出差

由出差导致的月球黄经变化量约为±1.274°（度），周期约为31.8天。经度的出差由下列表达式给出： +4586.45''sin(2D-ℓ)， 其中 D 是月球到太阳的平均角距离（它的距角，elongation），而 ℓ 是月球与其近地点的平均角距离（平近点角，mean anomaly）。

它来自以下两种运动的叠加：月球轨道偏心率以约6个月为周期的变动；以及月球近地点以类似周期进行的天平动。它是由太阳的作用引发的。

根据这两种均差的主要项的和式： +22639.55''sin(ℓ)+4586.45''sin(2D-ℓ). 可见，出差在新月和满月时与月球的中心差方向相反，在弦月时与中心差方向相同：

在新月和满月时，D=0°或180°，2D都等效于零度，上述和式化简为 +(22639.55-4586.45)''sin(ℓ)。

在弦月时，D=90°或270°，2D都等效于180°，出差项变号，上述和式化为 +(22639.55+4586.45)''sin(ℓ)。

In astronomy, evection (Latin for "carrying away") is the largest inequality produced by the action of the Sun in the monthly revolution of the Moon around the Earth. The evection, formerly called the moon's second anomaly, was approximately known in ancient times, and its discovery is attributed to Hipparchus or Ptolemy. The current name itself dates much more recently, from the 17th century: it was coined by Bullialdus in connection with his own theory of the Moon's motion.

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-ℓ), where D is the mean angular distance of the Moon from the Sun (its elongation), and ℓ is the mean angular distance of the Moon from its perigee (mean anomaly).

It arises from an approximately six-monthly periodic variation of the eccentricity of the Moon's orbit and a libration of similar period in the position of the Moon's perigee, caused by the action of the Sun.

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(ℓ)+4586.45''sin(2D-ℓ).

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(ℓ).

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(ℓ).