--- Kim Hyatt <kimharch@cut.net> wrote:
Does anyone know the relationship between the time when the solstices occur and the times of earliest/latest sunrise and/or sunset? <snip> Is it a matter of minutes, hours or days?
In northern latitudes, the earliest sunset is not at the winter solstice, but is a couple of weeks earlier around Dec. 8. The latest sunset is not at summer solstice, but occurs later during the first week of July. In northern latitudes, the latest sunrise is not at the winter solstice but occurs in later in early January. The earliest sunrise is not at the summer solstice but a week earlier around June 14. The time differentials involved are only a few minutes in the summer and about five minutes in the winter. Both effects stretch over a couple of weeks. IMHO, the best graphical representation of the relationship between the solstices and latest and earliest sunset and latest and earliest sunrise is at the "Other Phenomenon" page of Bob Urschel's outstanding "Analemma" website: http://www.analemma.com/Pages/OtherPhenomenon/OPRiseSetMovie.html http://www.analemma.com/Pages/OtherPhenomenon/OtherPhenomenon.html In a quantitative table, you can see this effect in the RASC Observer's Handbook. There is a table and hourglass graphic listing sunrise and sunset by date. The latest sunrise occurs in the first two weeks of January, after December's solstice. Monthly tables can be generated online at: http://www.sunrisesunset.com/custom_srss_calendar.asp The effect is due to timing interactions between the tilt of the Earth and its elliptical orbit around the Sun. Mathematically, the effect is captured in the "Equation of Time", which is graphed and on the Sky&Telescope annual planning chart that Patrick mentioned. - A plot of the change in apparent noon due to the Earth's elliptical orbit (blue) and the change attributed to the tilt of the earth's axis (purple): http://www.nmm.ac.uk/server/show/conMediaFile.6027 - A plot of both variations combined - http://www.nmm.ac.uk/server/show/conMediaFile.6628 - both plots the Royal Observatory website at Greenwich. http://www.nmm.ac.uk/site/request/setTemplate:singlecontent/contentTypeA/con... Wikipedia entry on the Equation of Time http://en.wikipedia.org/wiki/Equation_of_time A visual effect of this interaction is that if the position of the sun is measured at watch-time noon each day, it traces out the figure-8 pattern - the analemma. http://www.analemma.com/Pages/framesPage.html While the difference between the solstices and apparent noon may only be a few minutes, the total range variation in apparent noon and watch-noon is about 28 minutes throughout the year (or -+15 minutes around watch-noon). http://en.wikipedia.org/wiki/Image:Equation_of_time.png The sunset during the winter solstice is the only time I usually notice the inconsistency of sunset happening later in the days leading up to the shortest day of the year on Dec. 21. The earliest sunset is in the first week of December, but sunsets happen later in the day leading up to shortest day of the year on the winter solstice. It "feels" backwards that the sun would set later in the day leading up to the shortest day of the year. For sunsets during the summer, I usually don't notice the effect unless I check the newspaper and really watch the sunset closely as it continues to occur later each day during the week following the longest day of the year. If I was a "morning person," I would probably also "feel" the inconsistency of sunrise happening later in the morning in the week leading up to the longest day of the year at the summer solstice. The other effect I generally sense (being a pretty dense guy, it takes alot for me to notice anything) is the relatively rapid return of longer days after the Earth "turns" the winter corner around Dec. 8 and continuing through the end of January. Graphically, this is the steep 28 minute change in time of apparent noon seen in the Dec.-Jan. portion of the Equation of Time: http://en.wikipedia.org/wiki/Image:Equation_of_time.png This does not coincide with the winter solstice and there is no corresponding rapid change at the summer solstice, so the change never quite feels "right" to me. See also ________. Dec. 1998. "Sunrise, Sunset, and the Length of Day" Sky & Telescope. In conclusion, it's "only a matter of a few minutes" if you measure the change between watch-time noon and apparent noon around the solstices. It's "only a matter of a couple of weeks" if you consider the period over which the difference is observable. - Canopus56(Kurt) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com