NavList:

Francis,

The 6" slide rule seems to get close to 3 place precision which is enough to handle ex-meridian calculation needs. On ex-meridians within 30 minutes time of LAN the altitude is changing very little vs. time so meridian angle, declination, and latitude to the tenth of a degree is good enough for determining the minutes of correction to 2 places. The 6" slide rule is plenty good for calculating azimuth, amplitude, and prime vertical to within a few tenths of a degree as well.

As for lunars. The Hc can be calculated for the bodies in lieu of direct observation with the sextant. This Hc calculation doesn't need to be done as precisely as the rest of the lunar calculation. I will refer to Frank for a better explanation on this detail. 

Greg Rudzinski

From: Francis Upchurch
Date: 2016 Jan 1, 00:57 -0800

Happy new year Greg.

I'm intrigued by the accuracy you achieve with a pocket slide rule.I generally only get to 3 places on my 10inch linear, and therefore tend to use the Fuller 2 (4-7 places) for cel nav. I suspect like most of us you are a studeht of JS Letcher's Self Conatained Cel Nav withHO 218? 

I learn't my first cel nav from it and Hewitt's Book Common Sense Cel Nav.) in the 80s and read it again every year, always learning something new. 

I've always been a bit confused by his comments on page 92, suggesting that a standard (?10inch) trig slide rule is ok for clearing lunars with his formulae.

Anyone got any comments on this? I've only done it with the Fulller2, believing you need as much accuracy as possible for lunars.

What sort of results do you get with the intercept method and cosine formulae?

Any ideas appreciated.

Best wishes

Francis