Comet Hyakutake was an amazing comet. Although not particularly large, (the nucleus was only about 2km across) it was a very active comet and it passed within 15 million kilometres of the Earth on its way to perihelion. Around the time of closest approach the nucleus appeared to undergo some disruption which increased nuclear activity and brightness. In addition the comet contained a high proportion of ice which meant the comet was very active. Finally, when the comet was at it closest, the tail was side on to the Earth which gave observers the ideal view.
The comet was discovered in late 1995 as a 10th magnitude object low in the pre-dawn skies, in the constellation of Scorpius. The comet slowly moved northwestwards into Libra and brightened slowly.
Comet Hyakutake was due to pass close to the Earth in March 1996 on its way to perihelion. At this time it was expected to be a good object but no-one knew just how bright it would be, so its rate of brightening was followed closely. During January, it was clear that the comet was going to be a bright. It was already above magnitude 9 and displayed a moderately condensed coma.
During mid-February, the full moon made observations of the comet very difficult. When the moon moved out of the sky towards the end of February, the comet appeared much brighter than expected, and was developing a good tail.
It was at this time that most observers became aware of the comet, and added to the perception that the comet burst into view.
By the beginning of March, the comet was brighter than magnitude 0 and sported a tail nearly 10 degrees long.
By now the comet was moving rapidly northwards. As it did it continued to brighten and the tail continued to lengthen. Once the moon ceased to interfere with observations again, the comet was seen in all of its glory. It reached a magnitude of -2. The coma had a naked eye diameter of 1.5 degrees (3 times the size of the moon!!) and the tail reached an amazing 95 degrees to the naked eye!!!!!!!!
On March 26, the last night before perihelion that the comet was visible from the southern hemisphere, (although some of the tail was visible for several more nights) the view was spectacular. Near midnight, the head of the comet was visible about 10 degrees above the northern horizon, while the tail could be followed to past overhead where it was lost in the zodiacal band. The size of the comet's head and tail were such that the comet did not seem to belong to the stars. rather it seemed like it was something that was suspended in the sky only a few hundred feet above the ground. In the photograph below, the sheer size of the comet is graphically shown.
There has been much criticism of the reported lengths of the tail of comet Hyakutake, mostly by northern hemisphere observers who did not observe such great lengths. Some members of the German Comet Section have been especially vocal in their criticism, Where they try to show that such a tail length was not possible. However, while theirs is an interesting theoretical paper, it ignores 2 points. The first is that there were reports of such a long tail from a number of very experienced comet observers. The other is that most of these were from the southern hemisphere. This is an important point! From the southern hemisphere, the head of the comet during late march was very close to the northern horizon. This meant that close to mid-night the tail was pointing straight up, with the end close to the zenith, where the sky was darkest! From the northern hemisphere, the end of the tail was close to the horizon where it was lost due to normal horizon absorption.
As the comet approached perihelion, it started to fade and it was realised that the fantastic display had been the result of a disruption to the nucleus. It had been expected that the comet would continue to be a spectacular sight following perihelion, however it faded very quickly and this rapid fading, combined with its southern declination meant that few observers managed to see it. With the rapidly brightening comet Hale-Bopp, comet Hyakutake was quickly forgotten. This was very unfortunate as before the comet faded from view, it did provide southern hemisphere with some more fine views. The photograph below is one of the very few of comet Hyakutake taken after perihelion that I know of.
Comet Hyakutake was without a doubt, the most spectacular comet I have observed in over 35 years of comet observing.