Useful Definitions
Meridian
A meridian (or line of longitude) is the half of an imaginary polar great circle or great ellipse on the Earth’s surface, a coordinate line terminated by the North Pole and the South Pole. Each meridian is perpendicular to all circles of latitude. Meridians are half of a great circle on the Earth’s surface. The prime meridian is the meridian in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its anti-meridian (the 180th meridian in a 360° system) form a great circle. This great circle divides a spheroid into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.
Latitude
In geography, latitude is a geographic coordinate that specifies the north-south position of a point on the Earth’s surface. Latitude is an angle that ranges from 0° at the Equator to 90° (North or South) at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth.
Rhumb Line
In navigation, a rhumb line is an arc crossing all meridians of longitude at the same angle, that is, a path with constant bearing as measured relative to true north.
Great Circle
A Great Circle is any circle that circumnavigates the Earth and passes through the center of the Earth. A great circle always divides the Earth in half, thus the Equator is a great circle (but no other latitudes) and all lines of longitude are great circles. The shortest distance between any two points on the Earth lies along a great circle.
