# NavList:

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

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Re: getting GHA from MICA
From: Peter Hakel
Date: 2014 Mar 21, 15:26 -0700
Sean,

Try the attached spreadsheet aimed at (approximate) pre-computation of Hs from Ho. Input cells are green and the Ha you want is in yellow.

This spreadsheet is really just the reverse of the original alt_corr.xls:

which is why I retained the “Ho” labeling of cell B12, although here it really represents “Hc”.

Basically, plug in Hc where you would use Ha in the parallax and refraction corrections, and reverse their signs. As I recall, Frank made the point that lunar distance clearing is rather insensitive to getting the exact altitudes, so this result just may be good enough for your purpose.

Peter Hakel

From: Sean C <yhshuh---.com>
To: pmh099---.com
Sent: Friday, March 21, 2014 3:32 PM
Subject: [NavList] Re: getting GHA from MICA

"My later program, Lunar3, is better for navigation. It's primarily for lunars, but will also compute altitude intercepts.." -Paul
Speaking of lunars, I've been using all of these programs and Frank's web page to check my work after manually (on a calculator) working some lunars. If I understand correctly, the "corner cosines" Frank writes about should be derived from the apparent altitude of the body at the time of observation. I'd like to ask everyone how best to find Ha from Hc. Currently, I'm using Hc & HP to estimate the parallax in altitude, then using trial and error to arrive at the correct value. I do the same for refraction, but this is generally easier since the value's rate of change seems to be slower. I tried thinking of a way of reversing the equations, but since the correction is derived from the value I'm seeking (and I'm no good at math), I can't figure it out. Any help would be greatly appreciated.
Thanks,

Sean C.

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File: 127330.alt_prec.xls
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