The best observations were made around 11-12pm the evening of 25 March 1996 CST (26 March 1996 0500-0600 UTC). Observers were Brent and Jan Hugh.
Note that the total length of tail observed, adding sections 1 through 5 together, is about 85 degrees. At the time, astronomers indicated that this was impossible, using a calculation such as that shown on p. 162 of this article about Comet Hyakutake from the British Astronomical Association. Starting with the assumption that the Comet's tail streams radially outward from the sun, geometry shows that the longest possible observable tail length from Earth is about 57 degrees on March 25.0 UTC and 70 degrees at March 26.0 UTC.
Diagram from the Journal of the British Astronomical Association article about Comet Hyakutake
We definitely observed 70 degrees of tail and thought we'd observed up to 85 degrees.
So our observation was actually impossible! Or was it . . .
The Ulysses probe--in a complete different part of the solar system on a mission unrelated to Comet Hyakute--had an amazing chance encounter with Comet Hyakutake's tail in April 1996, detailed in this fascinating article. A closer examination of the dyanamics of the solar wind, which drives distribution of particles in the tail, shows that on March 25-16, 1996, and through Hyakutake's orbit, the tail had a distinct curve.
Snip from Imperial College London's animation of the about Comet Hyakutake tail. It curves distincly leftwards from the straight line extended out from the sun to the comet, meaning that the tail would extend further from an Earth observer's viewpoint.
This diagram shows the angle difference from the perspective of an Earth observer--about 14 degrees.
What this means for the evening of March 25/26, 1996--the night we were observing--is that the visible tail as seen from earth had a potential length about 14 degrees longer than that predicted by the simplistic comet tail model.
That means that the observable Hyakutake tail could have been up to 71 degrees on March 25.0 UTC and 84 degrees on March 26.0--around the time we made the observation sketched above. And we made our observation about March 26.25 when the observable tail would have been even a bit longer than 84 degrees.
You will note that the total length of tail sketched above is between 80 and 85 degrees--so almost exactly what this model predits!
Other observers from around the world, particularly those in dark-sky locations, reported similar tail lengths about this same time. Some of those observations are recorded in the article linked above. (Our location was only moderately good at best.)
Result: Eyewitness observation, thought to be impossible at the time, vindicated by SCIENCE years later!
The same Ulysses data analysis showed that Hyakutake's tail length is at least 3.8 AU (360 million miles). That's pretty long!
On a more personal note, the site of this comet and its tail, stretching nearly from horizon to horizon, was one of the most thrilling and remarkable things Jan and I have ever seen in the night sky. Most astronomical objects are rather small in angular size--Hyakutake was HUGE. It was very obvious in a completely visceral way that we were observing something that was some millions of miles in extent, and very close to us in relative terms.
I would rate the Hyakutake observation that night as one of the very top astronomical, scientific, or natural occurences I have personally seen--right up there with viewing the Comet Shoemaker-Levy 9 impacts, seen coming around the limb of Jupiter just minutes after they hit, in our small telescope in our front driveway.
Star chart generated by SkyMap.
You are welcome to visit these other pages I maintain:
Other Comet-related Pages:
The keeper of this page is Brent Hugh (brent[at]brenthugh dot com). Feel free to email him with any questions about, problems with, or complaints regarding the page.