aph·​elion | \ a-ˈfēl-yən How to pronounce aphelion (audio) , ˌap-ˈhēl- How to pronounce aphelion (audio) \
plural aphelia\ a-​ˈfēl-​yə How to pronounce aphelion (audio) , ˌap-​ˈhēl-​ \

Definition of aphelion

: the point farthest from the sun in the path of an orbiting celestial body (such as a planet) — compare perihelion

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Aphelion and perihelion are troublesome terms. Which one means a planet is nearest to the sun and which means it is farthest away? An etymology lesson may help you keep those words straight. Just remember that the "ap" of aphelion derives from a Latin prefix that means "away from" (the mnemonic "'A' for 'away'" can help too); peri-, on the other hand, means "near." And how are aphelion and perihelion related to the similar-looking astronomical pair, apogee and perigee? Etymology explains again. The "helion" of aphelion and perihelion is based on the Greek word hēlios, meaning "sun," while the "gee" of apogee and perigee is based on gaia, meaning "earth." The first pair describes distance in relation to the sun, the second in relation to the earth.

Examples of aphelion in a Sentence

Recent Examples on the Web Officially, aphelion occurs at 6:27 p.m. EDT on July 5. Eric Mack, Forbes, 5 July 2021 Our star is at aphelion on July 5, 2021, the point of the Earth’s slightly elliptical orbit that is farthest away from the Sun. Jamie Carter, Forbes, 10 June 2021 The closest of these run-ins occurs when Earth reaches aphelion, or its farthest point from the sun, and when Mars reaches perihelion, its closest point to the sun. Jennifer Leman, Popular Mechanics, 5 Oct. 2020 Apart from Mercury, no other natural object in our solar system is known to have a smaller aphelion—the point at which an orbiting body is farthest from the sun. Katherine J. Wu, Smithsonian Magazine, 21 Jan. 2020 Mars’ orbit is more elliptical than Earth’s, so the difference between perihelion and aphelion is greater. Dave Epstein, BostonGlobe.com, 11 July 2018 Over the past centuries, Mars’ orbit has been getting more and more elongated, carrying the planet even nearer to the sun at perihelion and even farther away at aphelion. Dave Epstein, BostonGlobe.com, 11 July 2018 Image On Friday, Earth will swing toward the outermost point in its orbit, known as aphelion. Shannon Hall, New York Times, 5 July 2018 After orbiting the Earth for 6 hours, a third-stage burn-to-depletion was completed at approximately 02:30 UTC Feb 7, placing the dummy payload in a heliocentric orbit having a perihelion of 0.99 au and aphelion ~1.7 au. Joe Pappalardo, Popular Mechanics, 8 Feb. 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'aphelion.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of aphelion

1656, in the meaning defined above

History and Etymology for aphelion

New Latin, from apo- + Greek hēlios sun — more at solar

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The first known use of aphelion was in 1656

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Cite this Entry

“Aphelion.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/aphelion. Accessed 8 Dec. 2021.

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More from Merriam-Webster on aphelion

Britannica.com: Encyclopedia article about aphelion


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