EM SC 100S
First Year Seminar

Understanding the Equinox and the Solstice


Understanding the Equinox and the Solstice

All matter is imbued with gravitational attraction. Both the earth and the sun exert gravitational attraction on each other, but because of its massive size, the sun’s gravitational attraction is far greater than that of the earth. Therefore, the gravity of the sun holds the earth in its orbit. If the gravitational attraction of the sun were equal to that of the earth, centrifugal force would cause the earth to escape its orbit.

As the earth revolves around the sun (in its orbit), it remains tilted at an angle that is 23.5 degrees from the vertical. Therefore, during the winter months in the northern hemisphere, the southern hemisphere is tilted toward the sun, and during the winter months in the southern hemisphere, the northern hemisphere tilts toward the sun.

When the direct rays bear down directly on the equator on March 20/21, the earth is experiencing the vernal equinox. After the vernal equinox, the most direct rays of the sun striking the earth continue to migrate north. On June 21/22, the most direct rays of the sun fall on locations situated at 23.5 degrees north latitude. This is the summer solstice. At that time, days are long in the far north while darkness prevails in the polar extremes of the southern hemisphere.  After the summer solstice, the earth continues on its journey around the sun and the most direct rays of the sun migrate south. On December 21/22 the sun’s most direct rays once again fall directly on the Tropic of Capricorn. This is the winter solstice (when the circle of illumination cuts directly through the north and south poles). Once the winter solstice has occurred, the path of the earth’s orbit again heads toward the Autumn Equinox. If the earth were not tilted on its axis, there would be no seasons.

Please watch the following (6:00) video explaining the solstices and equinoxes.

Check Your Understanding

Which of the following is true?

A. The tropics are a zone of convergence.
B. A permanent heat low exists at 30 degrees north latitude.
C. During summer, the earth is warmer because it is closer to the sun at that time.
D. The earth’s tilt on its axis and its revolution around the sun accounts for the seasons.

Click for answer.

The answer is D.