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Update: October 28, 2007: I just went outside to look at Holmes through my 'scope, and it's changed significantly since two nights ago! The cloud surrounding the nucleus is much larger (roughly the size of (but much brighter than) the nucleus of the Andromeda Galaxy, which I took a quick look at to compare), and has taken on a distinctive edge-brightened appearance, like a thick soap bubble. I suspect it will start to fade now; as it expands it will get thinner and reflect less sunlight. There appear to be two nuclei; one is bright and slightly off-center from the cloud, and the other a dimmer but sharper pinpoint significantly off-center. It's possible the second light is a background star: I'll observe again in a couple of hours and see if the comet has moved relative to it. The comet as a whole has moved noticeably in two days. I am not sure I understand the shape of the cloud; I would expect it to be significantly elongated, but instead it appears to be expanding as a spherical shell, like the whole comet blew off its surface. Very weird.
Update on the update: I decided to make a whole post about this after taking a new image of the comet.
So I took video of the comet on Thursday night, but it was really hard to get it centered and focused. I worked really hard, and I felt bad that it didn't turn out as nicely as I had hoped. I decided to post the video anyway (slightly edited) so that it wasn't a total waste of time... and it wouldn't have been anyway, because the view through the telescope (a 12.5 inch Dobs) was fantastic!
Here's the video:
I said in the video I thought I saw a hint of tail, but I think now it was an internal reflection in the 'scope. The Moon was very nearly full and playing havoc with the optics. The still image of the comet I posted in the video is from an earlier blog post, and the long shot image showing its placement in the sky is on my Flickr page. Flickr is loaded with great images of the comet.
It cleared up tonight, so I may try again. And Halloween is coming up, so I may haul the 'scope out and show Holmes to the costumed tykes...