What does evection mean?
Evection is a term from celestial mechanics denoting a significant periodic irregularity in the Moon's motion around the Earth. Because the Sun's gravity continuously tugs on the Moon's orbit, the orbit's eccentricity is not constant but oscillates, causing the Moon to run slightly ahead of, or behind, its predicted position over a cycle of about thirty-two days. This inequality amounts to just over a degree at maximum, yet it was large enough for Greek astronomers such as Hipparchus to detect, and Ptolemy incorporated a correction for it into his lunar theory. The word itself derives from the Latin 'evection', meaning 'a carrying out'. Today evection appears mainly in astronomy, physics, and histories of ancient science, where it stands as a striking example of how carefully pre-telescopic observers could chart the heavens.
nounIn astronomy, a periodic perturbation of the Moon's orbit in which the Moon's orbital eccentricity varies under the gravitational influence of the Sun, producing an inequality of about ±1° 16' in the Moon's longitude with a period of roughly 31.8 days.
- (Astronomy) A periodic perturbation in the Moon's longitude caused by variations in its orbital eccentricity due to solar gravitation, discovered by Hipparchus and later explained by Ptolemy.
"Ptolemy's account of lunar motion included the evection, though he did not recognize it as a distinct inequality."
"The evection is one of the largest periodic inequalities in the Moon's apparent motion."
"Early lunar tables had to correct for the evection to predict eclipses accurately."
The term is normally used as an uncountable mass noun; plural forms are essentially unattested.
"The evection was first clearly identified in Ptolemaic lunar theory."
Long before Newton explained it, ancient astronomers had already measured the Sun's subtle tug on the Moon's orbit — they called it evection.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of evection
Evection comes from Latin 'evection-', the stem of 'evection', meaning 'an act of carrying out or away', from 'evecere' ('to carry forth'), formed from 'ex-' ('out') plus 'vehere' ('to carry'). It entered English in the late sixteenth or early seventeenth century as astronomers sought Latin terminology for lunar inequalities. The same Latin root gives English words such as 'evict' and 'evident'.
How evection is actually used
A technical term confined almost entirely to astronomy and the history of science; rarely encountered outside specialist literature.
Easily confused with evection
Eviction is the legal expulsion of a tenant from property, while evection is an astronomical perturbation of the Moon's orbit.