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    Re: Early Land Navigation.
    From: David Pike
    Date: 2015 Feb 10, 16:37 -0800


    David Pike, you wondered:
    "What did you mean the sun is so low it makes the use of an artificial horizon impractical?"

    I believe he said that the Sun is so far south that it makes the use of an artificial horizon impractical. But you see... he is in the southern hemisphere, so a far southerly declination for the Sun actually implies that the Sun is too high for an artificial horizon. Assuming a sextant has a maximum practical angular measurement of 120° (some manage a bit more but we'll ignore that), that implies a maximum altitude of 60°. How did 19th century explorers/surveyors in Africa and Australia, for example, measure latitudes on land with a sextant when the Sun at noon was higher than 60 degrees?

    Oops! Silly me.  I thought it didn’t sound quite correct, but I’m afraid I let the sciatica still present from sitting on the wet snow over Christmas overrule my brain.  However, many sheets of sketch pad later, all might not be lost.  I think it’s now been shown that there are many better methods than the use of a nautical sextant for land navigation, but what if you only had a mirror artificial horizon and a sextant which measured up to a Hs of 60°, and you wanted to measure a Hs of 65°?  Last month, I managed to prove to myself that the error caused by a tilted artificial horizon mirror was equal to the angle of tilt http://fer3.com/arc/m2.aspx/Tilt-Errors-Artificial-Horizons-DavidPike-jan-2015-g29955 .  Why not use this to advantage.  If you modified your AH to be able to tilt to a measurable angle, you could increase the values of Hs you could measure.  E.g. in the photo below, Hs = 65°, the mirror is horizontal, so the angle to be measure = 130°.  This is outside limits.  If the mirror is tilted 10° towards the observer, the angle to be measured = 110°.  This is inside limits.  Hs = 110°/2 + 10° tilt error.  I.e. Hs is still 65° but measurable.  And I was going to go to bed early tonight.  Dave   



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