The Moon's Orbit
The motion of the Moon relative to the stars is direct, i.e. from West to East. It moves along the 'Celestial Sphere' at 13.2° per day, or about a half a degree per hour.
The Sun's pull on the Moon is twice as large as the Earth's pull on it, although the Sun is pulling the Earth and the Moon almost equally. Seen from a distance, the Moon's orbit is always concave to the Sun, i.e. a standard planetary orbit.
The two most significant irregularities affecting the motion of the Moon are
- Retrograde motion of the line of nodes the line of nodes being the line of intersection between the Moon's orbit and elliptic. This retrograde motion is in the plane of the ecliptic and has a period of 18.6 years.
- Advance in the line of apsides the line being the line joining perigee and apogee in the plane of the orbit.This motion is direct, it is in the palne of the Moon's orbit and has a period of 8.85 years (3232.6 days).
The eccentricty of the moon's orbit can vary. When the line of apsides points towards the Sun (which will happen when they coincide with the New or Full Moon, the eccentricity reaches a maximum.
On the other hand, when the line of apsides coincides with First or Third quater, they will be perpendicular to the direction of the Sun, the eccentricty will be a minimum.