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BAA Journal 2019 February

The mean distance from the Earth to the Moon

Journal issue: 2019 February
Pages: 27–28


What is the mean distance between the Earth and Moon? In this paper, the Earth–Moon distance is defined to be that between the centres of the two bodies; ‘mean’ herein refers to the mean of this value with respect to time.

Parallax and distance

For an observer on the surface of the Earth, the parallax of the Moon – at a given instant – is the difference between the observed position of the Moon's centre, and the position of this centre for an observer situated at the centre of the Earth. This parallax changes continually by reason of the motion of the observer, due to the rotation of the Earth. It is greatest when the Moon is on the horizon: the ‘horizontal parallax’.

Actually, the horizontal parallax is half the angular diameter of the Earth as seen from the centre of the Moon. By reason of the flattening of the Earth’s globe, the horizontal parallax of the Moon depends on the geographical latitude. It is largest for a point on the equator: the ‘equatorial horizontal parallax’; which is denoted here by EHP. Because the Earth–Moon distance is continually changing, the Moon’s EHP continually changes too. Its value for every day of the year is given in the great astronomical almanacs, such as the Astronomical Almanac published in the United States. (continued)...

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