# NavList:

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

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Re: What should I expect for a land based sight accuracy
From: Gary LaPook
Date: 2014 Jan 10, 10:51 -0800
Don't use the fix accuracy to evaluate the accuracy of your sextant work because this introduces additional error. For instance, if you have two sights, each with a one minute error, and a cut of 90 degrees the fix will show a 1.4 NM error. If the cut is 30 degrees then the fix will plot 3 NM from your location.

Just use your GPS coordinates as your AP and then the intercepts will accurately show the quality of your sextant work. The easiest way to use your GPS position as the AP is yo use the U.S. Naval Observatory website to do the sight reduction. Here is the link.

http://aa.usno.navy.mil/data/docs/celnavtable.php

Don't get lazy, however, continue to practice with other methods of sight reduction, such as HO 229, HO 249, log methods such as HO 208 and, of course, the Bygrave solution available here which has the advantage that you can work it from your GPS position as your AP:

gl

From: Greg Licfi <cfi@licfi.com>
To: garylapook@pacbell.net
Sent: Thursday, January 9, 2014 10:07 PM
Subject: [NavList] Re: What should I expect for a land based sight accuracy

Frank, Alan, Randal, Greg,
Thanks guys too cloudy tonight for anything Lunar or otherwise - I'll try again
when we get some clear sky's.
~Greg

On 01/09/2014 07:33 PM, Frank Reed wrote:

Greg,
Your average error here is 2 n.m. That's not bad, but there's plenty of room for improvement. A proper a.h. will help, of course. There are other observations you can try to test your sextant and your skill.
If the sky is clear tonight, try measuring the angle between the Moon and Jupiter. That is, make Jupiter just barely touch the clear (illuminated) limb of the Moon. It's a nice land-based observation, and it shouldn't be too uncomfortable with tonight's geometry. Wait until Jupiter is at least 5° high, and preset the sextant to about 63°. Then aim the horizon view at Jupiter, and rotate the instrument until the Moon pops into view. Adjust until Jupiter just touches the bright limb of the Moon as you rock it slightly back and forth. Record the time (nearest five seconds is fine), and record the angle. Let us know what you get. Also include your position to the nearest few miles. you can do this every night that the sky is clear.
When you're testing your sextant and your skill with it, it's important to separate out the various components that can lead to error. For example, you wouldn't want to use some paper method of clearing your sights (unless you're perfectly skilled at that task --and who is these days?) since that just introduces another source of error. Focus on observations and the instrument first. An angle that is precisely defined and exactly calculable, like the arc from Jupiter to the Moon at a known time and known location, provides a reference that we know is correct. If you measure that and get good results, then you can start adding in the other pieces of the puzzle.
-FER
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