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Analemma and equation of time

  1. Mar 9, 2012 #1
    Hi,

    In a regular analemma, you have 365 marks on the ground where the sunlight fall every day at true solar noon. The equinoxes and solstices are also represented on the analemma.

    To indicate the solstices, you don't have a point on the ground but a kind of ellipse. I think this is due to solstices that don't strictly occur the same day (20, 21 or 22 june / december) and also because near solstices the declination speed is very low, so the positions are too close to be represented individually, so an ellipse as an envelope of the different possible positions.

    Can you confirm or infirm ?

    Thanks
     
  2. jcsd
  3. Mar 9, 2012 #2

    Drakkith

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    Staff: Mentor

    What do you mean by a "regular analemma"?

    As far as I know, an analemma IS an ellipse. If you mark a spot each week or day for an entire year and then connect the dots you will make a figure 8 ellipse that shows the movement of the Sun in the sky for that year.

    http://en.wikipedia.org/wiki/Analemma
     
  4. Mar 10, 2012 #3
    I know analemma makes a figure 8 slightly deformed. The word "regular" wasn't appropriate, forget it !
    My question was (sorry if it was not clear enough) : each mark on the ground is a point for each day of the year but to mark the solstices we have a kind of ellipse containing several points. Why we have such an ellipse to mark solstices ?

    I hope it's more clear now.

    Thanks
     
  5. Mar 22, 2012 #4
    Anyone ?
     
  6. Mar 22, 2012 #5

    Drakkith

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    I have no idea what you are talking about. What ellipse are you referring to? The analemma makes a figure 8, so I don't see any ellipses other than the two ellipses that form the figure 8.
     
  7. Mar 25, 2012 #6
    Sorry for the misunderstanding, but english is not my mother tongue. I agree analemma makes a figuer 8. On this figure 8, you have on the floor 365 marks for the 365 days of the year. You have also other indications like months, equinoxes, solstices, etc.
    But, on the figure 8, the position of the solstice is not marked by a single dot but with a little ellipse (in dotted line). Why ?

    My first ideas were :
    - Solstice doesn't strictly occur at the same day (20, 21 or 22 june / december) for a given year.
    - in addition, near solstices the sun declination speed is very low, so the positions of the sunspot on the ground are too close day by day to be represented individually, as singles positions. So we draw an ellipse which the "envelope" of the different possible positions for solstice.

    I hope that my words are clearer now...

    Thanks
     
  8. Mar 25, 2012 #7

    Drakkith

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    Staff: Mentor

    Are you talking about an analemma for a single year, or multiple years?
     
  9. Mar 25, 2012 #8
    This analemma (where I see an ellipse for delimiting the solstice) is engraved in the ground, so i think it is an permanent analemma, for multiple years...
     
  10. Mar 26, 2012 #9

    Drakkith

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    Staff: Mentor

    I'm not sure, but I think your reasoning is correct in this case. Since the solstice occurs at different times each year it would not be a single spot.
     
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