NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: Douglas Denny
Date: 2009 Jun 20, 04:13 -0700
From: Douglas Denny
Date: 2009 Jun 20, 04:13 -0700
Agreed "altitudes" .. not declination for same refraction component. Don't know why I said declinations - brain not in gear whilst typing. I think it is called senility. And agreed refraction is not completely negligable for rigorous calculation. I also take the point the errors (being in height - altitude) are going to be greatest when the two stars are opposite each other towards 180 degree separation, and less so when the hour angle between then is smaller: my point is the difference in refractions is going to be much reduced when of comparable altitudes - and I should now say less than 90 degree sepatation in hour angle - and of a value not entirely way out and comparable to the accuracy of the measurement itself. If errors in measuring; and the errors inherent in the sextant itself are comparable with the errors in refraction then it is not worth worrying too much about it, as it is more a theoretical problem. Interesting to contemplate and correct for, but I would query the practicality anyway for serious measurement - unless lunars are your interest when refraction is incorporated in the procedure anyway. Errors in calibration of the scale on a sextant can sometimes exceed a minute of arc for example if poorly divided, but that can only be determined on a proper dividing circle table or optical dividing head. Attempting to use star separations to try to determine scale accuracy for example would not be possible due to the variables in the measurements themselves - including the refraction component even if calculated. Checking Polaris altitude (Northern hemisphere) is much simpler for a casual, simple, practical check of the sextant index error if that is all that is wanted. Douglas Denny. Chcihester. England. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---