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Sights around noon - correction for motion
From: Frank Reed
Date: 2009 May 4, 17:06 -0700

```I wrote yesterday (to Bill B):
"I wanted to mention that you are responsible for one of the few improvements
that I've added over the past few years to that method for finding longitude
with sights around noon. "

Just for general reference, I should probably have mentioned that this
"improvement" I mentioned was posted by Bill nearly a year ago. Here's the
relevant part from Bill's post (June 18, 2008):
"2. Plot the raw observations. X axis time, and y axis elevation.

3. Calculate the slope of north/south motion and declination changes.

This should be simple for list members.  At 12 kt it would be 0d 12!0 per
hour, and declination changes are listed in the NA by the hour.  If moving
toward the sun, the slope should be positive (subtracted from observed curve
to simulate the fixed-position LAN curve). Away from the sun, negative.
Declination increasing, slope component positive. Declination decreasing,
slope component negative.

4. Now plot the slope on the same sheet of graph paper used for the curve.
(Don't worry about a constant to shift the slope, plot it anywhere that is
convenient.) Plot it for an hour (easy), or if space is limited mentally
halve the x and y components.

5. Move the slope line with parallel rules (or any other plotting device) so
it is anchored at the first observation's x & y coordinates.

6. Using dividers or a drafting compass you can now graphically add or
subtract the slope component at any point along the x axis from the plotted
curve's y axis. Do so by successive approximation around the observed peak
until you construct the "static" LAN curve and have a second peak.

7.  Now you can fold the graph paper along the peak of the y axis of the
constructed LAN curve as per ..." etc.

The rest of that post is here:
http://www.fer3.com/arc/m2.aspx?i=105496

There are a bunch of posts on this general topic of longitude at noon back in
May and June 2008 which may be of interest.

-FER

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