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    Re: Sundog
    From: Brad Morris
    Date: 2013 Feb 16, 12:00 -0500

    Hi Greg

    How about this method?

    Frank mentioned it was about sunset.  The sun sets about 5:19 PM mean time, Latitude N41 on 10 Feb.  Unfortunately, there is no meta-data associated with the image so we cannot tell the precise time.  Frank has made specific statements about 10 Feb, stating that on this day, "it is noon at noon".  Therefore apparent time aligns with mean time. 

    "Azimuths of the Sun" 1934, gives a table for N41 latitude, declination of opposite name. The table clearly states that the sun sets at 5:10PM apparent time, at azimuth 108 deg 42 min.  Enter the table with declination (S14) and apparent time of sunset, less 20 minutes. Why 20 minutes?  For the 5 degrees of altitude.   The table entry for 4:50PM, Dec 14 gives 112 degrees 0 minutes.

    Since this is reckoned from N to W, the sun's true azimuth is 360-112=248 Degrees.  Add the sun dog offset of 22 degrees and we are facing 270 degrees.

    For a closer estimate, the mean time of the picture and the true observed altitude ( was it truly 5 deg?) would have been helpful.

    Brad

    On Feb 16, 2013 7:30 AM, "Greg Rudzinski" <gregrudzinski@yahoo.com> wrote:

    Brad,

    The sundog was given at 5 degrees elevation. Sundogs form horizontally to the Sun which means the Sun azimuth for 5 degrees altitude is used to then add 22 degrees. Using the amplitude will give you an azimuth for zero altitude and thus a slight error.

    Greg Rudzinski

    I get a different answer.

    Because this is sunset, we can use an amplitude of the sun calculation, as if we were calibrating our compass.

    Known on this day is the declination = S14 deg 3.7 minutes
    Latitude =N41

    Amplitude = A
    A= arcsin(sin(dec)/cos(lat))
    A= arcsin(sin(-14.061666)/cos(41))
    A= arcsin(-.3219332015)
    A= -18.7798 degrees

    Azimuth Z is the complement of Amplitude A so
    Z =90-A
    Z = 108.7798 Degrees

    In SW Quadrant
    Sun setting at 360 - Z
    Sun setting at 251.2202 degrees

    The problem statement is that "you are facing the sun dog", but not the sun. The sun dog must be north of the sun, since we are facing generally west and the sun dog is to the right of the setting sun.

    A sun dog forms at 22 degrees to the side of the sun. Therefore, we are facing (251.2202 + 22) degrees
    = 273.2202 degrees

    I am prepared to be completely corrected!

    Regards
    Brad

    On Feb 11, 2013 2:48 PM, "Frank Reed" <FrankReed---com> wrote:
    There was a very bright sundog yesterday at sunset here. Photo linked.

    Here's a little navigation puzzle: you're walking in a hilly area near sunset on February 10 in latitude 41 N. You see a sundog about 5 degrees high directly ahead of you but you can't see the Sun (behind a hill to your left). In what direction are you walking?

    -FER


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