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    Fish tag longitude
    From: Frank Reed CT
    Date: 2003 Dec 12, 18:19 EST
    Anyone remember reading about fish tags that found their longitude by underwater lighting? I think they were trying to track tuna. They wanted tags that could record the longitude of the fish. If I remember correctly, the purpose was to determine whether the east and west Atlantic populations comprised one stock. But GPS doesn't work underwater! So the tags carried a reasonably accurate electronic clock set to UT, a light sensor, a radio transmitter, and a simple processor and memory. Every day, the tags record underwater light level, and on average this peak occurs at local apparent noon (rather accurately when you have a whole day of light measurements). Then by comparing the UT from the tag's clock with the apparent time you get the longitude. The tags eventually drop off, bob to the surface, and radio their results. Pretty clever and probably one of the last serious scientific uses for celestial navigation.

    Frank E. Reed
    [X] Mystic, Connecticut
    [ ] Chicago, Illinois
       
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