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George Huxtable:

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The altitudes of the two stars have to be calculated. Then the refractions have to be ADDED, as I have been insisting all along, and you have contradicted. To get from a calculated altitude to what would be an observed altitude (if anyone had observed it) requires applying refraction in the OPPOSITE DIRECTION to what's needed when converting an observed altitude to a true altitude, as in reducing an altitude observation. The effect will be to produce an apparent angle between the stars that is always LESS than the theoretical value deduced from the almanac. George."
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George,
I am sorry I missed this posting of yours for some days somehow as I did not notice it. I wondered what had happened. I seem to be missing postings occasionally as they change so frequently.

Agreed: from calculated true altitude to a calculated apparent refraction, refraction is to be added, as in the zenith diagram in fact. I should have seen this immediately.
I was convincing myself of standard subtraction to get from apparent to true as in Lunar clearing of distance.

I have re-drawn the Rational Horizon Diagram as there were some mistakes on the previous one. I hope this diagram can make the rigorous method clearer.

Douglas Denny.
Chichester. England.
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