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Re: Star - Star Observations
From: George Huxtable
Date: 2010 Mar 11, 14:33 -0000
From: George Huxtable
Date: 2010 Mar 11, 14:33 -0000
Some comments about Douglas' latest posting. The initial question was posed by Brad Morris, not Bill Morris. Douglas asked- "You queried my previous postings. Are you now satisfied with my general case solution for the two star distance problem with included Zenith/Rational Horizon diagram? It should be a rigorous solution unless there are flaws I have not noticed. I do not incidentally, understand your method which mentions an addition of refraction. Refraction should be subtracted every time." ==================== There is a flaw you still have not noticed, and keep on restating, though I have done my best to point it out. The drawing, attached to your 10 March posting, shows an arc, in red, between the calculated positions of star 1 and star 2. The length of that arc is NOT the end-product that is needed to calibrate a sextant. What's needed is the distance between the APPARENT positions of the two stars, which have not been joined by an arc on your diagram. You wrote- "The refraction affects measured altitude and hence zenith distance which is what is effectively measured with a sextant. Refraction is always subtracted as the true star position is always below the apparent in altitude. Hence between two stars there is a slight increase in the true distance between them as opposed to the measured. (see diagram)." and (above) "I do not incidentally, understand your method which mentions an addition of refraction. Refraction should be subtracted every time." NO! 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. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
