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evection (ih-VEK-shuhn) - n., a perturbation of the moon's motion in its orbit due to the attraction of the sun.
If the earth and moon were all by their lonesome in space, without a sun, the moon's orbit would be a perfect ellipse. With the sun, the moon's pulled off that ellipse in a direction that changes throughout the year that's the earth's orbit around the sun -- so it has a wobble, or libration, of about 1.274°. This was known in ancient times (and its discovery attributed to Ptolemy). The current name was given by 16th century astronomer Ismaël Boulliau, in a treatise that expanded on (and attempted to explain) Kepler's elliptical theories, from the Latin ēvectiōn, a going up, from ēvectus, the past participle of ēvehere, to raise up, relating it to his hypothesis of conic sections and orbits.
---L.
If the earth and moon were all by their lonesome in space, without a sun, the moon's orbit would be a perfect ellipse. With the sun, the moon's pulled off that ellipse in a direction that changes throughout the year that's the earth's orbit around the sun -- so it has a wobble, or libration, of about 1.274°. This was known in ancient times (and its discovery attributed to Ptolemy). The current name was given by 16th century astronomer Ismaël Boulliau, in a treatise that expanded on (and attempted to explain) Kepler's elliptical theories, from the Latin ēvectiōn, a going up, from ēvectus, the past participle of ēvehere, to raise up, relating it to his hypothesis of conic sections and orbits.
---L.
