# NavList:

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

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Re: Which calculator to use for "arctan(tan(269))"?
From: Tony Oz
Date: 2017 Jul 6, 01:14 -0700
Calculate
<snip>
Pol{ [ cos(Lat) · tan(Dec) - sin(Lat) · cos(LHA) ] , [-sin(LHA) ] }

and then

Hc = cos-1( r cos(Dec) ) and θ = Zn

There is no need for a separate calculation of the altitude!!! One proviso, as Lars indicated, is that it only gives the correct answer for objects above the horizon.

I attempted to feed this formula the coordinates (Lat, λ) of my location and Singapore's: 60°N,30°E and 1°17'N, 103°50'E - to see what it gives.

To my surprise - the Hc obtained was exactly the GC distance between me and Singapore, coinsiding with the check calculation by the formula:

d = arccos[sin(Lat1)·sin(Lat2) + cos(λ1)·cos(λ2)·cos(Lat1 - Lat2)]

So, may be the Pol() calculator's function is much more usefull that I expected. I guess I must try it to see what Robin's and Lars's warning about being above the horizon means.

Regards,

Tony

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