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    Re: First Noon Sight: Astra IIIb
    From: Frank Reed
    Date: 2014 Apr 11, 14:10 -0700

    Looks good, Sean! And thanks for posting the 'selfie'. It's always nice to see who's out there in Internet-land.

    For your noon latitude, I figure dip+refraction correction nets to about -4'. Sun SD is +16'. So the corrected altitude right at the peak is 61°13.5'+12' or 61°25.5'. I subtract that from 89°60' to convert it to zenith distance. That yields 28°34.5'. As always (thinking out loud here), zenith distance is exactly equal to our distance from the place where the Sun is straight up and for a noon sight that distance is entirely north/south giving our separation in latitude from the spot where the Sun is straight up. The latitude where the Sun is straight up is its declination, by definition. So I add the z.d. to the Sun's dec at that moment, 8°28.1', (Lat=z.d.+Dec) and get 37°02.6', just a tenth of a mile from the location you gave. Can't complain about that! Naturally there's a big dose of good luck in this. Typical noon latitude errors from a location like that, using a good sextant, will be more like 0.5-1.0'. So once every five or ten noon sights, you might expect the kind of accuracy you had today. One way to get this sort of accuracy more regularly, is to shoot four or five sights in rapid succession during those several minutes of "hang time" right around noon. The Sun's altitude will not change perceptibly during that interval, so you have an angle which is essentially constant. If you measure it four times in a row and then average, you can normally expect to reduce random error by a factor of two.

    One little concern: I see you've got your times labeled as LMT. Those times are actually Eastern Time watch times (EDT), right? That's not Local Mean Time. LMT is equal to zone time only at the center of the standard time zones (or 15° east of each of those when we're on Daylight Time). Apart from those special longitudes, LMT varies smoothly with longitude, and is rarely the time displayed on clocks (not since the 19th century).

    For a bigger issue, you wrote,
    "After evaluating my sights, these are the only ones I considered"

    Okay... but how did you decide what sights were worth considering? Your table of data is extremely symmetrical with two altitudes before local noon paired with the exact same altitudes (to a tenth of a minute of arc) for sights after noon. This basically never happens. Did you adjust the altitudes? Or did you select these because they looked so nicely paired up? It's no concern for latitude right at local noon, but this sort of sight selection, trying to pick good sights from bad sights, can be very misleading.


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