MadSci Network: Earth Sciences

Re: Why is the day longer in December instead of January for Ottawa Canada?

Date: Tue Nov 28 01:40:00 2006
Posted By: Bryan Dunne, Instructor, Astronomy, University of Illinois
Area of science: Earth Sciences
ID: 1158951728.Es

The short answer is that it isn't. By only looking at sunrise times, you are making an assumption that the length of a day is always (noon - sunrise time) * 2, but that isn't true. To figure the length of the day you need to use (sunset time - sunrise time). For mid-northern latitudes, the day of earliest sunset is around Dec. 7th. The latest sunrise is around Jan. 4th. And the shortest day is Dec. 21st.

Of course, this begs the question "Why do the earliest sunset, latest sunrise, and shortest day of the year occur on different dates?". For an excellent answer to that question, I refer you to the following website by John Holtz

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