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    Finding Longitude from LST
    From: Duane Smith
    Date: 2020 Aug 16, 14:46 -0700

    I seem to have discovered a quick easy way to get Longitude from the star clock around Polaris.

    Accuracy depends on how well you guesstimate, measure, determine a stars clock position. The clock is  24 hour that runs Counter Clockwise. Straight above Polaris is LST 0 hours, straight below is 12 h. Straight level West is 6 h East is 18 hours. Get the clock position as close as you can, then add the stars Right Ascention.

    Clock time+RA of star, if the result is more than 24, subtract 24. This is your Local Sidereal Time. Determine the Greenwich sidereal time of the sight. If GST is less than LST add 24, GST+24, then subtract LST from GST. This is your Longitude in hours. If you kept everything in decimal, calculations are straightforward, with Longitude in decimal hours multiply by 15. This is your Longitude in decimal, convert to D.mmss if you like.

    How hard can this be? Caph, beta Cassiopeia, close to Schedar, #3, has an RA of 0h 10m 16s. When this star is on your meridian above Polaris, your LST is 0h10m16s. West of Polaris LST is 6h10m16s. Below Polaris LST is 12h10m16s. Pick any star you like, many astronomy apps can give you the RA of a star. With current accurate GST, you can get Longitude. It is easy to make a clock face to make measuring position more accurate.  With the crude one I made, to get within 10 minutes of actual LST is fairly easy, sometimes within 5 minutes or less. Two minutes gives Longitude within half a degree.

    I welcome comments, thoughts, criticism. If you find this of interest let me know.

    Duane Smith


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