An equinox is an astronomical event in which the imaginary plane of Earth's equator passes the center of the Sun, making night and day of approximately equal length all over the planet. The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the Equator. As a result, the northern and southern hemispheres are equally illuminated.

In the Northern Hemisphere, the Spring Equinox occurs around the 21st of March, and the Autumnal Equinox around the 23rd of September.

People first witnessed this curious phenomenon many years ago, so there are a lot of interesting traditions and superstitions connected with an equinox that still survive. In some cultures, the Spring Equinox marks the beginning of a new year. Others just say goodbye to winter and darkness, holding various spring festivals. The Autumnal Equinox is often called the Second Harvest. Pagans, for example, thank summer for the harvest and welcome the winter darkness, as days become shorter than nights.

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