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    Re: Easy compass check
    From: Bill B
    Date: 2015 Mar 19, 01:36 -0400

    On 3/19/2015 12:36 PM, Bruce J. Pennino wrote:
    > At spring and fall equinox the sun rises  due east (true) and sets due
    > west (true), declination  of sun  zero.
    Wish it were so, but it isn't. (I'll skip my sea monkey lament this
    round :-)
    * GP is based on the center of the sun, not a limb, so in a vacuum the
    upper half would be above the horizon when bearing was 90d or 270d true.
    * As we are not living in vacuum and there is refraction to consider.
    Due to this the sun is below the horizon when we see it set. It is
    moving at an angle to the horizon as it sets, so not 90 or 270 when we
    see it. Either way we most likely cannot view this instant due to the
    sliver being extinguished by the atmosphere.
    * Being picky, the declination change is trucking right a long at this
    time, so cannot be 0 at both sunrise and sunset. One or the other if
    hyou are in the right place.
    To be fair, you did say within a couple of degrees. My problem with
    using this to teach navigation is it discounts the rapid change in d,
    refraction and dip. Along the same lines, the length of day and night
    cannot be equal, even in a vacuum. So it perpetuates a myth by ignoring
    factors vital to cel nav.

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