Libration

Libration

Algorithms

Calculates the ephemerides for physical observation of the moon

procedure OpticalLibration(date: TDateTime; var latitude,longitude:extended);

procedure PhysicalLibration(date: TDateTime; var latitude,longitude:extended);

Description

The moon always shows the same side to the earth, however, the excentricity of the moon's orbit and the inclination of the moon equator to the ecliptic cause the actually visible part of the moon surface to be in fact 59% instead of just 50%, an effect called libration. This can be put into numbers by the selenographic coordinates at which the earth is in zenith, the so-called CoLongitude and CoLatitude. The main effect of the libration is because of the orbit, and is called the Optical Libration. The actual rotation of the moon changes slightly from the mean rotation, this also affects the libration, however, it is much smaller than the optical libration and always smaller than 0.04°. The Physical Libration, thus, is the addition of both effects.

Reference

These functions are based upon chapter 53 (51) of "Astronomical Algorithms".