QUESTION

December 21 is the shortest day of the year, but why don’t the earliest sunsets or latest sunrises of the year coincide with that date?

ANSWER

The common wisdom is that June 21, the summer solstice, is the longest day of the year and that December 21, the winter solstice,  is the shortest day of the year. And that is true.

To get the shortest possible day (daylight), we want a late sunrise and early sunset. The dates of latest sunrise and earliest sunset depends on latitude. Here’s what happens in Tomah, Wisconsin. The latest sunrise is January 3 at 7:37 AM. The earlier sunset is December 9 at about 4:24 PM.

Up to December 9, the sunrise gets later and sunsets earlier, so the days shorten. From December 9 to December 21, sunsets get later, and sunrises also get later, but advances faster than sunset, so days continue to get shorter.

From December 21, the solstice, to January 3, both sunrise and sunset continue to get later, but sunset advances faster, so the days get longer. After January 3, sunrise is earlier and sunset is later, so each day is longer. December 21 (sometimes December 22) remains the shortest day of the year, with about 9 hours of daylight and 15 hours of darkness.

Why aren’t the latest sunrise and earliest sunset on the same day, namely December 21, the winter solstice?

Two factors are involved. First, the Earth’s axis is tilted 23.5 degrees with respect to its plane of orbit around the sun. Second, the Earth’s orbit around the sun is not a perfect circle, but rather an ellipse or oval shape.  It is the first, the tilt thing, that is most important.

The time of day when the sun gets to its highest point in the sky is called solar noon and the time from one solar noon to the next one is called the solar day. The length of the solar day is not constant through the year. Around the winter and summer solstices (Dec 21 and June 21), it is a tad more than 24 hours and near the spring and fall equinoxes (Mar 21 and Sept 21), it is slightly less than 24 hours.

The length of the solar day is determined mostly by the rotation of the Earth on its axis, and a little bit by its revolution around the sun.

We don’t like to tell time using solar days, because we want every day to be the same, exactly 24 hours. So our clocks don’t run on solar time. Our clocks average out the variations in the solar day, making every day the same length, and so out clocks don’t agree with the solar day.

Solar noon rarely occurs exactly at clock noon. During the winter solstice, solar noon occurs at a slightly later time each day because the solar day is slightly more than 24 hours. When we talk about “earliest sunset”, we mean earliest according to our constantly running clocks. The difference between clock time and solar time create the phenomenon.  If sundials were used to tell time, the latest sunrise and earliest sunset would occur on December 21, the winter solstice. It can get a tad complicated.

There is a table of sunrises and sunsets for an entire year from the US Navy Observatory for any location on Earth at http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-one-day-us. Type in Tomah for location.