Thanks, Steve, for reminding me (and many others, I hope!) about this glorious and striking comet.
Several things went through my head when I saw your post:
Long, LONG tail....Over 100 degrees, as I recall.
Daylight visible: I recall seeing this large aquamarine spot in the sky during the hour or so before sunset.
Actually seeing a comet move through a telescopic field of view "in real time." All I had to do was stare at the field, and I could see Hyakutake crawl across the FOV among the stars bit by bit.
I'll have to see if I can find a print of my best photo of the Great Comet of 1996 and upload it here.
Thanks, and clear skies!
The sentence emboldened above is my doing to point out the following.
There has long been controversy regarding the visible length of Comet Hyakutake's tail when nearest its perigee. Given that what we saw was overwhelming composed of ionized gas, the tail should have been directed radially away from the Sun, just as all ion tails are. Throughout the comet's apparition no marked deviation from that specific trajectory was imaged so there was no suggestion of any deviation. That being so, the absolute maximum apparent visual dimension of the tail could not have physically exceeded about 70* (degrees)...even were the tail considered to be of infinite length! Nevertheless, there were reports of the tail spanning 100* and even 120*! Nevertheless, absolutely no photographic/imaging evidence substantiated the existence of any tail beyond 60*-70*. In addition, following the comet's perigee a number of visual observers continued to report extraordinary lengths for the tail, once again unconfirmed by images.
The seemingly puzzling situation is more easily explained than might at first seem to be possible, something addressed in detail in a paper I co-authored with a German colleague shortly following the end of the comet's apparition. The fact is that in combination the eyes and brain can play bizarre tricks on the observer. One of these occurring in situations of very low contrast is the extension of linear features, somethings to a considerable extent. And, just as with Percival Lowell's Martian canals, they can seemingly link otherwise physically unrelated low surface brightness, or contrast, objects along the extended path of the of a linear feature to ultimately result in a dramatic increase in the apparent overall length.
C/Hyakutake's tail was both imaged and visually seen by a multitude of individuals as 60*-70* long seen under very good skies. Observers were hampered by the fact that, first, the entire span of the tail could not easily be taken in with just a single view. It was actually necessary to scan the object with one's eyes to see it in its entirety! As diffuse and weak as the tail grew toward its terminus the actual point where it ended was quite impossible to define. Under such circumstances the expanse of the long, narrow, low surface brightness ray-like tail was easily imaged to extend well beyond its actual terminus. This was the situation encountered particularly by those reporting enormous tail lengths. And, secondly, further compounding the error was that the softly glowing Gegenschein lay directly in line with the vector of the tail's terminus just 20* or so away. Thus, the claims that the tail could be traced up to and even through the Gegenschein was most easily explained as a trick of the mind and not a physical reality.
The exceeding long tail reported by some following perigee, but prior to the comet's perihelion, can be considered the result of this same illusion, along with anticipation. Many of the observers involved in those sightings made the unusual observation that within minutes of the comet's head setting they lost sight of the previously seen 40*-50* tail standing vertical up in the western sky just minutes before. Neither could this residual tail be imaged after the comet's head had set. Now a check of the historical record addressing other extremely long-tailed comets of the past indicated that their tails, the outer parts being equally as faint as was Hyakutake's, remained visible sometimes for hours after the head of those comets had set. The only reasonable conclusion is that the observers, already well aware of the extraordinary lengths reported during the preceding fortnight, continued to have this in the back of their minds when they went out to observer following perigee...in addition to the illusion outlined previously compounded the situation!
Aint comet observing fun? 
BrooksObs
Edited by BrooksObs, 29 December 2016 - 02:41 PM.
