... Bad Astronomy . ... Bad Astronomy : Comets whiz across the sky in a few seconds. Good astronomy : Comets appear to move very slowly, usually taking days to noticeably move. ... The orbital periods of comets are measured in years; the most famous comet of all, Comet Halley (usually--- and incorrectly-- called ``Halley's Comet''), takes about 76 years to orbit the Sun just once. ... Now, when a comet gets near the Earth, it tends to be moving pretty quickly. ... Want to know more about comets? ...