Equinox

Equinox is a point in time of the year when Earth's axial tilt is inclined neither away nor towards the sun. It happens twice a year around the 21st of March and 21st of September, i.e. when Spring and Autumn start. Around these times of the year, the length of day is roughly equal to the length of night.
See Also: http://en.wikipedia.org/wiki/Equinox

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