|Moon, Venus, and other lights, 12/26/11.|
So writes John Donne in “A Nocturnal Upon St. Lucy’s Day, Being the Shortest Day.” Our shortest day this year is December 21. (It is currently falling on the 21 or 22 and will continue so until 2080, when it occurs on the 20th.) But oddly, the 21st will neither be our earliest sunset or our latest sunrise. The earliest sunset in Gettysburg occurred on Dec 7 (4:44 PM) and the latest sunrise will occur on January 4, 2014 (7:31 AM).
Why?? In a nutshell, though the tilt of the earth’s axis assures that the length of days changes predictably though the year, the entire day in local sun time is shifting against clock time due to regular changes in the speed of the earth around the sun.
For an explanation that will not fit in a nutshell, read this article from the U.S. Naval Observatory.