While this seems rather abrupt this is the moment that defines the start of autumn in the northern hemisphere and spring in the southern hemisphere.
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An equinox is different from a solstice, where the sun hits its northernmost or southernmost position.
While meteorologists say autumn begins on September 1 each year, the autumn equinox can fall on September 22, 23 or 24 – depending on when the day and night lengths most closely match.
On the equinox, day and night are both roughly 12 hours long all over the world.
The autumnal equinox is one of 2 days out of the year when at nearly all points on the Earth, outside of the north and south poles, the sun rises due east and sets due west.
A Slooh spokesman said: “Equinox marks the time, twice each year, when days and nights have equal length and the world moves through a time of harvest or renewal”.
Day and night aren’t exactly 12 hours long because the Earth’s atmosphere refracts, or bends light, in an optical illusion that brings more daylight than there really is.
There are a couple of reasons for this. Sunrise and sunset is defined as when the Sun’s top edge and not its center cross the horizon. We see this by the fact that solar noon, the time when the sun reaches its maximum height each day, drifts 10 minutes earlier during the month of September.
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Our daylight hours have been getting shorter since the summer solstice and will continue to until the winter solstice Sunday, December 21, at 9:49 pm MST.
