Sunrise and Sunset: Strange Behavior

If you track the daily rise and set times for the sun around the winter solstice, you would expect the latest rise and earliest set to occur on the shortest day of the year, the solstice.

But you would be wrong.

In Chicago, for example, the earliest sunset occurs on December 8 and the latest sunrise almost a month later, on January 3. And yet the shortest day of the year does indeed occur on the solstice, December 21.

What's going on?

The upper curve tracks the changing sunset times and the lower curve tracks rise times from November through January. The vertical green bars represent relative day lengths, varying from nine hours and 57 minutes down to nine hours and eight minutes (the timescale between morning and evening has been compressed on this graph for simplicity).

Even though the curves' "turnaround" points (Dec 8 and Jan 3) are offset by about four weeks, the curves still reach "closest approach" in between the two, on the solstice.

Now you might be wondering why the sun behaves so strangely. The answer is on page two.

©2000 Jeremy Kohler