Latitude constant latitude and constant longitude, which are constructed with reference to the rotation axis of the Earth. The primary reference points are the poles where the axis of rotation of the Earth intersects the reference surface. Planes which contain the rotation axis intersect the surface at the meridians; and the angle between any one meridian plane and that through Greenwich (the Prime Meridian) defines the longitude: meridians are lines of constant longitude. The plane through the centre of the Earth and perpendicular to the rotation axis intersects the surface at a great circle called the Equator. Planes parallel to the
International Latitude Observatory The data collected by the observatories over the years still has use to scientists, and has been applied to studies of polar motion, the physical properties of the Earth, climatology and satellite tracking and navigation. The final six observatories were located, in order of Longitude (E to W), in: International Latitude Observatory The International Latitude Observatories were a system of (originally) six observatories located near the parallel of 39° 08' north latitude. They were used to measure the variation in latitude that occurs as a result of the "wobble" of the Earth on its polar axis. The original six observatories