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Randall,

You asked:

"Will changing to a metal instrument such as an Astra markedly improve results?
Do direct calculations using the law of cosines give better results than Pub. 229 tables? "

1. Yes a metal sextant will improve your sights. Reasons-Index error will not change as much over time, mirrors are bigger, optics better made, less sensitive to temperature change, and can handle a good squeezing without deflecting the observation (just to name a few reasons). Astra, Tamaya, Plath, and Frieberger are all well made and can be had on Ebay. Budget about $450 and stay away from anything up for auction out side the U.S.

2. Yes a calculator will give better results than Pub. 229 but there is no need to be better than a tenth of a degree which 229 will do. Using a scientific calculator with trig functions is a good way to perform sight reduction. Worth memorizing the formulae. I like the following pair :

Hc= InvSin [ (Cos Lat.)( Cos Dec.)( Cos t) +/- (Sin Lat.)( Sin Dec.) ]
Az= InvSin [ (Cos Dec.)(Sin t)/(Cos Hc) ]

Greg Rudzinski

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