A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2020 Jan 17, 02:31 -0800
Laurence Holden You wrote: With somewhat over 200 sun sights with my AstraIIIB now, I rarely come up with an LOP under 20NM, and I can'tfigure out why. I've checked and rechecked the setting of my mirrors (I'm using the whole horizon mirror) and and they are correct. I am doing my sights in a pan of water. All I have left to wonder is if my calculations are somehow wrong. Would someone look over a recent sight reduction of mine and see if I am doing something wrong?
You don’t say if you’ve been observing the Sun, a star, a planet, or the Moon. If you’re observing the Sun using an AH, try putting one Sun over the other and forgetting about semi-diameters, I can never work out which limb is which, can you? It’s less accurate, but it should give you a better result than 20’. Better still, try observing a nice bright star, or an unmissable one because of its colour. Having been trained mainly using periscopic bubble sextants, I’m afraid I cheat by setting 2xHc on the sextant first. If you still see no improvement, forget calculating for the moment, and try comparing your Hs less instrument error with the value you should be getting from Frank Reeds GPS Anti-Spoof Android App (you can set it to AH, centre obs, zero ht of eye). I calibrate my peri-sextants this way. If you’re within a couple of minutes of arc, then there’s probably not much wrong with your sextant or your observing technique, and the error is probably in your calculations or plotting. If you do decide to forward calculations and don't fancy trying to type them, you could always photograph your hand written calcs and send them to NavList that way. DaveP