Latitude and Longitude

Locations on the earth's surface are specified in terms of the earth's axis of rotation. A great circle is any circle on the earth's surface whose center is the earth's center. The equator is a great circle midway between the North and South Poles. A meridian is a great circle that passes through both poles, and it forms a right angle with the equator. The prime meridian passes through Greenwich, England. The longitude of a point on the earth's surface is the angular distance between a meridian through this point and the prime meridian; the prime meridian is assigned the longitude 0°, and longitudes are given in degrees east or west of the prime meridian. Thus a longitude of 60°W identifies a meridian 60° west of the prime meridian.

The latitude of a point on the earth's surface is the angle between a line from the earth's center to it and another line drawn from the center to a point on the equator on the same meridian. Thus the latitude of the North Pole is 90°N, that of the South Pole is 90°S, and that of the equator itself is just 0°. A latitude of 60°N identifies a circle 60° north of the equator. Degrees of latitude and longitude are further divided into minutes [1° = 60 minutes (60')], and minutes are further divided into seconds [1' = 60 seconds (60')]. A nautical mile is equal to a minute of latitude, and is 6,080 ft in length. (A statute mile, used on land, is 5,280 ft in length.) Because of the way they are defined, parallels of latitude are equally spaced everywhere (hence the usefulness of the nautical mile), but meridians of longitude converge at the poles.

On December 22, the shortest day of the year in the Northern Hemisphere, the 23.5° tilt of the earth's axis means that no sunlight reaches any point within 23.5° from the North Pole. The Arctic Circle is the boundary of this region of darkness. On the same day, which is the longest day of the year in the Southern Hemisphere, there are 24 h of daylight at all points within 23.5° of the South Pole, and the Antarctic Circle is the boundary of this region of daylight. On June 22 the situations in the two hemispheres are reversed.

The Tropic of Cancer is the most northerly latitude in the Northern Hemisphere at which the sun is ever directly overhead at noon. The Tropic of Capricorn is corresponding latitude in the Southern Hemisphere. On June 22, when the North Pole is titled closest to the Sun and which is hence the day of maximum sunlight in the Northern Hemisphere, the noon sun is directly overhead 23.5 º north of the equator; hence the latitude of the Tropic of Cancer is 23.5º N. Similarly the latitude of the Tropic of Capricorn is 23.5 º S; the South Pole is tilted closest to the sun on December 22, when the noon sun is directly overhead at this latitude.