Latitude measures the vertical axis, which describes how far north or south a location is. The zero point for latitude is the equator. To the north, the values increase until reaching 90 degrees at the pole. South of the equator, latitude decreases, with 90 degrees being the other pole. The horizontal axis measurement is called longitude. Longitude describes how far west or east a location is. The zero point - location of George Biddell Airys telescope in Greenwich, London, to be the Prime Meridian. To the east of this spot, longitudes increase to 180 degrees on the other side of the earth. Similarly, longitude decreases to 180 degrees to the west.