# Evection

Evection causes the Moon's ecliptic longitude to vary by approximately ± 1.274° (degrees), with a period of about 31.8 days. The evection in longitude is given by the expression $+4586.45''\sin(2D-\ell )$ , where $D$ is the mean angular distance of the Moon from the Sun (its elongation, and $\ell$ is the moon's mean angular distance of the moon from its perigee (mean anomaly).
The evection opposes the Moon's equation of the center at the new and full moons, and augments the equation of the center at the Moon's quarters. This can be seen from the combination of the principal term of the equation of the center with the evection: $+22639.55''\sin(\ell )+4586.45''\sin(2D-\ell ).$ At new and full moons, D=0° or 180°, 2D is effectively zero in either case, and the combined expression reduces to $+(22639.55-4586.45)''\sin(\ell ).$ At the quarters, D=90° or 270°, 2D is effectively 180° in either case, changing the sign of the expression for the evection, so that the combined expression then reduces to $+(22639.55+4586.45)''\sin(\ell )$ .