Astronomy blog

Strange Phenomena with Sunrise and Sunset This Time of Year

By travel
12/10/2011, 7:54:14 AM

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I love and am fascinated by nature, weather, animals...basically anything associated with so called "Mother Earth". Something that caught my eye recently is how, around this time of year, the sunrise continues to get later each day while the sunset time does not change nearly as much.

Here are the next five days of sunrise and sunset times here in State College, PA: Sunrise: Sunset: Sat. Dec. 10th- 7:24 a.m. 4:43 p.m. Sun. Dec. 11th- 7:25 a.m. 4:44 p.m. Mon. Dec. 12th- 7:26 a.m. 4:44 p.m. Tue. Dec. 13th- 7:27 a.m. 4:44 p.m. Wed. Dec. 14th- 7:28 a.m. 4:44 p.m.

Notice that even though we have not reached the winter solstice, the sunsets are already getting later.

We all know that the shortest day of the year is the day of the winter solstice, usually Dec, 21. It seems reasonable that the latest sunrise and earliest sunset would both occur on the shortest day. But it's not so; the earliest sunset is generally a couple of weeks earlier and the latest sunrise is generally a couple of weeks later.

What explains this peculiarity? Why doesn't the shortest day have the earliest sunset and latest sunrise?

The time of day when the sun reaches its highest point in the sky is called solar noon. The period from one solar noon to the next is called the solar day. One solar day is the time it takes for the sun to return to its highest point in the sky.

The length of the solar day is not constant through the year. Near the winter and summer solstices the solar day is more than 24 hours long, and near the spring and fall equinoxes it's less than 24 hours long. So solar noon rarely occurs exactly at clock noon, it's sometimes before and sometimes after. The critical point is this: near the solstices, solar noon occurs at a slightly later time each day because the solar day is more than 24 hours.

Now the time from solar noon to sunset doesn't change very much near the winter solstice. Therefore, since solar noon is a little later each day, sunset (even assuming the exact same day length) is also a little later each day. And if sunset is a little later each day, the sunrise (again assuming the same day length) is also a bit later.

Obviously, this time of year the day length is not the same, it is getting shorter each day. If we had a "natural" clock, based on the sun (which is impractical in the modern world), then the sunrises would be getting later while the sunsets get earlier, thus resulting in less sunlight each day until the solstice. However, the "lag" caused by our clock not matching up with the sun causes the imbalance in the sunrise and sunset times. The days are getting shorter, just that we are getting the shorter days because the sunset times are later each day in magnitude than magnitude of the sunrises also getting later.

For example, if the sunset times are earlier each day by 60 seconds, and the sunrise later by only 30 seconds, we are still losing 30 seconds of day length each day.


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