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    Time of Sunset
    From: David C
    Date: 2020 Mar 13, 00:48 -0700

    I was outside in the sun this evening and noticed that the low sun was approaching the neighbour's chimney. The significance was that the chimney is due west and it was obvious that the sun was setting near west. I cannot see the actual sunset because I am in a valley  surrounded by hills. I have recently purchased copies of Burdwood and Davis and decided that as some light relief I would put the tables to use.

    First I went to a sun app and determined that the dec is 2.7° S and EOT -9.23.  I then  determined that the equinox is one week away.

    I am at lat 41S and my watch keeps NZDST which is UT + 13.

    From Davis  on 13th March 1920 Mean to apparant time is 9min 39 sec subtract. Say 10min subtract. 2020 - 1920 is a multiple of 4 so 1920 will give an approximate value of EOT.

    From Burdwood the sun sets between 6h 7min and 6 hour10 min. The sun's azimuth is between 87° SW and 86° SW i.e between 273° and 272°

    Now convert sunset time from apparant to NZDST.

    Sunset 1 =  6h 7m  + 13h + 10m  = 1917 hours

    Sunset 2 = 6h 10min + 13h  + 10m = 1920 hours

    According to the app  the sun is west at1916 and sets at 1943, It seems that I have calculated when the sun is west, not when it sets. Possible reasons for the error are:

    • I have made a mistake.
    • The assumptions introduce a 23 min error.
    • The app is incorrect
    • It is Friday the 13th.

    This excercise servs no practical purpose apart from allowing me to use two of the books in my collection.




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