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    Re: Happy Equinox Day
    From: Greg R_
    Date: 2007 Mar 22, 00:11 -0700
    > Over a few minutes spanning LAN there is very little change in the
    > body's altitude - they used to say the sun tends to "hang in the sky"
    > at this time.
     
    Correct - and amazingly enough, it still does...  ;-)
     
    > So differences in recorded altitude over those few minutes are mostly
    > caused by random errors in observation. By comparing a graph of them
    > with your horizontal line it may be possible to derive a better
    > altitude to adopt for calculation of your latitude than any of the
    > recorded sights (see: "An example of a slope", 10 March 07) .
     
    Ah, gotcha...
     
    When I do noon sights (or is it "noon shot"? Or either/or?) I use a nifty little multi-timer (http://www.invisibleclock.com) that I have set to go off every two minutes (with a different "standby" alert 10 seconds before the actual 2 minutes runs out). That saves having to look at the watch every few minutes, or the need to keep resetting some other type of alarm.
     
    But once I've determined the peak altitude, I just leave the sextant set for the duration of the "hang time" and verify periodically that the body (sun, in this case) hasn't started downwards yet from its peak. As a result, I've only got two "real" data points for the peak today (Hs = 55° 50.1') which both happened to fall on that 2 minute "time to take a sight" cycle.
     
    So in 20/20 hindsight I could have used those few minutes of "hang time" to take multiple sights as sort of a "self-error check" to make sure that I was getting consistent results over time... Good point, and I'll make a mental note to do that next time (and I guess to be realistic about it, reset the sextant to either zero or at least somewhere else far removed from the peak altitude reading).
     
    --
    GregR
     
     
     
    ----- Original Message -----
    From: "P F" <peter.fogg---.com>
    Sent: Wednesday, March 21, 2007 10:33 PM
    Subject: [NavList 2452] Re: Happy Equinox Day

    >
    > > > and then compare this pattern with the (in this case)
    > > > horizontal slope?
    > >
    > > Eh? Wouldn't a graph of the sights still follow the familiar parabolic curve
    > > leading up to and then following local noon? I wasn't aware that the
    > > declination being almost zero would have any affect on that (of course, the
    > > slope of the LOP *is* horizontal at meridian passage, but that's the case
    > > any time of the year as far as I know...).
    >
    > Over a few minutes spanning LAN there is very little change in the
    > body's altitude - they used to say the sun tends to "hang in the sky"
    > at this time.
    >
    > So differences in recorded altitude over those few minutes are mostly
    > caused by random errors in observation. By comparing a graph of them
    > with your horizontal line it may be possible to derive a better
    > altitude to adopt for calculation of your latitude than any of the
    > recorded sights (see: "An example of a slope", 10 March 07) .
    >
    > You could also (if you had a reason to) calculate the standard
    > deviation of those sights, which could be useful in terms of analysis
    > of the resulting position line (see recent thread on "Cocked Hats").
    >
    > Since you have the sights recorded it might be informative to go back
    > to them and try it out. This particular case lends itself easily to
    > the technique, since you don't even need to calculate a slope - it is
    > effectively a horizontal line over five minutes of time spanning LAN.
    >
    > Let us know the results ...
    >
    >
    >

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