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    Re: Celestial Navigation without a sextant.
    From: Frank Reed
    Date: 2008 Mar 09, 19:31 -0400

    You wrote:
    "Supposing one was in a small boat with an accurate timepiece and the
    necessary tables, how accurate could you determine your longitude by
    observing the rising or setting of the sun or any other celestial body?"
    This would work with the Sun only. The time of sunrise can be trusted to
    about one minute (worse at higher latitudes). So that would give you your
    longitude to the nearest quarter of a degree. This could certainly be
    useful. At minimum, it's a nice quick sanity check. Say you're sailing west
    across the Pacific in the tropics. Overnight you make sixty nautical miles
    according to a taffrail log (or as estimated from average speed and he
    duration of the night). The Sun should then rise about four minutes later
    than expected. If instead it rises six minutes late, you may have a nice
    current helping you along... More accurate observations later in the day
    will tell.
    You could not easily use the Moon for these observations since the Moon is
    only rarely clearly visible right at rise or set and even then you have to
    correct for longitude. It's an iterative process. You can't use the stars or
    even a bright planet like Venus because they're simply not visible until
    they're a degree or more above the horizon. Atmospheric extinction at the
    horizon is 12 magnitudes or more.
    And you asked:
    "Also, assuming you had a compass and were north or south of the tropics,
    would it be possible to estimate your latitude by taking a bearing of the
    setting or rising of a celestial body?"
    This is less accurate simply because it's tough to get a good bearing. How
    about timing the length of the day (after correcting for the longitude
    A few years back, I mentioned that there are fish tags that work on exactly
    these principles. These are tags attached to tuna that record underwater
    brightness every few minutes along with GMT. Peak brightness gives local
    apparent noon and thus longitude (this is determined by the symmetry of the
    light curve around noon, not the actual "peak" brightness which would be
    hard to pin down). Duration of brightness as a surrogate for the length of
    day gives latitude, though not as accurately. Why do something so
    "primitive"? In short, because GPS doesn't work underwater.
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