Two days, the Equinoxes. Equinox is derived from the latin words for "equal" and "night", and there are two of them a year, the Autumnal Equinox and the Vernal Equinox.
The roots are equi = equal, and nox = night. They refer to the days when day and night are of equal length.
At the time of the equinox, daytime and night time are as close to equal in duration as they can get.
The times of year when the days and nights are equal length is known as the equinox (literally means equal night). For 2010 the Vernal Equinox is March 20th and the Autumnal Equinox is September 23rd.
It depends where you are on the planet. In the Northern hemisphere, just two days are of equal length during the year - they are the spring & autumn equinox. However - if you're stood on the equator, every day will have the same length !
The summer and winter solstice have equal night and day.
One of them is named "March 21st", and the other is named "September 22nd". They are the days during which the sun appears to reach the two points among the stars known as the "equinoxes".
When night and day are approximately of equal length occurs twice per year. In the northern hemisphere these days are called the Vernal Equinox (20-21 March) and the Autumnal Equinox (22-23 September). However, as these seasons are reversed in the southern hemisphere, current usage is the March Equinox and the September Equinox.
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