NavList:

David, you wrote: 
"89°60' instead of 90°00'. That's a clever way of getting zenith distance".

Indeed it is, and in the late 18th and early 19th centuries, they took it one step further. For latitude by Noon Sun, they subtracted the altitude from an angle that I call the "navigator's right angle". It was 89°48'. It's a simple trick. By the normal rules, you're always adding 10 or 11 or 12 minutes to the Sun LL altitude to correct it. That's +16' for the Sun S.D., minus a few for dip and one or two more for refraction. So let's just call it 12' as a quick and dirty average. You then subtract that sum from 90° to get zenith distance. But we can save a step by taking the 12' from 90° in advance for all of our noon Sun sights. Then our calculation becomes z.d. = 89°48' - altitude ...where the altitude is now just the directly observed altitude of the Sun's lower limb. This was a common and very clever approach to the calculation even as late as 1900, but by then it was considered a bit lazy. Lecky rants about it in "Wrinkles". :)

Frank Reed

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