether the brightness has changed.
I recommend you keep a journal of your observations. Include sketches of the comet and written descriptions of the object and of the observing sessions in general. If you're a shutterbug, the easiest way to photograph a comet is to use fast film, an exposure time of less than half a second, and an average lens. [Editor's note: For details, see the fall 1996 Universe in the Classroom or Rick Dilsizian's article in the January 1996 Astronomy magazine.] If you have access to a telescope, try to use it as well, but keep the power low. The telescope will enable you to study subtle structural changes in the comet's tail(s) and coma, but it's not necessary.

Incorporate your observations into a document with the following information: name of the project (such as, "Observing Comet Hale-Bopp"), your name or the name of your group, details of the observing location, mailing address, telephone number, and email address, if available. We welcome reports from observers of all ages in all countries. In your report, please provide accurate time and date information and details of your observations. We want to know where you were for the observations, when you were there, what the weather and skies were like, and, of course, written commentary on your work. This helps us as we select the Guest Observer, and it helps readers to understand what you studied.

Please submit your completed report by Jan. 31, 1997 by email to or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203-3630. The selection committee will choose one report to publish in an upcoming Mercury. Send your comments and suggestions to me at or Department of Physics and Astronomy, Middle Tennessee State University, Murfreesboro, Tenn. 37132.

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. His astronomical research focuses on cataclysmic variables. White writes a monthly astronomy column carried by newspapers in Tennessee. His email address is


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