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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Time Sights 11/15/2013
From: Frank Reed
Date: 2013 Nov 15, 10:09 -0800

These are nearly the worst possible circumstances for time sights. You do not shoot time sights when the Sun is near the meridian. Let's consider your sight at 16:25:17 with an altitude of 34° 00.9' (after dividing by two). We can subtract 1.5' for refraction. That yields 33° 59.4' (assuming this is center-to-center and not limb-to-limb). We can easily calculate the longitude where the Sun would have that altitude at that latitude at the stated UT. It's right around 76°31'. But consider the error bars. The angle as measured may be assumed accurate to +/-2' for a good, well-calibrated Davis sextant. The altitude, since it's half the angle, can be considered accurate to +/-1'. We should look for the longitudes where the Sun's altitude would be 1' higher and 1' lower. In this case, it's 10' of longitude higher or lower. That means that a ONE minute error in the altitude is translating to an error of 10' of longitude or about 8 nautical miles in that latitude. And since the Sun is so close to the meridian, a ONE minute error in the estimated latitude would yield almost the same error in altitude and therefore the same large error in longitude.

-FER
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