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

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Re: Lunars and accuracy generally
From: Greg Rudzinski
Date: 2013 Jan 2, 15:13 -0800

Attached is an image of the Sun using a 200 mm lens that is calibrated to .0975 minutes of arc per pixel at the center of the field. Today's solar diameter measures 335 pixels x .0975 = 32.66' diameter by camera.

To compare is a set of index error observations of the Sun on and off the arc using a Tamaya Jupiter sextant with a 7x35mm scope.

Off Arc
32.9'
32.7'
32.7'
32.4'
32.5'
Average 32.64' off arc

On Arc
32.4'
32.4'
32.6'
32.7'
32.7'
Average 32.56' on arc

32.64' + 32.56' = 65.2'
65.2'/2 = 32.6' solar diameter

I did try my hardest to be precise on the above measurements.

So what do I think my absolute precision is based on today's observations ? As a personal rule of thumb I double the range so with a range of 0.5' I can expect to be able to observe to ± 0.5'. Lunar distance measurements are harder than measuring the Sun limb to limb for index error so a tenth or two may have to be added so that ± 0.6' would be my expected precision on a single lunar observation from land doing everything right.

As for Cook on his voyages I think his 30' of longitude precision is pretty good if done from a deck of a bark using the equipment of the day.

Greg Rudzinski

P.S. Caution- Chasing precision can become an obsession. At some point error must be accepted.

[NavList] Lunars and accuracy generally
From: Frank Reed
Date: 2 Jan 2013 10:55
I received an email from a former NavList member, a couple of days ago.

He quoted me as follows:
"For lunars today, using a well-adjusted sextant with a 7x scope (with good weather, on land), I get a typical standard deviation on individual lunars of about one-quarter of a minute of arc --roughly two-thirds of my individual lunars are as accurate as that."

And then he wrote:
"You won't want to know. Nor care. But I would like YOU to know...
I SIMPLY DON'T BELIEVE YOU.
I would concede an SD of +or- half a minute of arc might just be possible in perfect conditions - but pushing it at that. I think you are peddling snake-oil.

Note: I have replaced the bold italicized words in his original email with upper case here. For those of you who were not following NavList messages from June 2009 through October 2010, you may not recognize the style. Others will surely know that this was from that most miserable of online character-types, a "know-nothing know-it-all". He ranted about the accuracy of lunars in October 2010 but based his conclusions on incompetence at every level: in his observations and in his calculations, too.

Nonetheless, I post this because I think it's an interesting expression of the degeneracy into which celestial navigation has fallen. There are people who are so incompetent at adjusting and using their sextants and working even the simplest of calculations that they actually do not believe it is physically possible to measure angles with a standard deviation of a quarter of a minute of arc. In another fifty years, pseudo-historians of the subject will probably be writing that celestial navigation was never more accurate than 10 nautical miles, but hey, that's "good enough" for those twentieth century primitives.

I repeat a little challenge I made earlier. Can you measure the diameter of the Sun today (or in the next few days), now at its largest for the year? Get out your sextant and give it a try. How close are you to the correct value? Do you get better results with a more powerful scope? If you're way off, where do you think you're losing accuracy?

-FER

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