Locations on Earth can be specified using a spherical coordinate system.
The geographic (“earth-mapping”) coordinate system is aligned
with the spin axis of the Earth. It defines two angles measured from
the center of the Earth. One angle, called the *Latitude*,
measures the angle between any point and the Equator. The other angle, called
the *Longitude*, measures the angle
*along* the Equator from an arbitrary point on the Earth
(Greenwich, England is the accepted zero-longitude point in most modern
societies).

By combining these two angles, any location on Earth can be specified. For example, Baltimore, Maryland (USA) has a latitude of 39.3 degrees North, and a longitude of 76.6 degrees West. So, a vector drawn from the center of the Earth to a point 39.3 degrees above the Equator and 76.6 degrees west of Greenwich, England will pass through Baltimore.

The Equator is obviously an important part of this coordinate system; it
represents the *zeropoint* of the latitude angle, and the
halfway point between the poles. The Equator is the *Fundamental
Plane* of the geographic coordinate system. All Spherical Coordinate Systems define such a
Fundamental Plane.

Lines of constant Latitude are called *Parallels*. They
trace circles on the surface of the Earth, but the only parallel that is a Great Circle is the Equator (Latitude=0
degrees). Lines of constant Longitude are called
*Meridians*. The Meridian passing through Greenwich is the
*Prime Meridian* (longitude=0 degrees). Unlike Parallels,
all Meridians are great circles, and Meridians are not parallel: they intersect
at the north and south poles.

### Tip

Exercise:

What is the longitude of the North Pole? Its latitude is 90 degrees North.

This is a trick question. The Longitude is meaningless at the north pole (and the south pole too). It has all longitudes at the same time.