Astronomical longitude
The astronomical longitude of a point on the earth's surface corresponds to its geographical longitude , but does not affect its position , but its perpendicular direction . It is the angle between the meridian plane of a point and the meridian plane in Greenwich, and equally the difference between the two sidereal times (local sidereal time minus Greenwich sidereal time).
The name "astronomical" comes from the fact that it can - like the astronomical latitude - only be determined by astronomical measurements ("astr. Location "). The astronomical longitude or latitude differs from the geographical longitude or latitude by the influence of the vertical deviation . The latter represents the deviation of the local vertical from its ideal direction (undisturbed by mountains / valleys).
The vertical deviation is caused by the mass irregularities of the earth's body and can reach about 50 ″ (0.005 °) in the high mountains (see also geoid ). Their components (north-south and east-west) are usually designated with the symbols ξ and η .
The relationship exists between astronomical latitude and longitude and their geographical (better: ellipsoidal ) equivalents
φ′ = φ + ξ und λ′ = λ + η/cos φ
For an exact geoid determination , one needs the vertical deviation in as many places as possible, and the astronomical coordinates φ ′, λ ′ are the tools to determine them. With a universal instrument (astronomical theodolite ) or with a zenith camera, one looks for the point of penetration of the extended plumb line in the starry sky (“astronomical” or “true zenith”); its celestial coordinates ( right ascension and declination ) can then be converted directly into the values φ ′, λ ′ .