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    Re: Can you work a Polaris problem just like any other star?
    From: Frank Reed
    Date: 2014 Jan 4, 20:00 -0800

    Greg, you wrote:
    "am I wrong in thinking that can you work a Polaris problem just like any other star?"

    Yes, certainly. All sights are all the same from the perspective of modern celestial navigation. The coordinates of Polaris changes more rapidly due to precession, but the significance of this can be exaggerated. Given all this, there's also no particular reason to prefer Polaris over any other second magnitude star. We don't need Polaris.

    And you wrote:
    "I know that the USNO Nautical Almanac has a set of tables [...]"

    These are traditional tables, and they're kept in the almanac primarily because of tradition. There are other ways of handling this that are just as good (among many, see for example the attached tables). Note that the calculation for latitude by Polaris is usually SHORTER than a complete sight reduction by hand so it does have that benefit.

    You wrote:
    "I began to think about this because I just bought a program called 'AstroNav' it is a almanac program that goes from 2000BC up to 8000AD"

    Well, it CLAIMS to have that data. :)

    And you wrote:
    "I once modified a nocturne to show the error but I don't think it worked exactly correct."

    It should have. This is the fundamental idea behind the short tables for Polaris. Greg Rudzinski made up some neat little pocket cards that function as nocturnal combined with Polaris correction data.

    You concluded:
    "I believe that early explorers (on land) used to sit up late into the night
    with a sextant and mercury horizon waiting for Cassiopeia and Ursa Major
    to be parallel to the horizon (happens twice in 24 hours) supposedly the
    error is zero at that time, and they could read the latitude directly, after
    correcting for IE, etc."

    Sounds like that might be a legend more than a reality. The correction for local sidereal time (or equivalently orientation of a noctural) is easy enough to get either by calculation or by observation. You don't need to wait for a specific hour, and they knew this historically. There IS a specific advantage for taking the observation when the local sidereal time is 18 hours (just another way of saying that certain stars in Cassiopeia and Ursa Major are aligned horizontally), but only if you have no access to current astronomical data. Have a look at the attached star chart and tables. These are tables I created for my classes. Notice what happens when either 6 or 18 is up/down. The correction for latitude at that orientation is easy and also unchanging over the decades (and it's no coincidence! this is a property of precession). For any other orientations, you just read out the correction based on the year. Easy.


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