An altitude correction applied to moon altitude observations because of the relative proximity of the moon to the earth. The true altitude of a heavenly body is a measure of an arc of a vertical circle between the celestial horizon of an observer and the true direction of the body from the earth's centre. For all celestial bodies except the moon, the directions from the earth's centre are regarded as being the same as those from an observer on the earth's surface. For the moon, however, the angle subtended at the moon between lines which terminate respectively at the observer and at the earth's centre may be as much as nearly a degree of arc. The parallax correction for a moon sight depends upon the distance between the earth and moon, the latitude of the observer, and the altitude of the moon. Tables of parallax for moon observations are normally included in all compilations of nautical tables and almanacs.
Subjects: Maritime History.