17P / Holmes – Not As Faint As You Might Think

I had the opportunity to make an interesting comparison of Comet Holmes’ visibility from Phoenix and from Flagstaff this weekend. While in Phoenix, I tried to view the comet from a fully moonlit, light-polluted sky and was not surprised that I could not see it with the naked eye. With 15 x 70 binoculars, it was just barely visible as a softly brighter patch amid the gray light of a washed out sky. Going to the 8″ Dob, the comet was invisible again. I assume this is because the very large size of the comet and narrower field of view didn’t allow comparison to the surrounding sky the way the binoculars do. I had hoped to show it to family while we were visiting, but it was obviously a bomb under those conditions.

Last night from my back yard in Flagstaff before moonrise, the comet was once again readily visible to the naked eye. Through binoculars, it was monstrous and well-defined. As it continues its inexorable expansion, its surface brightness continues to drop, which is really killing its visibility under bright skies. I imagine this is why so many observers have been saying it’s getting dimmer if they have moderate levels of light pollution to deal with. Brian Skiff noted on the amastro list that he estimates that the comet’s total brightness has not changed much, although the increase in size has obviously dropped its surface brightness accordingly. He compared it to M31 and noted: “An easy comparison to make is with the Andromeda Galaxy, V = 3.5, which is way fainter than the comet and lots smaller, though not so different in surface brightness.”

So, as the next new moon cycle approaches, there will be more opportunities to photograph and sketch this wonderful thing blossoming above and beyond the orbit of Mars. I don’t know how much of that I’ll be doing myself as I continue to try and recover mentally and physically from all the lost sleep I suffered on it a few weeks ago.