Moon distance calculation methods

The Earth-Moon separation changes continuously because the lunar orbit is not a perfect circle. Distance and angular size describe one UTC instant; the neighbouring perigee and apogee are the local minimum and maximum distances around it.

Instantaneous geometry and orbital turning points are different quantities. A current distance reading does not by itself identify an apsis, and apparent size depends on its stated observer convention.

Earth and Moon distance diagram on an astronomy workbench

Calculation sequence

How this method is divided

01
The reported distance is geocentric: center of Earth to center of Moon. Apparent angular diameter is the geometric width of the lunar disc from Earth's center at that distance. Sub-Earth libration and geocentric ecliptic coordinates describe additional geometry, while local observing conditions remain separate.
02
For apsides, the calculation identifies distance extrema on both sides of the input. Previous and next perigee and apogee remain distinct result pairs. The nearest event is chosen by elapsed time, so it need not be the event with the smallest or largest distance among all values shown.

Working sheets

Two focused guides

01Moon Distance and Angular SizeSee how geocentric Earth-Moon distance determines the Moon's angular diameter, with an exact-instant example and clear observer conventions.

02Lunar Perigee and Apogee CalculationLearn how lunar perigee and apogee are found as local geocentric distance extrema, and how previous, next, and nearest apsides are selected.

Method selection

Where this method fits

Use this method when the required reference point is the centre of Earth.
Apparent angular size here is a geometric angle, not a camera crop or a horizon-perception effect.
Use the position calculator for an observer-specific topocentric range and altitude.

Put the method to work

Use the working calculation

Evaluate another exact instant with the calculator described here, or return to the overview for the shared time conventions and supported era.

Open Moon Distance Calculator

Method overview · Calculation sources