The Moon is in orbit around the Earth, and, like a planet orbiting the Sun, its-motion is along an ellipse with semi-major axis 384 390km and eccentricity 0.0549. Because of the ellipticity of the orbit the distance-between the Earth and the Moon varies between a minimum of 356400km and a maximum of 406700km. The point of the orbit closest to the Earth is called PERIGEE and the point furthest away APOGEE. These terms are also used for artificial satellites in orbit around the Earth and correspond to the perihelion and aphelion of an orbit around the Sun. .The Moon goes round its orbit in the same direction as all the planets go round the Sun, in a SIDEREAL MONTH of 27.3217 days, after which time it returns to the same place among the stars. Because of the Earth’s motion round the Sun, the Moon takes 29.5306 days to return to the same position relative to the Sun; this interval is called a SYNODIC MONTH . As the Moon orbits the Earth it turns a varying proportion of its illuminated face to the Earth; this gives rise to the PHASES of the Moon.

The Moon’s orbit is inclined at 5° 9′ to the plane of the Earth’s orbit (the ecliptic). The orbit has an ASCENDING NODE, where the Moon crosses the ecliptic from south to north and a DESCENDING NODE where it crosses from north to south. The gravitational pulls of the Sun and the planets perturb the Moon’s orbit and this has two important effects. First, the perigee moves round the Earth with direct motion, i.e. in the same direction as the Earth rotates, with a period of 8.85 years. (This is also the direction in which each of the planets orbits the Sun and the Moon orbits the Earth.) Second, the line joining the nodes, which is the line along which the orbits of the Earth and the Moon intersect, has RETROGRADE motion in the ecliptic, i.e. the opposite of direct motion, with a period of 18.61 years, the so-called NUTATION PERIOD. This motion has a counterpart in the nutation of the Earth’s axis, a slight nodding of 9.210 arc sec with the same period. The time between successive passages of the Moon through the same node is consequently less than a sidereal month and is equal to 27.2122 days, the DRACONITIC, Or NODAL MONTH.

As the Moon goes round its orbit, it broadly presents the same face to the Earth; this is due to the equality of its rotation period of its period of revolution, the sidereal month. The part of the moon seen from the Earth is not exactly the same because of several wobbles, or LIBRATIONS. The Moon’s equator is inclined at-an angle of 6° 41′ to its orbit which causes a libration in latitude of 6° 41′. The Moon rotates on its axis at a uniform rate, but, in accordance with Kepler’s second law, the Moon’s motion in its elliptical orbit is not at a uniform speed and consequently there is a libration in longitude of 7° 36′. As the Earth rotates on its axis, a terrestrial observer sees the Moon from different positions; he can thus see round the edges a little and this is called a daily libration. The combined effect of these librations, and a number of much smaller ones, is to make 59 per cent of the Moon’s surface visible from the Earth at least some of the time; of this 41 per cent is always visible and 18 per cent occasionally comes into view.

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