# NavList:

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

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Re: Calculating Ha from Hc
From: Frank Reed
Date: 2014 Mar 26, 08:44 -0700

Sean, you wrote:
"I have devised the following method for "reverse engineering" parallax in altitude"

Hmmm. But what is your purpose here? What are you trying to calculate?

You also wrote:
"It has been stated that, in terms of lunars, getting the altitudes exactly right is not crucial."

That's right. For 'average' lunar circumstances, an error of +/-6 minutes of arc in the altitude of the Sun or other body will not lead to an error greater than a tenth of a minute in the cleared distance. An error in the Moon's altitude is much less important when the distance is nearly 90° (and this leads to many interesting results, but we'll set that aside for the moment). Of course both altitudes can be observed easily enough at sea. Note that for unusual (not 'average') lunars, the requirement for accurate altitudes increases. For example, if you shoot short distance lunars (less than 20°) then you need greater accuracy in the observed altitudes, but these were not recommended historically in any case.

" However, after comparing my calculations of a recent lunar to the results from Frank's online calculator and also interpolating the "observed time" myself, I found that it does matter...however little."

I'm not sure what you're getting at here. What are you doing with the observed time? If I interpret that phrase 'however little' literally, then I would say 'yes'. A small change leads to a small change. But is it significant?

I should mention that the altitude of the other body, especially if it's the Sun, may ALSO be used for a time sight (to determine local time) and if that's the case, then we want the altitude of the other body with as much accuracy as we can reasonably achieve. This is not essential. If we have a simple watch with a second hand available, or something equivalent, we can take a time sight whenever convenient before or after shooting the lunar and then just adjust for the elapsed time.

"The white text on a black background is a little hard on the eyes. When I look away from my laptop screen, I see lines everywhere."

Yeah. Me, too. We're getting old! :)

-FER

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