The equator forms a great circle , or a maximum circumference of the earth, because it splits the earth into two equally sized halves, the northern and southern hemispheres. Like all great circles, the equatorial plane passes through the center of the earth. Latitude is measured as the angular displacement from the equatorial plane formed by a ray extending from the center of the earth to the point in question. Every point on the earth's surface has a latitude value. Since they never cross one another, they are often referred to as parallels.
All longitudes form half of a great circle.
They all intersect at the north and south poles. Longitude is measured as the angular displacement away from a universally agreed upon reference, the prime meridian . All points of the earth surface have a longitude value. Always be sure to indicate whether it is east or west of the prime meridian, in the eastern or western hemisphere. Each point on the earths surface is defined by a unique set of latitude and longitude values.