Sky Talk October 2010: Periodic Comet Hartley

We have never featured a comet in this column. One reason is the well-known fact that the visibility of comets — both returning (previously found) and newly discovered ones — is typically highly uncertain. Another is that this column has a long lead-time (normally two months) and a new comet may suddenly appear after an installment has already been submitted. But this month we do feature a “reliable” returning comet that should put on a nice show for stargazers using binoculars, small telescopes — and possibly even the unaided eye!

Short-period Comet Hartley was discovered in 1986 by Malcolm Hartley. It orbits the Sun every 6 ½ years and will come closest to Earth on its next trip sunward on October 20th, remaining visible all night (and most of the month as well!). To follow its path, use the Scientifics’ Star and Planet Locator. The best instruments for viewing it will be your Edmund binoculars and, especially, the Astroscan wide-field telescope at its low magnification of just 16 power. It may be possible when nearing its brightest on the 20th (predicted to be 5th magnitude) to glimpse it with the unaided eye. But as mentioned below, the Moon will also be a factor to contend with.

As October begins, you’ll find this fuzz-ball in the constellation Cassiopeia, just south of its familiar W-shaped figure. On the evening of the 8th into the morning of the 9th, it will lie very close to the famed Double Cluster in Perseus. This celestial encounter should be a truly wondrous sight through the Astroscan! If you watch the comet closely over a period of several hours, you’ll actually be able to see its motion relative to the cluster itself. Then by mid-October, Comet Hartley will be located just south of the bright star Capella in Auriga. Unfortunately, the Moon will be nearing full and definitely interfere with the comet’s visibility. And here, telescopes definitely have the advantage over binoculars, with their greater light-gathering and magnifying powers. Using the Astroscan’s 30x eyepiece will help reduce the bright sky background, improving contrast between the two objects, while its generous aperture will soak up lots of extra photons from the comet itself.

Comet Hartley then moves from Auriga into neighboring Gemini, but a Full-Moon on the 22nd will make tracking it somewhat of a challenge for the following several nights. By the end of October, you’ll find this interloper located about midway between Gemini’s two bright stars, Castor and Pollux, and radiant Betelgeuse in Orion as it slowly moves away and fades from view.

In closing, it should be mentioned here that there’s a long tradition of amateur astronomers discovering comets by sweeping the sky with small, wide-field telescopes like the Astroscan. While the professionals have the market pretty much cornered today with automated sky surveys, there may be a comet out there with your name on it! All you need is a good star atlas (to help avoid misidentifying existing deep-sky objects, The Cambridge Double Star Atlas offered by Edmund being excellent for the purpose), lots of patience, a clear night — and quite a bit of luck!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.