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?

Astronomy magazine subscribers can read the full article for free. Just make sure you're registered with the website.