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Halley's comet

[hal-eez or, sometimes, hey-leez]
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noun
  1. a comet with a period averaging 76 years. In this century it was visible to terrestrial observers just before and after reaching perihelion in 1910 and again in 1986.
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Origin of Halley's comet

named after Edmund Halley, who first predicted its return

Pronunciation note

The common pronunciation for both the comet and the astronomer after whom it is named is [hal-ee] /ˈhæl i/. This is the pronunciation usually recommended by astronomers. However, several variant spellings of the name, including Hailey, Haley, and Hawley, were used interchangeably during the astronomer's own time, a period when spellings even of proper names were not yet fixed, and corresponding pronunciations have survived. The pronunciation [hey-lee] /ˈheɪ li/ in particular remains associated with Halley's comet ; it is less likely to be heard as a pronunciation of Edmund Halley .
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for halley's comet

Halley's Comet

noun
  1. a comet revolving around the sun in a period of about 76 years, last seen in 1985–86
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

halley's comet in Science

Halley's comet

[hălēz, hālēz]
  1. A short-period comet that orbits the Sun once every 76 years. It is visible to the unaided eye and last appeared in 1986, when close observation by spacecraft showed that its nucleus measures about 16 km (10 mi) by 8 km (5 mi) and is composed of water ice, stony minerals, and organic compounds. Its next appearance will be in the year 2061.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.