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[ ih-vek-shuhn ] [ ɪˈvɛk ʃən ] Show IPA Phonetic Respelling


a periodic irregularity in the moon's motion, caused by the attraction of the sun.

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More about evection

Evection, “a periodic irregularity in the moon’s motion,” comes from Latin ēvectiō (stem ēvectiōn-) “a going upwards, flight.” Ēvectiō is based on the verb ēvehere “to carry forth, move forth,” from ē (or ex) “out of, from, beyond” and vehere “to carry, drag.” Vehere has two common stems in English: veh-, as found in vehement and vehicle, and vect-, as in convection and vector. Distant relatives of vehere in English include way, wagon, weigh, wiggle, and even Norway (literally “north way”). Evection was first recorded in English in the 1650s.

how is evection used?

The point at which a moon and its planet come closest together, known as periapsis, is not constant …. When the period of this precession matches the time it takes the planet itself to orbit the sun, the sun’s gravity will distort the moon’s orbit. This is evection.

“Saturn’s rings and several of its moons may be recent creations,” The Economist, March 22, 2018

Jeremiah Horrocks[’s] … life was short, but he accomplished a great deal, and rightly ascribed the lunar inequality called evection to variations in the value of the eccentricity and in the direction of the line of apses, at the same time correctly assigning the disturbing force of the Sun as the cause.

George Forbes, History of Astronomy, 1909
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[ kuh-nik-yuh-ler ] [ kəˈnɪk yə lər ] Show IPA Phonetic Respelling


pertaining to the rising of the Dog Star (also called Sirius) or to the star itself.

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More about canicular

Canicular, “pertaining to Sirius, the Dog Star,” is equivalent to Latin Canīcula “Sirius” plus -āris, an adjective-forming suffix. Canīcula literally means “little dog” and is based on canis “dog,” plus the feminine diminutive suffix -cula (compare English -cle or -cule, as in molecule and particle). Canis survives today as French chien, Italian cane, and Portuguese cão, but Spanish can has declined in favor of perro, of unclear origin. A direct descendant of Canīcula is French canicule “heat wave,” which previously referred to the dog days of summer. This period of the year takes its name in both English and Romance languages from the appearance of Sirius in the northern sky, and the co-occurrence with hot weather gives dog days as well as French canicule their heated sense. Canicular was first recorded in English in the late 14th century.

how is canicular used?

Sirius was then above the horizon during daylight hours, so it was believed the star’s heat was added to that of the Sun to give a run of scorching temperatures. The interval from early July to mid-August of Dante’s “great scourge of canicular days” is still referred to as the dog days of summer in modern times.

John Flannery, “What to see in the sky in April: Venus and Mercury to meet in a conjunction,” The Irish Times, April 1, 2021

​​Now beat the pulse and burned the flame of that canicular time when Sirius, rising with the sun, adds his glow to the lesser planet for earth’s delight.

Eden Phillpotts, The Girl and the Faun, 1916
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[ uh-pas-truhn, -tron ] [ əˈpæs trən, -trɒn ] Show IPA Phonetic Respelling


the point at which the stars of a binary system are farthest apart.

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More about apastron

Apastron, “the point at which binary stars are farthest apart,” is based on Ancient Greek apó “away, off, apart” and ástron “star, constellation.” Apó, which remains apo- in English before most consonants (as in apology) but becomes ap- before vowels and h (as in aphorism). Distant relatives of apó, all featuring a telltale labial (using the lips) consonant, include English after and off, Latin ab “away from,” and Sanskrit ápa “away, off.” Ástron is based on astḗr “star” (compare the recent Words of the Day asterism and astronaut), which is a cognate and synonym of English star and Latin stella (compare the recent Words of the Day circumstellar and stellate). Apastron was first recorded in English in the early 1840s.

how is apastron used?

[A]s their orbits are so eccentric that when at apastron the stars are twice as remote from each other as at periastron, they will for the next three and a half centuries continue to slacken their pace, until they shall have reached the most remote points of their orbits…

Thomas N. Orchard, The Astronomy of Milton’s ‘Paradise Lost’, 1896

Owing to the great eccentricity of the orbits of double stars, such stars are anywhere from twice to nineteen times as near to each other at periastron as they are at “apastron,” or point of greatest departure.

Isabel Martin Lewis, Astronomy for Young Folks, 1922
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