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evection(i-vek -shŏn) The periodic (31.807 day) inequality in the motion of the Moon that may amount to a displacement in longitude of up to 1°16′20″.4. It arises from changes in the eccentricity of the Moon's orbit (0.0432 to 0.0666) that are brought about by solar attraction. Evection was discovered by the Greek astronomer Hipparchus. See also annual equation; equation of center; parallactic inequality; variation.
the greatest disparity—that is, deviation from motion according to Kepler’s laws—in the moon’s motion around the earth, caused by perturbation by the sun. Discovered by Hip-parchus in the second century B.C., evection was detected by Ptolemy in the second century A.D. and was correctly explained by I. Newton. The change in the moon’s longitude owing to evection reaches 1°16’26” and varies with a period of approximately 31.8 mean solar days. The perturbing force that causes evection also changes the horizontal parallax of the moon periodically.