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    Re: Star to Star with Stellarium
    From: Frank Reed
    Date: 2012 Aug 19, 08:47 -0700


    A few years back I discovered a rather neat mathematical cancellation that makes it easy to correct star-to-star distances for refraction whenever both distances are above 45 degrees, as in the example you've described. Refraction lifts all stars. For higher altitudes, the refraction is nearly proportional to the distance from the zenith. But mathematically, when you contract distances in proportion to the distance radially from some center, you're contracting all distances, whether aligned radially or not, in a simple proportion. For standard refraction, it's "one part in 3000" or equivalently 0.1' for every 5 degrees of distance. And again, this applies regardless of the orientation. If the un-refracted distance is 10d 0.0' then the refracted distance is 9d 59.8' whether one star is directly above the other, or they're both at the same altitude, or any orientation in between. All distances are compressed by the same amount. This only works when both stars are above about 45 degrees. Below that altitude, the refraction changes in a more non-linear fashion so you need to do a more involved calculation.


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