[E' kwi noks'] one of two days of the year when the Sun is directly above the Equator, meaning that the days and nights are of nearly equal length everywhere on Earth

The term equinox comes from a Latin word meaning "equal night."

The equinoxes occur on March 20 or 21 and on September 22 or 23. In the Northern Hemisphere, the March equinox marks the beginning of spring and is often called the vernal equinox, while the September equinox, also known as the autumnal equinox, marks the beginning of autumn. The two equinoxes are "reversed" in the Southern Hemisphere.

The time interval from the March equinox to the September equinox is longer than that between the September equinox and the next March equinox. This time difference results from the Earth's elliptical orbit around the Sun. The Earth moves faster in its orbit when it is closer to the Sun, and slower when farther away. And, because the distance between the Earth and the Sun is shortest in January, the Earth completes the semicircle from the September equinox to the September equinox faster than it does the opposite semicircle.

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