The sun will cross the equator heading south at 9:44 a.m. Sept. 22 marking the end of northern hemisphere summer and the beginning of fall.

Jimmy Westlake

The sun will cross the equator heading south at 9:44 a.m. Sept. 22 marking the end of northern hemisphere summer and the beginning of fall.

Jimmy Westlake: Autumnal Equinox

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Autumn officially arrives for the northern hemisphere this year at 9:44 a.m. Sept. 22. How do astronomers determine the precise moment marking the beginning of autumn?

Our season of autumn begins the instant the sun crosses the equator on its way south. Thanks to the 23.5 degree tilt of the Earth's axis of rotation, the sun spends half of the year shining directly onto the northern hemisphere and the other half of the year shining directly onto the southern hemisphere. It reaches its highest point in our sky June 21, the summer solstice, and its lowest point Dec. 21, the winter solstice. Separating these two extremes are two days during the year called the equinoxes, six months apart, when the sun shines directly down on the Earth's equator. Equinox is a word that means "equal nights" and is used to describe these two special days of the year when every location on Earth experiences exactly 12 hours of sunshine and 12 hours of darkness, and the sun rises exactly in the east and sets exactly in the west.

You might have noticed recently that the sun is rising later in the morning and setting earlier in the evening than it did in mid-summer and, if you are very observant, you also might have noticed that it is rising and setting much farther to the south than it was in mid-summer. Here in the northern hemisphere, we have just enjoyed six months of long, sun-filled days, but now, it's the southern hemisphere's turn. With each passing day, the noontime sun will sink lower and lower in our sky from now until the winter solstice Dec. 21. Then, it will begin moving northward and cross the equator again on the vernal equinox, March 21, bringing spring with it.

Oh, and one more thing - that business about balancing an egg on its end only on the day of the equinox? It's only a myth. You can just as easily balance that egg on any day of the year. But don't take my word for it. Give it a try and see for yourself.

Happy equinox.

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