Equatorial-horizontal parallax

As Equatorial horizontal parallax is the greatest parallactic to the displacement of a line of sight from the earth moon designated.

It is the angle at which the earth's radius appears from the moon and averages 3422 "(0.95 °), but can be 8 percent larger or smaller because of the moon's elliptical orbit and its changing distance. It occurs for an observer on Earth's equator then, when the moon is on the horizon is, and lets him about his twice the diameter appear deeper than the center of the earth the. on - or setting moon moves in front of the stars in opposite directions.

A similar effect occurs at the north and south poles of the earth when the moon is close to the celestial equator . This horizontal parallax , however, is 11–12 "smaller because the polar semiaxis of the earth ellipsoid is 21 km or 0.3 percent shorter than at the equator. The astronomer Jérôme Lalande discussed this question with Parisian academy colleagues in the 18th century .