Astronomical coordinates -latitude jА and longitude lА are used to determine the position of points on the surface of the Earth.
Astronomical latitude jА on the surface of the Earth is the angle between the plane of the celestial equator and the plumb line at that point.
Plumb line - a line that at each point in space coincides with the direction of the gravitational force at that point. Plumb line is measured directly using special tools.
Astronomical longitude lА point on the Earth's surface is called the dihedral angle between the planes of the prime meridian and the heavenly (celestial) of the meridian of the point.
Plumb line does not generally coincide with the direction normal to the surface of the ellipsoid. At the same time in different points of the Earth to the plumb line deviation from the normal (the deviation of the slope) is different, it depends on the type of accommodation of different density mass in the body of the Earth. The largest deviations are observed in the mountainous regions and near the coastline of the oceans and seas. Examples of such variations can lead to regions such as, for example, Baikal - 40 "in the Caucasus - 35".
The approximate solution of problems that do not require high accuracy, it is assumed that the directions of the plumb line and the normal. This assumption can be considered negligible difference between the values of astronomical and geodetic coordinates in this case, they share the name - the geographical coordinates.
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