There are comets and then there are comets. I didn’t know it when I caught my first glimpse of Comet West back in the spring of 1976. As I gazed at the brilliant nucleus and twin tails of that magnificent visitor, I was looking at a Great Comet. Astronomers like to use the term “Great Comet” for exceptionally bright comets, but there is no official classification as such.

Nonetheless, both in the minds of amateur astronomers and in the historical record, it’s pretty clear when a comet is great. You know a Great Comet when you see one. It’s a brilliant, naked-eye spectacle that causes the most casual viewer, your neighbor Fred or your Aunt Martha, who has never really observed the sky, to look up at the glowing specter and say, “WOW!” – or maybe even something a little stronger.

A number of factors influence how bright and well developed a comet will become. First and foremost is how close the comet will be to the Sun at perihelion. The closer the comet is to the Sun, the brighter it will be, as it will warm and outgas significantly more as it is closer to the Sun. Because of the inverse square law, normal objects are only one-quarter as bright when they are twice as far away from the Sun.