Time of Sunset, Sunrise, Shortest Day ``` Name: Bob Status: student Grade: 6-8 Location: PA Country: USA Date: Winter 2012-2013 ``` Question: In Pittsburgh, PA, in local newspaper and on calculated table found on the World Wide Web, the sun rise times after December 21 continue to occur later until about January 10. Should the sun rise be earlier after December 21? Sun set times have been getting later since December 7 (when I started keeping track based on the newspaper). Should they have been getting earlier only until December 21? Replies: Your question is more easily asked than answered!! Many questions need to be addressed: ?How do you define the terms: ?Sunrise? and ?Sunset?? Those terms are not unique. For example, you might mean the Sun rise/set at the time when: the first, the middle, or last image of the Sun at the longitude where the observation is being made. The precision of this time depends upon the resolution of the instrument being used. But there are other complicating issues. The light from the Sun, either rising or setting, is distorted, ?bent?, by the atmosphere. Whether you consider the ?apparent? image is the one used in the definition, or whether it is the ?undistorted? image is used changes the apparent Sun that also depends upon atmospheric conditions. The ?length? of the day (whatever its date, is a parameter to be determined). Uhhmm! I forgot to mention, ? Which calendar is being used?? Also the ?so called? longest/shortest day is rounded to the nearest day/night. But that can change (+/- ) a day since the calendar date is discrete, not continuous. Also the effect of the Moon?s gravitational influence on the length of the day, and hence the sunrise/sunset, in the model I have described, has not been taken into account. The bottom line is that the sun rise/set is not a precisely defined term. It is a useful approximation, just (for example) the times of ?high? and ?low? tides. Do not try to over-explain a very complex phenomenon. You can find an analysis on the web site: http://en.wikipedia.org/wiki/Sunrise_equation . However, the fact remains that the terms are approximate. Vince Calder Dear Bob, Do not worry; all is as it should be. The sunset gets earlier until early December, and sunrise gets later until mid January. This averages out to the shortest day on or about December 21. Good question! Sincerely David H. Levy Click here to return to the Astronomy Archives

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