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    Polaris correction tables
    From: Frank Reed
    Date: 2013 May 2, 10:21 -0700

    Gary L wrote:
    "I have attached the "Q" table from the 2013 Air Almanac, this is all you need for reducing Polaris shots."

    Here's the link to the file again:
    http://fer3.com/arc/img/123783.polaris%20q%20table.jpg

    The same table, by the way, is found in the every-five-year issues of H.O. 249 vol. 1. I just taught a class in "Intermediate Celestial Navigation" using Pub.249 this past weekend. It went well, but there's no escaping the tedium of plotting! :) I showed them how to enter the Q Table, of course.

    But the tables in the Air Almanac and Pub.249 are really the long way around to get that simple "Q" correction for the effect on latitude of the offset between Polaris and the North Celestial Pole. They both require the computation of "LHA A", the Local Hour Angle of Aries. Now this is not difficult. There are tables readily available for the purpose. But it's still extra work. The LHA of Aries is actually identical to what astronomers call the "Sidereal Time". And just as the observed Declination at the zenith is the observer's latitude, so the observed Right Ascension at the zenith (or indeed anywhere on the meridian) is the observer's Sidereal Time. That's a big mouthful that says that the LHA of Aries is an OBSERVABLE property of the sky, up to a certain level of accuracy, which is really sufficient for this Q correction. All you have to do is look at the stars above Polaris when you take your sight. If you can determine the orientation of Cassiopeia or the Big Dipper or other northern stars relative to the meridian by a simple visual observation, then you can read out the LHA of Aries, direct from the sky, to within about five degrees accuracy, which is plenty good enough to enter the "Q" table or a similar Polaris offset table.

    A few years back, Greg Rudzinski published a Polaris correction card which he called a "Latitude Wheeler". It's a little star chart showing Polaris, the Big Dipper, and Cassiopeia in a small circular chart. One can use such a chart to read off LHA Aries and then enter the Q table. But Greg's chart instead notes the Polaris offset right there directly on the circular chart. You hold it up, match the sky, and there's your Q. He also has a nice short refraction table and a dip table. This is all listed on a small, laminated yellow 3x5 card (the size of a common US index card). Here's a description and photo of one his cards:
    http://fer3.com/arc/m2.aspx/Emergency-Navigation-Rudzinski-jul-2012-g19945
    Note that the card is a little out-of-date. It would be possible to produce a long-term version by including a table of corrections on the opposite side for different years.

    For my introductory navigation classes, I provide students with a simple star chart of the northern sky with "RA" marked around the rim and then a table that provides the Q offset for five year intervals over 50 years (why not?) for a given RA of the meridian (which is identical to a given Sidereal Time which is equivalent to an observed LHA Aries). This is basically the same thing as Greg's "Latitude Wheeler" but it's not quite as compact.

    -FER


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