Autumn has begun

| September 23, 2011 @ 8:00 am | 4 Replies

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.

 

Category: Alabama's Weather

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