A great circle around the earth dividing the
Northern and Southern hemispheres. Starting
line for latitude. A great circle is a circle that is the
intersection of the surface of a sphere with a
plane passing through the center of the sphere.
Parallels of latitude (L) are used to locate points
north or south from the equator (0° L) to the poles
(90° L). The suffix "north" or "south" is an
essential part of the description and must always
be included. Parallels of latitude are always
parallel to the plane of the equator.
A line running from the North to South Pole
through Greenwich, England and the starting line
for the measurement of longitude.
Meridians of longitude (LO) are used to locate
points east and west. LO is based on great
circles passing through the poles. These great
circles are divided in half by the poles, with each
half being assigned a value of east or west. LO is
measured in degrees of arc, from 0° to 180° east
or west from the prime meridian. The suffix "east"
or "west" is an essential part of the description
and must always be included.
Using a "grid" composed of lines of latitude and longitude superposed over the earth,
you can understand how any point or position on earth can be accurately plotted.