Autumn officially begins. The autumnal equinox, or the time when the sun is overhead at the equator, occurs at 4:51 am CDT today. To a close approximation, there are 12 hours of daylight all over the world. You can see this fairly well on the U.S. Naval Observatory’s “Geochron”-style map for today at noon CDT (left), and compare it with June 21 (right).
As we move into fall, the days are getting shorter in a hurry. We will have 12 hours and 7 minutes of daylight today, compared to 14 hours, 22 minutes in late June. We are losing about 2 minutes of daylight a day. The shortest day will be December 22, with only 9 hours, 56 minutes of daylight.
Remember, the sun will start setting for the winter in the arctic regions over the next few weeks. The length of day changes much more dramtically the farther north you go (in the Northern Hemisphere), even in the U.S. Below is a chart showing approximate length of day at various cities. (Miami in yellow, BHM in green, Boston in blue).
More on changes in sun angle and incoming energy later.
