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    Re: Sadler
    From: Frank Reed
    Date: 2008 Dec 4, 08:33 -0800

    George H., you wrote:
    "One thing that struck me was the rarity, in that office, of anyone with a 
    direct practical interest, or any personal experience, in navigation. I would 
    have thought that having a grizzled old retired seadog on-hand might have 
    been rather useful, when choosing the bast layout for tables. I wonder 
    whether that was the case, also, with their counterparts in the US."
    I don't know if it was the same in the US. But it is my impression from 
    reading various historical reviews that they got the input they needed from 
    numerous interviews and surveys. I think an old seadog as authority might 
    have been more trouble than good -- you might end up with a lot of strong 
    personal opinions declared to be common practice 'throughout the seven seas'. 
    It is worth noting that whatever process the two offices used to get input on 
    the design and layout in mid-century (of the almanacs at least), it was very 
    successful. The Nautical Almanac today is nearly identical to the first 
    edition of the new almanac published in 1958. About the only component that 
    could be counted as a failure (in my opinion) would be the short sight 
    reduction tables which were added in the 1980s --as was discussed a few weeks 
    ago, no one seems to use them.
    Regarding Sadler's comments on his dip experiments, you asked:
    "Why should the sea surface not be an equipotential near the shore? Is he
    talking about gravitational effects from the adjacent land-mass? Or about
    the effects of tidal inertia? Or what?"
    The tides are only equipotential surfaces in static approximations. Just 
    consider that it can be low tide at one port a few hundred miles from another 
    where it's high tide. 
    "Are these effects, whatever they are due to, likely to be big enough to affect dip observations?"
    Maybe in some unusual areas with large tidal ranges (possibly inluding the 
    seas around Britain) you might want to worry about that at least 
    hypothetically. In order for this to make any difference, you would need a 
    significant difference in the height of the tide between your observing 
    location and the horizon. Suppose there's a five foot difference, which is 
    about the most I can imagine. If your observing location is a hundred feet 
    high, then the horizon is about ten nautical miles away. At that distance, 
    five feet amounts to about 0.3 minutes or arc which would, in fact, be 
    relevant to detailed observations of dip. In addition, the curve of the sea 
    surface might be somewhat flattened or somewhat more rounded than the 
    curvature of a sphere (not much though). But I suspect that Sadler's 
    discussion of tides was just a speculation on his part --something that he 
    hoped might explain away his unsuccessful experiments. Unfortunately, dip is 
    variable, especially close to shore, and it's the atmosphere's temperature 
    profile on a small scale that drives this, not the phenomena of tides.
    Incidentally, you could also worry about the heights of hills and valleys on 
    the ocean's surface. None of them that I have examined occur rapidly enough 
    to affect dip. For example, near the margins of the Gulf Stream, the sea 
    height changes by 4 to 6 inches in a distance which, in unusual cases, is as 
    short as ten nautical miles. But half a foot seen from ten n.m. distance 
    subtends an angle of less than two seconds of arc. So we don't have to worry 
    about that.
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