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    Re: Correcting declination for longitude
    From: Frank Reed
    Date: 2019 Jan 18, 11:14 -0800

    Paul, you wrote:
    " In sixty years of Pacific navigation I have never applied that correction"

    Yes, you have! It just doesn't seem that way.

    Calling it a correction "for longitude" was a way of describing an adjustment for "estimated Greenwich time". We all know that the Sun's declination changes during the course of one day from one Greenwich midnight to the next. Around the equinoxes, the rate is just about one knot, one minute of arc per hour. In modern celestial navigation, we simply look up the declination for the correct hour and possibly minutes of UT (GMT). Three hundred years ago, they didn't know Greenwich Time so they calculated it from their best estimate of the dead reckoning longitude. That might be wrong by 15° implying a one-hour error in Greenwich Time (or equivalent), but this would imply only one mile (one minute of arc) error in the Sun's declination as opposed to a dozen minutes if the correction was ignored completely.

    By the way, if you read old navigation books or look at old calculations in logbooks and elsewhere, this "estmated Greenwich time" is sometimes referred to as the "Reduced Time" occasionally labeled "RT".

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

       
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