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    Re: What should I expect for a land based sight accuracy
    From: Frank Reed
    Date: 2014 Jan 9, 16:13 -0800


    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.


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