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Night, day nearly same length during equinox

Fall is coming up. What's the science behind the fall equinox?

— Mike G., Medford

We're glad you asked, Mike. There are two equinoxes every year, one in March and the other in September. These are standout days because, according to www.timeanddate.com, it's when "the sun shines directly on the equator and the length of day and night is nearly equal."

The autumnal (or fall) equinox happens the moment the sun crosses the celestial equator from north to south. The celestial equator is the imaginary line in the sky above Earth's equator. The fall equinox usually takes place on Sept. 22, 23 or 24 each year.

But why is this important? Earth's axis is always tilted at an angle of around 23.4 degrees, so at any other time, either the northern hemisphere or southern hemisphere is tilted slightly more toward the sun. During the equinoxes, the tilt of Earth's axis is perpendicular to the rays of the sun. On the equinoxes, day and night are practically the same length — 12 hours.

— Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. To see a collection of columns, go to mailtribune.com/youasked. We’re sorry, but the volume of questions received prevents us from answering all of them.