Holmes symmetry, fans, and orbits

David Kingsley

When I first looked at the comet on Wed, I also thought the nucleus was offset from center. However, the Celestron microguide reticle eyepiece that I have been using to follow the size of the comet has both a graduated linear scale, as well a series of nested concentric circles of different sizes (see reticle image below). As soon as I centered the near stellar nucleus in the smallest circle of the bullseye pattern on Wed night, it was clear that the bright circular comet disk (and the outer much dimmer envelope) were symmetric around the stellar point of the nucleus, congruent with the other larger circles of the bullseye pattern. Only the small bright fan coming off the nucleus was offset from the center. But that's clearly enough to drag your eye away from the center and give a strong visual impression that the entire nucleus is offset.

The actual angle of projection of the asymmetric fan presumably reflects the current direct antisolar direction. I found a NASA web tool very useful for visualize the current orbit (see web liink below). We really are looking almost straight out at the comet, which is near opposite us away from the sun (as Heather and Michelle were discussing earlier this week). Not quite straight out however, and the earth will catch up and pass the comet in terms of sun-earth-comet angles over the next few weeks. If the position angle of the fan shape is really just tracking the anti-solar direction, then the orientation of the bright fan projecting from the nucleus should also subtly change from a projection SSW of the near stellar nucleus (currently), to a projection that points due South of the nucleus as the orbit alignment changes.

WIth the rapid expansion of the shells surrounding the comet, they will probably soon get harder to see in terms of brightness per unit area. (I thought the very dim outermost envelope was already much harder to see Saturday night than on Wed, Thurs, or Friday). However, the stellar core and fan are so much smaller and brighter than the envelopes, that we may be able to follow them for quite a while.

And the moon is finally shrinking. I can't wait to see what Holmes looks like as 3rd quarter and new moon approach in November.

Check out the NASA /JPL orbit visualizer tool for Comet Holmes here:

http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=17p&orb=1

(be sure to play with the vertical scroll bar on the right hand side to view the geometry from different perspectives)

The reticle pattern of the Celestron Micro guide eyepiece can be seen here. (For some reason, Celestron's own web site only shows the eyepiece, but not the reticle itself):

http://tinyurl.com/36nevs

This has been a very useful 12.5 mm eyepiece for measuring things accurately during visual observations (looking at angles and distances in double stars, determining the position of lines in stellar spectra, and, more recently, for following the size and symmetry of Comet Holmes).

--David Kingsley