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    Celestial Orange
    From: Frank Reed
    Date: 2015 Feb 16, 14:24 -0800

    Three weeks ago, I posted elsewhere a photo of my "celestial orange". You can use an orange as a makeshift globe and plot circles of position on it for an approximate fix. If you plot the equator and meridians on it in permanent ink, and use water-soluble ink for your circles of position, it's reusable. You can erase an orange! Since the original post, my celestial orange has dried out a bit, but it still works just fine (and it's still erase-able). Conveniently, it has shriveled to a near perfect nine inches in circumference which means I can use a common US-ruled index card as a latitude scale where each line on the card is 10 degrees (a quarter inch is 10° of latitude or distance). Here we see a fix from two Sun sights taken on July 11, 2014 aboard the historic whaleship Charles W. Morgan: 42° 05' at 13:17:50 GMT and 67° 19' at 15:58:00. Crossing the circles of position yields a position, as read from the orange, of 40° N and 67° W. The actual position was 42.1° N, 70.3° W. Not bad... for an ORANGE!!! The error in the fix from the primitive plotting technique is about 190 nautical miles. Obviously if I had a 45-foot wide perfectly spherical orange, I could get a fix with an accuracy of one nautical mile. No sailing vessel should leave port without a giant orange.

    Frank Reed
    ReedNavigation.com

    PS: In practice, if 45-foot wide oranges are not available, use an app... or tables of logarithms... Same geometric problem, minus the juice.



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