# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: "Vernier acuity" of horizon IC tests**

**From:**Frank Reed

**Date:**2009 Jul 4, 18:09 -0700

Greg, you wrote: "An unorthodox method that I use for adjusting and determining index error is to tweak the side error adjustment just enough to split a star into two images with the sextant set at zero (with a bright star I even use a weak shade to present a point of light instead of an asterisk of light). With the two stars side by side I seem to do a better job of zeroing my index error." Bill B., if I remember correctly, suggested using fainter stars for this, which strikes me as equivalent to your use of shades to eliminate the flares in the image. I've tried this, and I agree that it's effective. And yes, I've also found, and others seem to agree, that leaving a little so-called "side error" is helpful with these sorts of observations. Just as you say, it's easier to align the images side by side than to superimpose them. And you wrote: "A fussy navigator would probably tweak the side error back but I leave mine off so that I can periodically recheck index error. Question: What kind of errors get generated on high altitude observations if side error is a few minutes out?" Yeah, you're right about the 'fussy navigator' here, and I think this is mostly because so many navigation manuals insist on it. They emphasize that one should eliminate "side error" with the same insistence as the other adjustable errors. But this is really over-kill. If the side-by-side gap as seen through the sextant is k minutes of arc, and the observed angle is d, then the error in minutes of arc in any angular measurement is given by error=(1/6876)*k^2/tan(d). So suppose k is 5' which is probably more than you need for the trick you describe above. Then if d is 30', the error is about 0.4 minutes of arc. That's the error you would find in measuring the diameter of the Sun or Moon using the sextant with that large side-by-side gap. But suppose that d is ten times bigger, 5 degrees. Then the error would be just about ten times smaller, only 0.04', negligible, and dropping roughly in inverse proportion to the measured angle. So there are really no practical observations affected by "side error" at all. When this has come up before, I think I said that checking IC by the Sun's diameter on and off the arc would be influenced by this error, but it's not true. The error cancels out. So don't fuss over that side gap. It doesn't matter. -FER PS: The error formula above if everyhing is done in pure angles (in "radians") would be error = (1/2)*k^2/tan(d). Dividing by 3438 turns one factor of k into a pure angle and leaves the other in minutes of arc so the result is in minutes of arc. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---