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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » How to take horizontal sextant sights

   
Author Topic: How to take horizontal sextant sights
David Burch


 - posted March 21, 2004 05:44 PM      Profile for David Burch           Edit/Delete Post 
How to take horizontal sextant sights

These sights are used for finding position or distance off. The 3-body position sight is one of the most accurate piloting techniques. It can be executed to very good precision with a simple, inexpensive Davis Mark 3 sextant.

In this sight we need to measure the angles between 3 bodies: L on the left, M in the middle, and R on the right. They can be anywhere from 20° or so to 100° or so apart. There are various criteria for choosing the best bodies or landmarks, but that is not the subject here, other than to note they should be roughly at the same elevation.

We will add illustrations to this as time permits. We have just finished a class on this that pointed out the value for some notes to review before doing the practice. So this is just a first step in that process. By the way: in the example we did in class, a remarkably good compass bearing fix was off of our precise GPS position by some 130 yards, but the 3 body fix taken as below was as close to the GPS position as we could plot on the chart scale we had.

Step (1)
Check and adjust the index error. Set the sextant to 0° 0' and view any distant horizontal line with the sextant vertical. If the direct view on the left (no glass) and the reflected view (in the mirror) do not make a straight line, then adjust the screw on the index mirror until the horizon is a straight line. (more on this process later... in contrast to cel nav sights where 1' is roughly 1 mile, this index adjustment does not need to be super accurate for these piloting sights)

Step (2)
Set sextant to 0° 0', and hold it horizontal with the right hand, roughly parallel to the shoreline with the mirror on the top, and look straight toward target L on the left. It should appear just above the center of the mirror.

Step (3)
With the left hand, slide the index arm away from you slowly, watching the shoreline in the mirror move to the right in the mirror. The idea here is to bring the center of mark M into view in the mirror just below the center of mark L. See next step.

Step (4)
Unless you are very lucky, mark M will not appear in the mirror without a slight adjustment of the angle of the sextant relative to the horizon. That is, if you hold it perfectly parallel to the shoreline, you will not see mark M appear, but just the shoreline below it or terrain or sky above it. So this crucial step is to rotate (roll) the sextant slightly till you see mark M or some part of the shoreline near mark M that informs you which way you have to go: more to the right or more to the left.

Step (5)
Once you have them both in sight, L in the direct view, M in the mirror below it, carefully adjust the index arm until they are precisely lined up, and then read and record the sextant scale for your answer: the horizontal angle between L and M.

Step (6)
Then do the same thing again, replacing L with M and M with R, to obtain the second answer: the horizontal angle between M and R.

--david

From: Starpath, Seattle, WA


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