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Horizontal Parallax The maximum value of the parallax angle of a celestial body, corresponding to its observation on the horizon. It is only used for moon-sight reductions (Workforms, Box 2) and is tabulated in the Nautical Almanac. Geometrically, HP is the angle between two lines, one from the center of the moon to the center of the earth, the other from the center of the moon to the edge of the earth. This angle is about 56', but it changes slightly from day to day as the distance to the moon changes along its elliptical path around the earth. See Parallax. Abbreviation:  HP

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