Equinox is one of two special days of the year. On these two days, day and night are almost the same length everywhere on Earth. One equinox happens on March 19, 20, or 21. The other happens on September 22 or 23. Image to left: The equinoxes happen when the sun's rays shine directly on the equator, the imaginary line around the middle of the earth. In this picture, the dotted lines show where the sun's rays hit the earth directly at different times of the year. Credit: World Book diagram

The length of day and night changes throughout the year because of the way Earth moves around the sun. In winter, days are short and nights are long. In summer, days are long and nights are short. In between, at the start of spring, there comes a time when day and night have the same length. This time is the spring equinox. The other equinox happens on the first day of autumn. It is the autumnal equinox. How to cite this article: To cite this article, World Book recommends the following format: "Equinox." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.