NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star - Star Observations
From: George Huxtable
Date: 2010 Mar 13, 22:30 -0000
From: George Huxtable
Date: 2010 Mar 13, 22:30 -0000
Douglas Denny wrote- "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." Comment from George- Yes, that looks perfectly good, to me. The red arc, between the true positions of the two stars, is what has been calculated from Almanac coordinates. Then the (smaller) apparent arc between the stars, the angle that is actually seen in the sky and compared with the sextant reading, is shown by the black arc between the apparent star positions. Just as it should be. 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. ----- Original Message ----- From: "Douglas Denny"To: Sent: Saturday, March 13, 2010 9:43 PM Subject: [NavList] Re: Star - Star Observations Initial Post George Huxtable: Copy: 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." Ends. ================= 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. ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------