The autumnal equinox, or the official beginning of fall (when the sun is directly overhead at the equator and headed south), occurred at 0904 GMT (4:04 am CDT) this morning. So, today everyone on earth will have approximately 12 hours of daylight and 12 hours of darkness. We’ll actually have a little bit more than 12 hours, due to refraction of sunlight by the atmosphere for a minute or so after the sun has crossed the horizon.
The “geochron”-style image above shows the areas of earth receiving daylight and darkness at 8:00 am. Notice that the sun is at the equator, and is equal pretty much in the northern and southern hemispheres. (In summer, there was a lot more of the light concentrated in the northern hemisphere, with 24-hour sun at the North Pole and longer days here, and 24-hour darkness at the South Pole and shorter days in Australia and most of South America and Africa.)
The days are getting shorter, and they are doing so at a rapid pace right now. We have already lost a little over 2 hours of daylight since the peak in late June (blue curve). Notice also that the number of minutes of daylight we gain/lose (dashed black line) is very negative right now (09/23), as we are losing about 2.1 minutes of daylight per day. By November 6, when we switch back to standard time, we’ll only have about 10 hours and 41 minutes of daylight.
