9.2 Latitude and longitude

9.2 Latitude and longitude

The intersection of a sphere and a plane is a circle The intersection of a sphere and a plane is a circle. If the plane passes through the centre of the sphere, the circle is called a great circle. All other circles are called small circles.

To locate the position of any point on a plane (flat) surface, we use a system of coordinates with perpendicular axes called the Cartesian system. This cannot be done for the curved surface of a sphere. To locate the positions of points on a sphere, we use a system based on great and small circles.

The circles perpendicular to the polar axis are called parallels of latitude. All parallels of latitude are small circles, with the exception of the Equator (0°), which is the only great circle that is perpendicular to the polar axis. Parallels of latitude are measured in degrees north or south of the Equator. In the left-hand diagram on the facing page, P has latitude 50°N. Great circles that pass through the North and South Poles are called lines of longitude. They are split into semi-great circles called meridians of longitude. The Prime Meridian (0°) passes through Greenwich in London. Longitude is measured in degrees east or west of the Prime Meridian to make the angle less than 180°. In the top right-hand diagram on the facing page, P has longitude 20°W.

When the position of a point is stated, the latitude is stated first and then the longitude. So, in the diagram above, the position of P would be stated as 50°N20°W.

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