Sub-Earth Point(glossary entry)
The sub-Earth point is the location on the surface of the Moon where a particular point on the Earth is directly overhead. It can be found by drawing an imaginary line from the center of the Moon to the center of that point on the Earth. It is normally described by giving the selenographic longitude and latitude of that point.
- If not otherwise specified, the sub-Earth point would normally be assumed to be referenced to the center of the Earth. However, it is almost invariably meant to mean the location of the sub-Earth point for an observer at a specific location on the Earth's surface -- something that is more properly referred to as the Sub-Observer Point.
- The numeric values of the longitude and latitude of the Sub-Earth point are the same as the quantities referred to as the librations. If computed for an imaginary observer at the Earth's center, the result is the geocenric librations. If computed for a specific observer on (or above) the surface, the result is the topocentric librations.