# NavList:

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

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Re: Curve fitting for noon Sun
Date: 2020 Jan 12, 19:52 -0500
Rafael wrote: My initial celestial navigation project was to try to determine latitude by noon sun, to start with an easy measurement

He did not write: The point of this process is to determine the UT of meridian passage.

So it is not "Wrong. Wrong. Wrong."

On Sun, Jan 12, 2020, 7:33 PM Frank Reed <NoReply_FrankReed@fer3.com> wrote:

"Plot your points as shown, on a piece of paper.  Fair in a curve with a pencil.  Fold the paper so the left and right halves are superimposed. "

The part about "fairing in a curve" was common in late 20th century books that described this process. Do not do it! It spoils the whole thing. Plot the points. Then fold (lightly) and hold up to a bright light. Slide the paper back and forth until the before noon/after noon points line up as well as possible. Then crease the fold. If you draw the curve first, then no matter how much you pretend to ignore it, you will be inclined to superimpose your sketched curves rather the observations points themselves.

"Further, you can skip just about all of this for the sun's meridian passage.  The sun will hang there, with hardly any perceptible change.  Simply determine when you think LAN will occur as a function of your longitude and time zone."

Wrong, wrong, wrong. The point of this process is to determine the UT of meridian passage. You then calculate GHA at that UT. This is the observer's longitude (after applying a small correction for relative N/S motion).

Frank Reed

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