The longitude of a point P on the Earth’s surface is the angle, measured in degrees east or west, between the meridian through P and the prime meridian through Greenwich. If the meridian through P and the meridian through Greenwich meet the equator at P′ and G′ respectively, the longitude is the angle P′OG′, where O is the centre of the Earth. Longitude and latitude uniquely fix the position of a point on the Earth’s surface.