EVECTION (Latin for "carrying away"), in astronomy, the largest inequality produced by the action of the sun in the monthly revolution of the moon around the earth.
The eccentricity determined in this way is more than a degree in error, owing to the effect of the evection, which was unknown to Hipparchus.
Hence the evection is then - 1.2° sin g, and consequently has the same argument g as the equation of centre, so that it is confounded with it.
Equation of the centre and evection are, at quadrature 6.29° sin g+I 27° sin g= 7.56° sin g.
Thus, in consequence of the evection, the equation of the centre comes out 2° 30' larger from observations at the moon's quarters than during eclipses.
It was thus found by Ptolemy that an additional inequality existed in the motion, which is now known as the evection.
If we put g for the moon's anomaly or distance from the perigee, and D for its elongation from the sun, the inequalities in question as now known are 6.29° sin g (equation of centre) +1.27° sin (2D-g) (evection).