by Dennis Wellnitz, Astronomy Department, UMCP
Before dawn on 30 January 1996 in southern Japan, Yuji Hyakutake was scanning the eastern sky with his powerful 25x150 Fujinon binoculars when he discovered the comet which now bears his name, his second comet discovery in as many months. (The comet he discovered in December 1995 is known as Comet Hyakutake (C/1995 Y1).) At that time, the comet was about 10th magnitude. Within two days several observatories had confirmed his discovery and obtained the changing position of the comet with high accuracy. Within a week there was enough data for the determination of a preliminary orbit. A short time later, a pre-discovery plate which had been taken in early January helped to provide a much better determination of its orbit.
The orbit for this comet indicates that it will reach perihelion (closest approach to the Sun) on 1 May 1996, at a distance of only 0.23 AU. Before that, on 25 March, it will make a close approach to the Earth, coming as close as 0.11 AU (about 10 million miles). Because it will pass so close to the Earth, the comet may become rather bright, though it will also be rather large, with a coma perhaps the size of the full moon. Judging by a similarly close approach by Comet IRAS-Araki-Alcock in 1983, Comet Hyakutake should be easily visible by eye in dark skies away from city lights as a hazy patch in the sky, and a good sight through binoculars if it remains condensed towards the nucleus. There may be a faint tail which could be as long as 20 or 30 degrees, but it will likely be visible only where the sky appears very dark. During its closest approach to the Earth, it will be most visible from the Northern hemisphere because it will pass almost directly over the north pole of the Earth. But much later, after perihelion, it will be best viewed from the Southern hemisphere.
Unfortunately, it is difficult to predict the brightness of a comet, because visually we mostly see the sunlight reflecting from the dust coming from the comet. But comets vary considerably in the amount of dust and gas they produce, and generally produce more after perihelion than before. Also, comets which are approaching the sun for the first time generally brighten less rapidly than those which have approached the sun many times before. Recent observations indicate that this comet may be brightening at the rate expected for a new comet on its first approach to the sun, and therefore may not become as spectacular as some have predicted.
After its close approach to the Earth, the comet will appear to become smaller and dimmer because it will be getting further from the Earth, but eventually its continuing approach to the Sun will cause it to appear brighter once again.
Before its closest approach to the Earth, Comet Hyakutake will be most easily visible at about 4 am local standard time. Near its closest approach to Earth it will move into the northern sky and thus become visible all night. After the closest approach, it rapidly moves to the evening sky, and will be most visible after sunset, moving steadily nearer the sun throughout the month of April.
Three finding charts accompany this note1: one for late February and most of March, following the comet as it moves closer to the Earth; a second one for its passage near the North Celestial Pole at the end of March; and a third to follow it on its journey toward and around the sun in April and May. Comet symbols show the predicted direction of a tail and mark the predicted location of the comet at 0 hours UT on the given date; this is at 7 pm EST or 8 pm EDT on the previous calendar date for observers in the Eastern US Time Zone, due to the difference between local time and UT.