NavList:

Geoffrey, you wrote:
"However, getting better errors on your position fix than around 1nm is not just a case of using a better instrument to measure altitudes, you also need to increase the accuracy of all the other variables as well."

OR, as an alternative, carefully selecting sights where inaccuracy in those variables is less problematic. For example, if we stick to altitudes about twenty degrees or higher, then we can ignore temperature/pressure corrections. And if we stick to altitudes close to the meridian, like Bruce's Moon sights here, then all those issues of accurate time determination become much less important. Each second of time yields 0.25 minutes in altitude change strictly only for sights due east or due west right on the equator. It's a good rough approximation over a much broader range of conditions. But, as long as we stick to sights near the meridian, an observer can worry much less about small corrections to time. Even without any better values for input quantities, the spread in Bruce's observations, showing a standard deviation (yes, from a small sample) of about 0.2', seems larger than we should expect from theodolite observations. That's nearly what I get for lunars with a handheld sextant. There's a little wobble somewhere!

-FER

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