Hale-Bopp: On Comets and Magnitude

(This column first appeared in the March 24, 1997 Buffalo News.)

Comet Hale-Bopp is certainly getting what is commonly called "a good press." Newspapers, magazines, and television are all providing information about this phenomenon. Here I offer, in addition to sighting instructions, only a few sidelights.

Indeed this comet deserves the publicity. Its close approach and naked-eye visibility is an extraordinary event. And now you don't have to get up before dawn to see it.

Get out to observe Comet Hale-Bopp yourself, if only to be able to say years later, "Oh yes, I saw it." Even if you start with that attitude, I promise that you will be impressed. (With binoculars you'll be amazed.) The best nights for viewing begin March 26 and continue to April 9. The best time is 8:15 p.m. The comet will be a bright smear in the northwest about 20 degrees above the horizon. During that period only the "dog star" Sirius in the southwest will be brighter.

Brightness is , of course, a matter of comparison and astronomers have developed a scale for that property called magnitude. To complicate matters the magnitude scale works in reverse: the lower the magnitude, the brighter the star appears to us. Thus the first magnitu de star, Aldebaran, is brighter than the second magnitude North Star, Polaris. Over time and with improved instruments, this magnitude scale has been refined considerably and gives us a very accurate -- but more complicated -- measure. The formal basis fo r the scale equates 5 units of magnitude to 100 times in brightness. That does not, however, mean that a difference of 1 magnitude is 20 times brighter. Instead magnitude multiplies like compound interest. An object one magnitude less is about 2.512 times as bright. (You can check that those two measures are equivalent with a calculator. Multiply 2.512 by itself five times and you'll get 100.)

I have been following with great interest the increasing brightness of this comet. (If you have access t o the World Wide Web, you can find this information and much else at http://galileo.ivv.nasa.gov/comet/.) When I first saw Hale-Bopp in mid-February, its magnitude was about 1.5 -- even then brighter than the North Star. But now its magnitude is negative. It measures -0.8 and is still increasing in brightness as it approaches the sun. That difference in magnitude means that it is now over eight times as bright. And it will almost certainly brighten to a magnitude of less than -1.0. Sirius's magnitude is - 1.5 (twice as bright as the comet is now) and the only still brighter objects will be below the horizon during this window of viewing opportunity. Our moon, when full, has magnitude -11.8, making it over 20,000 times brighter than the comet, but it won'tr ise on March 26 until after 9 p.m. On April 10, however, it will again appear in the evening to light the sky and lessen the comet's effect.

Comet Hale-Bopp has another designation to astronomers. It is Comet C/1995 O1. The C means that its orbit al period is greater than 200 years -- it would be P otherwise. The O tells when in 1995 it was discovered. Each letter represents a half-month, A the first half of January, B the second half, etc., with I omitted. Thus O represents the last half of July. Finally, the 1 means that it was the first comet found during that time period.

On the next available cloudless night, do get out to see Comet Hale-Bopp a.k.a. Comet C/1995 O1.


The image above is, of course, Comet Hale-Bopp. It was taken in early March by Denis DiCicco for Sky & Telescope Magazine. It, many other images, and much more information about this visitor from outer space may be found at the Jet Propulsion Laboratory's Comet Hale-Bopp Home Page.