Weird Science: Polar Circles And Tropical Circles

SF Fig. 1.9. Polar circles and tropical circles are shown in black lines ending in arrows. The equator, 30° N, 30° S, 60° N, and 60° S latitude lines are all shown in white.

Image by Byron Inouye

In addition to the equator, there are four other prominent circles (parallels) of latitude used to distinguish areas of the sun’s interaction with the earth. The polar circles are located near the poles of the earth, at 66.6° N and S latitude. These are called the Arctic Polar Circle and the Antarctic Polar Circle (SF Fig. 1.9). These circles denote the most northern and southern locations where, at least once a year, the sun remains continuously above the horizon for 24 hrs.

In the winter, the areas above the Arctic Polar Circle, and the areas below the Antarctic Polar Circle, experience days when the sun doesn’t rise. In the summer, however, they experience days when the sun doesn’t set! Closer to the equator, at 23.4° N and S of the equator, the Tropic of Cancer and the Tropic of Capricorn are located at the most northern (Cancer) and southern (Capricorn) positions where the sun is directly overhead at least once a year.

SF Fig. 1.10. The location of the Tropic of Cancer crossing a Mexican highway changes over time. The years on the signs show the movement of the Tropic of Cancer.

Image courtesy of Robert González (retrieved from Wikipedia)

The positions of these circles depend on the tilt of the earth’s axis in relation to its orbit around the sun. Today, the earth is titled at approximately a 23.4° angle, which is called its axial tilt. However, the angle of the axial tilt is not fixed; the axial tilt varies based on a number of complex cycles. The axial tilt of the earth is currently decreasing at a very slow rate. As the axial tilt decreases, the positions of the polar and tropical circles shift. This means that the polar circles are slowly migrating towards the poles and the tropic lines are slowly migrating towards the Equator (see SF Fig. 1.10).