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    Re: December Solstice
    From: Jim Thompson
    Date: 2003 Dec 19, 07:30 -0400
    Frank - Nice explanation.  I especially like "the time of local apparent noon is sliding about as the year passes".  Now, if only I could get EqT clear in my head for the time diagram of my first noon sight reduction (more on that later, after I finish the webpage describing my dilemma).
     
    Pierre - Your post intrigued me, so I generated the following data for each day in the period you mention at your position, using  the shareware almanac from the website http://home.att.net/~srschmitt/almanac.htmlI am plotting the results on an Excel graph to demonstrate more clearly what you noticed.  It doesn't answer your question, but it should help us newbies to see the phenomenon more clearly, making it easier to understand the explanations at the analemma site.  Output from the almanac generator looks like this:
     
    Friday, December 12, 2003
    Location: 45°00' N, 75°00' W
    Sun : RA = 17h17m33s, Dec = -23°04'50", distance = 0.98455 AU
    05:42 R : astronomical twilight begins
    06:18 R : nautical twilight begins
    06:55 R : civil twilight begins
    07:29 R : rise, azimuth: 122.66
    11:54 R : transit, azimuth: 180.00, altitude: 21.96
    16:18 R : set, azimuth: 237.30
    16:52 R : civil twilight ends
    17:29 R : nautical twilight ends
    18:05 R : astronomical twilight ends
    Length of day: 08hr 49min

    Jim Thompson
    jim2{at}jimthompson.net
    www.jimthompson.net
    Outgoing mail scanned by Norton Antivirus
    -----------------------------------------

    -----Original Message-----
    From: Navigation Mailing List [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Frank Reed
    Sent: Friday, December 19, 2003 3:05 AM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: December Solstice

    Pierre Boucher wrote:
    "Please comment on the facts that:
    Starting on Dec 13 (9 days before DSO) Sunset time is later day after day
    and It is only from Jan 9 (18 days after DSO) that Sunrise time stars to be earlier day after day"

    Right. The earliest sunset occurs BEFORE the solstice, and the latest sunrise occurs AFTER the solstice. This is a little difficult to explain without pictures, but there a couple of things you should consider. First, on the solstice, the day is shortest. That much is guaranteed. The time from sunrise to sunset is least. If you take the actual clock times of sunrise and sunset and find the time halfway between those, you will discover that it's not noon --not 12:00:00. That's really why the times of earliest sunset and latest sunrise are off --the time of local apparent noon is sliding about as the year passes. Local *apparent* noon is not the same as the noon we measure on our clocks because the Sun in the sky is not a perfect clock. It runs fast during certain parts of the year, slow during other parts. As I say, it helps a lot if you can see this with pictures, so I would direct you to a web site: www.analemma.com . It has lots of great information relevant to this issue. If you still have questions after reading there, I would be glad to talk more, and I am sure other people on the list would help, too.

    Frank E. Reed
    [X] Mystic, Connecticut
    [ ] Chicago, Illinois
       
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