The Moon's Orbit



Apparent Motion

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

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.

Important Perturbations

The two most significant irregularities affecting the motion of the Moon are

Evection

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.