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    Star - Star Observations
    From: Brad Morris
    Date: 2010 Mar 9, 13:48 -0800

    Gentlemen

    I have been playing around with star-star observations to determine the accuracy of the sextant arc. Calculating the star to star distance, without refraction is not a challenge, nor is correcting for refraction when both stars are on the same side as the zenith. Derive GHA Aries for the observation instant, apply SHA objects, determine instantaneous altitude for both objects using spherical trig, compute refraction correction based on altitude for both objects, create delta refraction correction and finally, subtract from star to star distance to get the observable distance for my location. It sounds like a lot of work, but I have set up a spreadsheet that uses the Celestron SkyScout as inputs. I just point at two stars, enter some data from the SkyScout (in Right Ascension & Declination), and the observable distance from my location at a known time is the instantaneous result. All the mindless tabular work is done by the spreadsheet. I don’t really even need to know which stars they are, as long as the Celestron does! Of course, I have checked my spreadsheet against some hand done calculations to check to see if it is working the way I expect it to…and it is.

    Here is the dilemma. When I get to larger angles, I need to go beyond my zenith. For example, I have been looking at Polaris vs Sirius. My latitude is about 40 degrees north. So Sirius is to my south, Polaris, naturally, is to my north. The nominal distance works out to about 106 degrees 20 odd minutes (forgive me, I don’t have the exact numbers in front of me). Because each object is on either side of my zenith, both objects will appear to be lower in the sky compared to the horizon, due to refraction. Yet because they oppose each other in azimuth, the observable distance between them should be larger by the sum of the refraction corrections, not reduced by the difference of the refraction corrections. That is, compute the true distance without refraction. Since each object is lowered by refraction, but in opposite directions, shouldn’t we add the refraction corrections to the nominal distance to obtain the observable distance?

    Best Regards
    Brad

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