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The graticule, the mesh formed by the lines of constant latitude and constant longitude, is constructed by 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 in 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 orthogonal to the rotation axis intersects the surface in a great circle called the equator. Planes parallel to the equatorial plane intersect the surface in circles of constant latitude; these are the parallels. The equator has a latitude of 0°, the North pole has a latitude of 90° north (written 90° N or +90°), and the South pole has a latitude of 90° south (written 90° S or −90°). The latitude of an arbitrary point is the angle between the equatorial plane and the radius to that point.
The latitude that is defined in this way for the sphere is often termed the spherical latitude to avoid ambiguity with auxiliary latitudes defined in subsequent sections.

In 1687 Isaac Newton published the Principia in which he included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of an oblate ellipsoid.[5] (This article uses the term ellipsoid in preference to the older term spheroid). Newton's theoretical result was confirmed by precise geodetic measurements in the eighteenth century. (See Meridian arc). The definition of an oblate ellipsoid is the three dimensional surface generated by the rotation of a two dimensional ellipse about its shorter axis (minor axis). 'Oblate ellipsoid of revolution' is abbreviated to ellipsoid in the remainder of this article: this is the current practice in geodetic literature. (Ellipsoids which do not have an axis of symmetry are termed tri-axial).
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