Positions on Earth are determined using two numbers that represent latitude and longitude. These numbers are actually two angles, measured in degrees (°), minutes of arc ('), and seconds of arc (''). On a globe of the Earth, latitude lines circle parallel to the equator, and differ in length depending on their location. The longest line is at the equator (latitude 0 degrees); the shortest lines—actually pinpoints—are at the poles (90 degrees north at the North Pole; 90 degrees [or -90 degrees] south at the South Pole). In the Northern Hemisphere, latitude degrees increase as you move north away from the equator; in the Southern Hemisphere, latitude degrees increase as you move south away from the equator.

Longitude lines, or meridians (once called “meridian lines” and eventually shortened to “meridians”), are those that extend from pole to pole, slicing the Earth like segments of an orange, with each meridian crossing the equator. In the Western Hemisphere, longitude increases as you move west from Greenwich, England (0 to 180 degrees). In the Eastern Hemisphere, longitude also increases as you move east from Greenwich, England (again, 0 to 180 degrees). All points on the same line of longitude experience true noon (and any other hour) at the same time. But note: Longitude lines are not to be confused with time zones, most of which follow a more erratic demarcation. (For more information on time zones, see below; for more information about latitude and longitude with regard to polar coordinates, see “Geometry and Trigonometry.”)