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    Equation of time blues
    From: John Huth
    Date: 2011 Oct 17, 18:33 -0400

    In the course I teach, Primitive Navigation, I like to keep things simple.    But.....evidently not simple enough.   Attendance in lecture was dropping, and I had a nagging sense that they weren't really assimilating the material.   I decided to give a pop quiz in lecture.

    The lecture before I had given a handout and talked about the equation of time.  I even have a memorization trick, based partly on David Burch'es trick - the trapezoidal approximation.  I use "chocolate 14-16" as the mnemonic.   As a guy, I like chocolate and reluctant to part with it, but quick to hoard it.   On Feb. 14th - valentine's day, I'm slow to give up the chocolate to my girlfriend by 14 minutes, but on Halloween, liking chocolate, I'm quick to get it, by 16 minutes.   +/- 2 weeks on either side of those dates, and then remember that 3 months earlier the +16 becomes a -6 from Halloween, and three months after Valentine's day the -14 becomes a +4 minutes.

    Then just connect the bars and the graph works pretty well.

    Point is - no, close to Halloween the sun is close to being about 16 minutes fast relative to mean solar time.

    So, I did an exercise over the weekend - quick and dirty equal altitude measure - getting 12:30 EDT for the meridian passage - then converting it to mean solar, then longitude.   I drew the Equation of Time on the board, worked through the exercise, mentioning....again....the real sun is 16 minutes fast.

    I worked it through to get longitude.

    Then I talked about refraction for awhile, then pulled out the pop quiz.   80 points for  being there.  15 points if you could give the value for the equation of time for today.   I left the graph on the board, with the +16 minutes prominently displayed.   

    Alas, about half the class didn't even know the answer even though it was clearly displayed on the board and audible groans arose when I pointed this out.   What were they doing during the discussion?   Face-booking on their laptops.

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