Horizontal parallax

The horizontal parallax ( HP ) of a celestial body is the apparent size of the earth's radius as seen from there . Double this angle would be the parallax , around which two observers, who are on earth as antipodes opposite, see the celestial body shifted against the fixed star background. For both observers, however, the celestial body would be on the horizon .

The HP decreases with the reciprocal value of the distance from the celestial body. It is about 1 ° (0.9 ° to 1.03 °) for the Earth's moon , much less for planets, and is specified in astronomical yearbooks and ephemeris in order to convert specified positions to the observation location or to select suitable observation locations, e.g. for star occultations .

The great parallax value of the moon helped the astronomers of antiquity to get an idea of the cosmic distances - because already on the north and south coasts of the Mediterranean the moon appears clearly shifted against the starry sky . Between Northern Europe and South Africa, this already accounts for two apparent moon diameters.

The average HP of the Sun , 8.794 ", was formerly deputy parameters for distances in the solar system , see solar parallax . Since the 1960s -Jahren you but does not measure the distance of the tracks more with the horizontal parallax of planets or asteroids, but through radar maturities to Venus and Mars, in conjunction with laboratory measurements of the speed of light .

The HP of the moon or a planet depends not only on the distance, but also on the exact shape of the earth . Since the earth is not an exact sphere, but flattened by 0.3 percent at the poles , the greatest possible HP for observers occurs at the earth's equator . This maximum value is called equatorial horizontal parallax and played a role in some scientific disputes as early as the 18th century when measuring degrees (see Jérôme Lalande , Paris Academy, Boscovich and others).

Horizontal parallax

See also