Wednesday, October 31, 2007

20071030 - 17P/ Holmes Observing Report

Prior to a brief session, I attended the monthly planetarium show, Celestial Highlights, at New York's Hayden Planetarium. Comet Holmes was given some time both as an object in Perseus and in a series of slides given at the end of the sky tour.

The presenter described Comet Holmes as difficult from the city with unaided eye but revealing in binoculars and scopes. I wanted to disagree but it was his show. He followed up with three images of the comet (one by Pete Lawrence) and for contrast, showed Comet Hale-Bopp with two tails trailing long and high into the sky.

After the show, I went for a brief session to TotL to take in the comet naked eye and with the 22x60 binoculars. I carried the Tele Vue Ranger to get tighter shots of the comet but it wasn't unpacked.

On my way around the Great Lawn I ran into Mark the Literary Agent. He stayed and we observed the comet and the passing people watching the two guys watching a comet. I didn't invite people to the bins nor did anyone ask but a few. One was Ken the Pilot who stopped to say, "Hello." He peeked in the bins and said, "Oh that's the comet. Not that impressive." I described the same features to look or that I described to Mark earlier.

My observation is that comet 17P/Holmes appears to be dimming. I compare to delta Per which is celestial-South, or around 9:00PM EDT, directly to the right naked eye. Their brightness looks nearly equal. The pedestrians that stopped to observe it naked eye agreed. They added the description that it looked fuzzy compared to the other stars (naked eye). Same observation to me. Late last week, the comet's brightness was almost midway between the brightness of delta Per and Mirfak, alpha Per. The comet's size is obviously growing.

Location in field of view
There are two ways that I help the pedestrians look for the comet, one from the direction of Cassiopeia and the other from Capella in Auriga. Around 9:00PM EDT, Capella has risen almost 25­° above the horizon due manhattan-north. The brightest star in the area is unmistakable and difficult to miss. With an extended, open right hand put your thumb on Capella and rotate your pinky up to about a one o'oclock direction. You'll notice another bright star, Mirfak. Consider Mirfak the apex of a tall, isosceles triangle. Two fainter stars are seen below Mirfak. Relax, give it a moment or two for your eyes to adjust to see the triangle. The base star to your left, may appear fuzzy, since this is the comet. Easily seen from the city as many pedestrians made it out with a little coaching.

In the binocular 2 degree field of view, the comet appears near the inside right corner of a right triangle. These three stars: mag 6.18-HIP17772; mag 7.72-HIP17713; and, mag7.55-HIP17476. In my mental image 17772 & 17713 are the base of the right triangle.

The separation between the two base stars is 23'15". I estimate that separation to be 2 1/4 ~ 2 1/2 comets yielding a size of 9.3' ~ 10.3' for the head of comet.

I didn't perform an in-out test for brightness. That is choosing stars of similar magnitude and bring them out of focus so that the star's size increases to s large as the diameter of the comet. If the comet's brightness appears equal to the star's de-focused brightness then the comet's brightness is assigned a value of the star used for testing.

The outer coma exhibits an uneven brightness, a darkening can be seen on the leading edge side. It is arcuate and extends almost halfway around the central condensation (what I have been calling the nucleus.)

As the comet grows larger its surface brightness decreases. The brightness is distributed across a greater surface area of the comet. (Consider the brightness and the appearance of M33.) This results in less contrast with the background sky and appears dimmer to the eye.
With unaided eye, compared to delta Per, 17P/Holmes' brightness is approaching delta's brightness, Late last week it was almost midway between alpha Per (Mirfak) and delta Per. Over the past few days I feel it is dimming, due to the explanation above.

The comet appears round with three sections of varying brightness. The coma appears with a bright central core which is about two-fifths of the comet's diameter and off-center relative to the outer coma ring. It is south of the leading (drift) direction. Within that brighter central condensation on its trailing side, a stellar condensation is evident. Its obvious in most photographs of the comet. I did not see any evidence of a tail.

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