From the August 2013 issue

Score your favorite comets

Using Joseph Marcus’ comet scorecard, find out if your favorite comets are Great or just good.
By | Published: August 26, 2013
PANSTARRS (C/2011 L4) was the first cometary visitor of 2013. How would it rank on Joseph Marcus’ comet scorecard? // Randall Kayfes
The term “Great Comet” does not have a technical definition. In broad strokes, a comet is Great if it’s bright and memorable enough to enter the public consciousness (and not just the consciousness of amateur astronomers). But that’s all pretty subjective. In Joseph Marcus’ October 2013 article, he proposed a numerical way to determine whether a comet was Great — and if it was Great, precisely to what extent. He created a scoring system that takes into account the five factors that add together to make a comet bright. By figuring out how long observers could see a comet without a telescope, how close a comet was to Earth, how close it was to the Sun, what its absolute magnitude was, and how small its forward scattering angle was when it appeared most brilliant, a curious person can rank comets relative to each other. According to Marcus’ tally, a comet must earn nine points to enter the Great hall of fame. How do your favorite comets stack up?

To gather the information you need to create your fantasy league of all-star comets, use the resources below:

Donald Yeomans of NASA’s Jet Propulsion Laboratory (JPL) created a page called “Great Comets in History.” It gives the number of days comets were visible to the naked eye, dates and distances of their close approaches to Earth and the Sun, as well as the date and magnitude of their most spectacular appearances. The only drawback is that it only lists comets widely considered Great, so you won’t be able to check on less popular objects. has gathered information on comets (and asteroids) from across the Web and built an impressive database filled with juicy details like mass and rotation period. Yum. You even can compare these quantities with the demographics of the rest of the cometary population.

The JPL Small Body Database lets you pick the numbers you’d like displayed. You can search only for a specific comet’s details, or you can query for every number available on all comets.

And finally, Gary Kronk’s Cometography website has orbital data for many comets, as well as more qualitative, historical stories of bygone comets and images observers have made over the years.

So check out these pages, gather the stats you need, and then pick your lineup of Greats or almost-Greats!