SKYWATCH

April 1997

by Steve Stefanik

Comet Hale-Bopp reached perihelion (its closest point to the sun) some 137 million kilometers away on April 1. It is presently in the constellation Andromeda about 20 degrees above the northwestern horizon at dusk. As predicted, it has reached a naked-eye brilliance of -0.5 magnitude, rivaling Sirius as the brightest star in the sky. It has a dual fan shaped tail, a blue ion tail and a yellowish-white dust tail extending about 3 degrees behind. The ion tail is caused by solar radiation ionizing molecules of gasses that are sublimating (changing from solid ices directly to gasses) as it approaches the sun. The dust tail is caused by the solid rocky debris that is being released as the ice thaws. The most amazing feature of this comet is its size. The nucleus or "head" is some 40 kilometers in diameter which can easily be seen with just your eyes as a brilliant white star-like object. It is traveling more than 158,000 kilometers per hour and covering about 12 minutes of arc per day (there are 60 minutes of arc in one degree of angle) so if you haven’t been following it on a nightly basis it will appear to move to the left southeastward about one degree every 5 days so you better get out there soon if you want to see it at its best! It will be at about the same altitude above the horizon the first two weeks of April but will begin to become lower and lower in the sky by the end of the month as it heads toward the winter constellations of Taurus and Orion which are beginning to set.

Tonight Hale-Bopp heads into the rich starfields of the constellation Perseus and passes within 1.5 degrees of M34, an open star cluster of about 40 stars, many of which can be seen in 10X50 binoculars. The tail of the comet should sweep right through the cluster making a spectacular view. On the nights of the 8th and 9th a new waxing crescent moon should make a pleasing photographic opportunity. Just mount a 35 mm camera equipped with a 50 mm lens on a tripod, set the lens to its widest opening, and open the shutter for 10-20 seconds using 400 speed film.

The moon will interfere with observing the comet from the 14th through the 23rd. On the 24th, a two day past full moon won’t rise until after the comet has set. Look for Hale-Bopp just above M45, the Pleiades open star cluster which looks like a miniature "dipper" in Taurus.

By the 30th, Hale-Bopp will only be about 15 degrees above the horizon, considerably dimmer, and its tail should appear considerably shorter due to fore-shortening as it begins to head away. This is the last month to get a good look at this "once in a life-time" comet. It won’t be seen again for another 2,400 years.

While you are out looking at the comet, you might want to see if you can spot Mercury. Look for the -0.6 magnitude yellow-orange planet about 10 degrees above the western horizon about 30 minutes after sunset. On the evening of April 8th, it will be about 7 degrees to the right of the waxing crescent moon. It will remain in view until the 28th of this month.