SKY WATCH March 1996

by Harry Adams


The long, deep sleep of winter has lasted 20,000 years. Gently rolling hills covered with frost have slumbered undisturbed in this solitude. No winds play over this barren scene. No sounds break the silence. Above the sky turns endlessly. The stars rise above a horizon that seems to be too close and set again soon after.

The Great Light, so long a tiny spark indistinguishable from the rest of the stars, at last begins to warm the bleak, desolate landscape. As the temperature begins to rise, the atmosphere starts to stir from its frozen state. At first gently, then with more vigour, tiny geysers form , lofting dust and debris high up above the surface, never to return. Soon the sky is filled with a strange, ghostly light. The temperature rises more rapidly now. The eruptions have become continuous as the geysers spew forth the substance of this world.

A faint glow is the first warning the inhabitants of the system's third planet have of the existence of the intruder. Giant eyes capture the emissions of this tiny object as it crosses the orbit of the system's fourth planet. An alert is sent out and hundreds of others ready themselves for the encounter. A comet is coming.

The scene just described is unfolding in our solar system at the present moment. Last January, Yuji Hyakutake was searching for comets from Japan using enormous 25x150 mm binoculars when he found the one that now bears his name. Comet Hyakutake C/1996 B2 is currently a binocular object in the constellation Libra. Its surface is currently undergoing changes much like those outlined above.

The star chart shows the comet's path during most of March. Note two departures from the usual monthly chart. This chart shows the east horizon and is for 23h EST on the 15th of March. Tick marks on the comet's path and the dates beside them indicate midnight EST at the start of that day. For example, to observe the comet at 9pm on the evening of the 25th, find the location marked by the letter "X" between the tick marks for the 25th and 26th. As usual, brighter stars are represented by larger circles and zenith is indicated by the letter "Z" near the top of the chart. The horizon is the curved line near the bottom of the chart.

C/1992 B2 has a reasonable chance of becoming an interesting naked eye comet. I know, I know, you have heard it before. Since comet Kohoutek, all predictions about comets are usually couched in very pessimistic language. Even Shoemaker-Levy 9 was down-played until it proved to be better than even the most optimistic astronomers had anticipated. The fact is, our models for comet behaviour have improved since the 1970's but they are still models and every new comet is an unknown quantity until it has completed its encounter with the Sun.

Here is what we know. The comet's orbital period is between 10,000 and 20,000 years, so it has been to the inner solar system before. This is good since first time comets tend to be poor performers. It is also brightening rapidly but this could just be a temporary outburst. If it does continue to brighten, then it could get as bright as the stars in the Big Dipper (+2) or Jupiter (-2). It is also going to pass only 15 million kilometres from Earth around March 25th. This is about 40 times farther away than the Moon but only 10 percent of the distance to the Sun. This means it will appear very large. It also means that its light will be spread out over an area as large or larger than the Moon at full phase, making it appear fainter.

So we will have a large, fairly faint comet racing across the night sky during March and April. If things go a little bit better than this, we could have a moderately bright comet. I won't say what I would really like to happen. Hope for fireworks but don't expect them is my best prediction for brightness. The tail of the comet will be much fainter than the nucleus and coma. Since its line of motion lays almost along our line of sight, the tail will be very short until close to its closest approach to us on the 25th.

Binoculars will provide the best views from in town. If you have a dark sky you will have much better views, both with binoculars and the naked eye. Give your eyes lots of time to dark adapt. And keep your fingers crossed.

A presentation on Comet Shoemaker-Levy 9 and its impact with Jupiter in 1994 will be given by Doug George of the Royal Astronomical Society of Canada at Mackenzie High School on March 29th at 8 pm. Watch the Club News section for more details.

The Sun rises at 7:13h on the 15th and at 6:50h on the 30th. Sunset occurs at 17:35h on the 15th and at 17:55h on the 30th.

Venus continues to shine at magnitude -4.2 and may be seen about 42 degrees above the west-southwest horizon at sunset. Although the fraction of the illuminated surface visible to us drops from 59 percent on the 15th to 52 percent on the 30th, the decrease in the planet's distance from Earth increases its apparent size from 20 arcseconds to 23 arcseconds which leaves it a little brighter overall.

Jupiter shines at magnitude -2.0 in the morning sky. On the 15th it rises at 3:16h and is 19 degrees above the south-southeast horizon at sunrise. On the 30th it rises at 2:23h and stands 20 degrees high at sunrise.

Moon FM -5th , LQ - 12th, NM - 19th, FQ - 27th

Back issues of Sky Watch are now available on the Deep River Astronomy Club's world wide web page at http://intranet.on.ca/~rbirchal/draco.html .

References: Burnham's Celestial Handbook, R.Burnham, Jr., General Publishing Co., Toronto. The Observer's Handbook of The Royal Astronomical Society of Canada, edited by Roy L. Bishop, University of Toronto Press Incorporated. Star chart produced using Guide 4.0 software by Project Pluto, with permission.


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