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measurements that define the position of a point on the earth’s surface. Latitude (ø) is measured by the angle between a plumb line passing through a given point and the plane of the equator, and longitude (x) is measured by the dihedral angle between the plane of the meridian of a given point and the plane of the prime meridian. Latitude and longitude are determined from observations of heavenly bodies with the aid of angle gauges (for example, the universal theodolite and the sextant), which are placed with the help of a leveling weight, and from the comparison of local time (determined by astronomical observations) with universal time. Geographic coordinates determined in this way are called astronomical coordinates of a point on the earth’s surface.
Latitude is counted from 0° to 90° on both sides of the equator. In the northern hemisphere the latitude is considered positive, in the southern hemisphere, negative. Longitude is counted from the prime meridian from 0° to 360° either to the west (west longitude) or the east (east longitude—by international accord, positive). A system counting from 0° to 180° to the east and west of the prime meridian is also used.
By international agreement the base meridian (so-called prime or zero) is the meridian passing through the old Greenwich Observatory in Greenwich (London) before its transfer to Hurstmonceux Castle. Previously, meridians through other points, including the Island of Hierro (the Canary Islands) and the Paris and Berlin observatories, were used for this purpose at various times. In Russia during the 19th century longitude was calculated from the meridian of the Pulkovo Observatory.