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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sun SD, Sextant precision, SNO-T
From: Alexandre Eremenko
Date: 2004 Oct 10, 16:12 -0500
From: Alexandre Eremenko
Date: 2004 Oct 10, 16:12 -0500
Can anyone tell me more precise value of the Sun radius on October 10? (This quantity is called "semidiameter, or SD" in the almanach. The almanach gives for October 10 the value of 16.0', but I noticed that it gives 16.1' beginning from October 12, and the refraction table implies the average value of 16.15 for October-March. The "Astronomical Almanach" or maybe some software could probably give it more precisely (I need one more digit). The reason I am asking: This morning I have fixed all (hopefully!) problems with my inverting scope, and tried to fing the index correction from Sun observations (with a fixed scope and more experience than I had last time:-) The results are the following: (everything is in angular minutes. "UL" is the sextant reading when the reflected image touches from above, -LL is when it touches from below. "4SD" is the sum of these readings, "-IC" is the difference divided by 2, the negative of the Index Correction.. 1. Rotating the drum in the decreasing direction: UL 32.5, 32.3, 32.5, 32.4, 32.3, LL 32.0, 32.1, 32.0, 32.1, 32.0, 4SD 64.5, 64.4, 64.5, 64.5, 64.3, Almanac: 64.0 -IC 0.25, 0.1 0.25, 0.15, 0.15 Average: -0.2 2. Rotating the drum in the increasing direction: LL 32.0, 32.0, 32.0, 32.1, 31.9, UL 32.2, 32.5, 32.6, 32.5, 32.5, 4SD 64.2, 64.5, 64.6, 64.6, 64.4 Almanac: 64.0 -IC 0.1, 0.25, 0.3, 0.2, 0.3 Average: -0.2 These results indicate that my index correction is something between -0.2' and -0.3' (Which is consistent with my previous Sun observations, but NOT with the star observation; star observation give consistently 0.0'. Today will repeat star observations with my newly fixed inverting scope. The problem with the Sun observations above is that my measurements of Sun's SD indicate that it is closer to 16.1' rather than 16.0'. This explains my question in the beginning. I have to multiply the almanac value by 4. This increases the almanac rounding error by the factor of 4. (The Russian manual says: if your measurement of 4SD is within 0.4' from the almanac value, your measurement is OK. Otherwise it should be repeated. My measurement is on the boundary:-) (My apologies in advance to those list members who consider all this activity "splitting of a hair":-) Alex.