The Mathematics of the Longitude - Department of Mathematics

The most important line of latitude is the Equator (0°). The North Pole is
90° North (90°N) and the South Pole is 90° South (90°S). All other lines of
latitude are given a number between 0° and 90°, either North (N) or South
(S) of the Equator. Some other important lines of latitude are the Tropic of
Cancer (23.5°N), Tropic of Capricorn (23.5°S), Arctic Circle (66.5°N) and
Antarctic Circle (66.5°S).
Lines of longitude are imaginary lines which run in a north-south direction,
from the North Pole to the South Pole (Figure 2.2). They are also measured
in degrees (°).
Figure 2.2 Lines of longitude.
Any circle on the surface of a sphere whose plane passes through the center
of the sphere is called a great circle. Thus, a great circle is a circle with the
greatest possible diameter on the surface of a sphere. Any circle on the
surface of a sphere whose plane does not pass through the center of the
sphere is called a small circle.
A meridian is a great circle going through the geographic poles, the poles
where the axis of rotation (polar axis) intersects the earth's surface. The
upper branch of a meridian is the half of the great circle from pole to pole
passing through a given position; the lower branch is the opposite half.
The equator is the only great circle whose plane is perpendicular to the
polar axis. Further the equator is the only parallel of latitude being a great