Word of the Day

: July 4, 2020

aphelion

noun af-EEL-yun

What It Means

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

aphelion in Context

"Our planet reaches aphelion only once a year, and the event typically falls approximately 14 days after the June solstice, which marks the first day of summer for the Northern Hemisphere and the first day of winter for the Southern Hemisphere. Similarly, perihelion happens two weeks after the December solstice." — Hanneke Weitering, Space.com, 4 July 2019

"Currently about 34 AU from the Sun, Pluto is still slowly approaching its aphelion, the farthest point in its orbit from the Sun, where it will lie nearly 50 AU from our star." — Alison Klesman, Astronomy, 3 Apr. 2020

Did You Know?

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.