While the designation of a great comet is subjective, astronomers and skywatchers tend to agree on one key criterion: visibility to the naked eye. “My personal definition of a great comet is very simple: If you go outside in a dark location and look in the correct direction towards the comet, it will be obvious with just a glance, no searching, and no optical aids,” says David Schleicher, an astronomer at Lowell Observatory.
(Asteroids vs. comets: How do they differ, and do they pose a threat to Earth?)
Most comets never reach that level of brightness, remaining visible only through binoculars or telescopes. “Generally, a great comet only comes along every half-dozen to a dozen years,” says Schliecher. “But, rarely, one might see two great comets separated by only a year’s time or less.”
Because the term lacks a formal definition, there’s no official list. Still, a few standouts are widely recognized, including Comet Hale-Bopp (C/1995 O1), which was visible for 18 months, and the 1910 appearance of Halley’s Comet, which was visible during the daytime.
Why predicting great comets is so difficult
After astronomers spot a new comet, they quickly calculate its orbit by observing its ever-changing position relative to known objects, such as stars.
