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    Re: Kollsman periscopic sextant mount
    From: Frank Reed
    Date: 2020 Sep 22, 11:31 -0700

    Bill Lionheart, you wrote:
    "I was amazed when I first went to UC Berkeley in the 1980s to hear kids were taught to do physics in obsolete units"

    They weren't obsolete in US engineering and manufacturing. But the case you cite was an outlier. By the early 1980s nearly all physics education, outside of engineering schools has switched to SI. Of course, those who then went on to make money in engineering had to study up on the frumpy, unfashionanle units later. Poor things... making fathoms of doubloons and cursed with antique words. Must have been very painful for them. ;)

    "I had only met this with my dad who could do foot pounds per fortnight"

    Foot-pounds per fortnight... good for laughs... and as ridiculous as joules per week. In fact, of course, even here in the US of A, power is almost always quoted in SI watts and energy in most practical contexts in the slightly off-SI "kilowatt-hours". But ya know, the unit that really killed the old system in physics was the standard mass unit: the slug. Slug is an awful word. You don't even have to get passed the first two letters, and you know it's an awful word. Nearly all words that begin with "sl" are slimy, sleazy things. Only a slave to slander or a slovenly slut would slaughter physics with slugs. Probably invented by some slouch at Slytherin House.... 

    There's a fairly good Wikipedia article on the US system of units: https://en.wikipedia.org/wiki/United_States_customary_units. A useful quote from the article: "Americans primarily use customary units in commercial activities, as well as for personal and social use. In science, medicine, many sectors of industry, and some of government and military, metric units are used."

    So let's get back to navigation, OK? That's a message to everyone commenting in this thread. How do issues of units impact navigation?

    For navigation-related issues, normal weather conditions will continue to be described in traditional units for the foreseeable future. Temperatures in °F and pressure in inches Hg. It's conceivable that the USCG (and USN maybe) could issue a decree announcing that all weather data will henceforth be listed in SI units. That might tip the scales in navigation practice. I don't think such a decree is likely anytime soon. For general use, until purely digital devices are 100% ubiquitous, the installed base of thermostats and other devices displaying °F is much too large to switch to °C. The cost would be prohibitive and the benefit near zero.

    The most important unit in navigation, the nautical mile, is more metric than traditional despite the sound of it. In fact, with just a slight change in the order of events, the nautical mile might well have ended up as the fundamental length unit in the original metric system. The definition of the kilometer was modelled directly on the well-known definition of the sea mile or nautical mile. There is a simple integer number of these along a meridian of longitude, specifically from equator to pole. There are 90·60 or 5400 nautical miles from equator to pole. The original metric plan was to ditch the 90° right angle with 60' to each degree and replace that 100 degrees each divided into 100 sub-units. In that system the sub-units, analogous to sea miles, would be known as kilometers. Thus there are 10,000 km from equator to pole, and that implies that the nautical mile ans the kilometer have an almost exact integer relationship: 54n.m.=100km (both units have been slightly re-defined over the centuries, and this relationship is now off by a mere 8 meters, about 26 feet). Of course, astronomers and others objected strongly to any re-definition of angular norms, and they preserved degrees and minutes as standard measures. But suppose they had won the day just a little earlier... If the designers of the new systems of units had decided on day one that degrees and minutes of arc would remain in place, then they could have adopted the nautical mile directly as the new "kilometer". This would have worked just fine, and the derived "meter" would have ended up a bit more than six feet long, about the same a nice old-fashioned fathom. The nautical mile could easily have qualified as the basis of all length measurement.

    Note: the "nautical mile-ish" definition of the meter based on the length of the meridian from pole to equator was the original definition. Of course it's been re-defined several times (with each re-definition carefully containing the old definition), and in the modern SI, the meter is defined in terms of the second, such that the speed of light has the exact value 299,792,458 meters per second. But it's still true that 54 nautical miles = 100 kilometers with only an insignificant difference. 

    Frank Reed

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