While the designation of a nice comet is subjective, astronomers and skywatchers have a tendency to agree on one key criterion: visibility to the bare 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 by no means attain that stage of brightness, remaining seen solely via 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 time period lacks a formal definition, there’s no official record. Still, a few standouts are widely known, together with Comet Hale-Bopp (C/1995 O1), which was seen for 18 months, and the 1910 look of Halley’s Comet, which was seen in the course of the daytime.
Why predicting nice comets is so tough
After astronomers spot a new comet, they shortly calculate its orbit by observing its ever-changing place relative to identified objects, comparable to stars.
