# 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 9, 23:31 -0000

I suggest that Brad Morris and Peter Hakel are taking the wrong tack. Their procedure will work only if the two stars have the same (or opposite) azimuths. Only then can one calculate the angle between them, and then, as the next step, apply the appropriate correction for refraction by simple arithmetic. But in the general case, the job has to be done differently, to get the right answer. First, obtain the predicted position of star1, in altitude and azimuth. Add the appropriate refraction correction, to get the apparent altitude. Then do the same for star 2. Now, using those two apparent positions, calculate the angle between them using spherical trig. Then, the result will adjust itself automatically for refraction, depending on how the two azimuths differ. There are other ways to make the same calculation, but that' the simplest, conceptually. It's a similar process to the clearing of a lunar distance, except that it's simpler because there are no semidiametrs and limbs to worry about, nor any effects of parallax to account for. 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: "Brad Morris"To: Sent: Tuesday, March 09, 2010 9:48 PM Subject: [NavList] Star - Star Observations Gentlemen I have been playing around with star-star observations to determine the accuracy of the sextant arc. Calculating the star to star distance, without refraction is not a challenge, nor is correcting for refraction when both stars are on the same side as the zenith. Derive GHA Aries for the observation instant, apply SHA objects, determine instantaneous altitude for both objects using spherical trig, compute refraction correction based on altitude for both objects, create delta refraction correction and finally, subtract from star to star distance to get the observable distance for my location. It sounds like a lot of work, but I have set up a spreadsheet that uses the Celestron SkyScout as inputs. I just point at two stars, enter some data from the SkyScout (in Right Ascension & Declination), and the observable distance from my location at a known time is the instantaneous result. All the mindless tabular work is done by the spreadsheet. I don’t really even need to know which stars they are, as long as the Celestron does! Of course, I have checked my spreadsheet against some hand done calculations to check to see if it is working the way I expect it to…and it is. Here is the dilemma. When I get to larger angles, I need to go beyond my zenith. For example, I have been looking at Polaris vs Sirius. My latitude is about 40 degrees north. So Sirius is to my south, Polaris, naturally, is to my north. The nominal distance works out to about 106 degrees 20 odd minutes (forgive me, I don’t have the exact numbers in front of me). Because each object is on either side of my zenith, both objects will appear to be lower in the sky compared to the horizon, due to refraction. Yet because they oppose each other in azimuth, the observable distance between them should be larger by the sum of the refraction corrections, not reduced by the difference of the refraction corrections. That is, compute the true distance without refraction. Since each object is lowered by refraction, but in opposite directions, shouldn’t we add the refraction corrections to the nominal distance to obtain the observable distance? Best Regards Brad ---------------------------------------------------------------- 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 ----------------------------------------------------------------