You can't see them, but they're there. Points and lines on a map define not only where you are, but also when you're there. Navigators still rely on these imaginary lines to get where they are going. You can use them, too.

The Antarctic Circle

The Antarctic Circle lies three-quarters of the way between the equator and the South Pole.

The Arctic Circle

Three-quarters of the way between the equator and the North Pole lies the Arctic Circle. Above this line is the Arctic region, where nights last for 24 hours in the middle of winter. It is known as the Land of the Midnight Sun because in summer the sun never sets.

DEW Line

The DEW (distant early warning) line is a 3,000-mile line of radar stations north of the Arctic Circle. It should notify the U.S. and Canada of the approach of enemy planes or missiles.

The Equator

This imaginary circle goes around the middle of the earth for 24,902 miles. It divides the Northern Hemisphere from the Southern Hemisphere and is exactly half way between the North and South Poles.

The International Date Line

An imaginary line where the date changes one day when passed. It is one day earlier east of the line than it is on the west.

Meridians

Imaginary lines that run north and south on a map from pole to pole. Meridians express degrees of longitude, or how far a place is away from the prime meridian. The prime meridian runs through Greenwich, England. Longitude is used together with latitude to form a grid on which it is possible to locate any place on the earth.

Parallels

Imaginary lines that run east and west on a map. Parallels represent degrees of latitude, or how far a place is away from the equator. The equator's latitude is 0° and the poles are 90° south and north. One degree of latitude equals about 69 miles.

The Tropic of Cancer

A parallel line of latitude that is a quarter of the way from the equator to the North Pole. During the summer solstice, the sun is directly overhead.

The Tropic of Capricorn

This line of latitude is a quarter of the way from the equator to the South Pole. During the winter solstice, the sun is directly overhead.

The Hemispheres

The equator divides the earth into two halves, or hemispheres. The Northern Hemisphere is the half of the earth between the North Pole and the equator. The Southern Hemisphere is the half of the earth between the South Pole and the equator.

The earth can also be broken up another way: into the Eastern Hemisphere and the Western Hemisphere. The Western Hemisphere includes North and South America, their islands, and the surrounding waters. The Eastern Hemisphere includes Asia, Africa, Australia, and Europe.