We all know that the moon always presents the same face to observers since it takes as long to turn once on its axis as it does to complete one orbit of the Earth.

The Moon's orbit is elliptical rather than circular, so it speeds up near perigee (closest to Earth) and slows down near apogee (farthest from Earth) in accordance with Kepler's laws. The Moon's speed of rotation about its axis remains essentially constant from month to month as a consequence of the conservation of angular momentum.

The Moon's orbit is also tilted to the ecliptic plane and to the Earth's equator by about 5°. As a consequence of these factors, the Moon appears to 'nod' from side to side and up and down during a lunar month, and it is possible to observe about 59% of the Moon's surface over a period of time, although we can only see 50% at any one instant.

The term given to this 'nodding' is "Libration". There are in fact three types of libration involved in the moon's motions. Libration in latitude is due to the Moon's axis being slightly inclined relative to the Earth's. The moon’s poles will appear to be alternately tipped slightly toward and away from the terrestrial observer over a roughly four week cycle.

Diurnal libration is due to the observer being up to four thousand miles to one side of the Earth-Moon axis on the surface of the Earth - a significant proportion of the centre-to-centre distance. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to the west and then to the east.

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west oscillation.

Although the Moon always presents the same face towards the Earth, due to its rotation and revolution being locked to the same period, the combined effect of these different librations allows us over time to see some 59% of its surface.

LunarPhase Pro displays numerical Libration information in the Libration window. The information is presented in two formats - as distinct North-South and East-West rotations in degrees (e.g.: N-S: 3° 15' 04"; E-W -1° 38' 18") and as an overall figure in a specific direction (e.g.: 4° 2' 41" in PA 275° 07' 21" [PA stands for Position Angle]). It is this position that is indicated by the yellow dot in the Current Phase Window.

Clicking on the (
4) icon in this window calls up the Lunar Libration Diagram screen.

LunarPhase Pro also calculates the selenographic position of the Sun, which determines what part of the lunar surface is illuminated. From this, the Selenographic CoLongitude is calculated. This is the CoLong figure shown in this window. The figure is typically used for calculating sunrise and sunset times at various lunar features. LunarPhase Pro does this for you. See Sunrise/Sunset at Lunar Features.

The final figure presented in this window is the Position Angle of the Moon's Axis of rotation.