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The Autumnal Equinox

The Autumnal Equinox in the Northern Hemisphere occurs on Friday, meaning summer is over and fall will begin. Since the internet is global, the date is now often called the September Equinox. The exact equinox point will be at 4:02 PM Eastern Daylight Time, your mileage may vary. We say that the equinox is the day the sun crosses the Earth's equator, but the sun doesn't really do anything. What happens is that the tilt of the earth, relative to its orbital plane, reaches the point at which the Southern Hemisphere receives more sunlight than the Northern Hemisphere. If you live at the equator, the noonday sun sways a bit north and south over the year, but is directly overhead at both equinoxes. Within the Arctic and Antarctic Circles, the sun is up all summer and gone all winter. Most of us live somewhere between those two extremes, and enjoy long summer days and long winter nights. If you want to get deeper in the details of the equinox, astronomer Guy Ottewell explains what is happening in the graphic above. 

Our sky scene above, for an hour after sunset on equinox day in mid-U.S.A., happens to be about three hours after the instant of the equinox. You can see that the anti-sun, as we can call the point 180 degrees from the sun, appears to be just on the opposite crossroads of ecliptic and equator. Actually it’s a little way past, the fraction of a degree that the sun moves in three hours. The anti-sun point is already a little way into the northern celestial hemisphere, as the sun is a little way into the southern.

Earth is hurtling away from the point we mark as antapex of Earth’s way, 90° to the right of the anti-sun point. As Earth curves on along its orbit, both of these points will shift to the left: the anti-sun point higher into the northern sky, the sun itself deeper into the southern, our hemisphere of Earth deeper into autumn.

Read more about the equinox at Guy Ottewell's Universal Workshop. -Thanks, Walter!

(Image credit: Guy Ottewell)

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