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    Re: Measuring Dip in the 18th Century
    From: Gary LaPook
    Date: 2014 Jan 1, 00:28 -0800


    I am attaching a page from Wright's 1599 book, Certaine Errors in Navigation, that has the declination table for the Sun for March. You will notice that the equinox occurs on March 10th or on March 9th in leap years. When I first read this ( in 1980, prior to the internet) I couldn't understand how Wright could have gotten these dates so wrong. The mystery is explained. The Gregorian calendar was adopted in 1582, seventeen years prior to the publication of Wright's book so he should have been using the modern dates. But, England did not adopt the Gregorian calendar until 1752 so Wright's dates were on the Julian calendar, 11 days out of step with the modern dates. I have also attached the June page showing Wright's determination of the Sun's maximum declination as 23° 30' north while the modern value is 23° 26.3' a difference of only 3.7'. I have also included Wright's explanation of how he determined the Sun's declination.

    http://en.wikipedia.org/wiki/Gregorian_calendar

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