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It is not really surprising that Wright's values for days near the
solstices - where declination changes only slowly - are pretty accurate,
because for that you only need a fairly exact value for the obliquity of the
ecliptic, which is the maximum value of the solar declination at the time of
the solstices. As far as I remember Abraham Zacuto - and following him the
Portuguese manuals of the early 16th century as well as Pedro de Medina,
1545 - used a value of 23 deg 33min; Regiomontan gave it as 23 deg 30 min in
the late 15th century. He was followed in this by Pedro Nunes half a century
and Edward Wright  a century later. William Bourne ("A Regiment for the
Sea", 1574)used a value of 23 deg 28 min, by the way.

You can calculate the obliquity of the ecliptic using the formula given in
the "Explanatory supplement to the Astronomical Ephemeris..." (London 1961),
p. 169:
23 deg 27 min 08,26 sec - 46,845 sec *T - 0,0059 sec*T*T + 0,00181 sec*T*T*T
(where T denotes centuries from 1900.0).

Much more critical are dates around the equinoxes where declination changes
very fast. I checked the accuracy of the tables in the oldest German
navigation manual (of 1578) many years ago and found that they showed a
systematic error of 5 - 6 min around the equinoxes, whereas the faulty value
for the obliquity of the ecliptic leads to an error of 3 min at the
solstices.

Wolfgang

-----Urspr�ngliche Nachricht-----
Von: NavList@navlist.net [mailto:NavList@navlist.net]Im
Auftrag von glapook@pacbell.net
Gesendet: Montag, 10. Dezember 2007 20:50
An: NavList
Betreff: [NavList 4281] Re: Certaine Errors in Navigation Corrected

Gary adds:
It is interesting to compare Wright's almanac with the modern one. The
 sun's declination today, December 7, 2007 at noon in England is 22�
 35.7' south. If we were using Wright's almanac for the same day,
which
 would be November 27th in the column for 1607 (found on page 118), we
 get his value of 22� 38' south and a modern almanac program gives it
as 22� 39.0', only one minute differece!

gl

On Dec 10, 11:44 am, glap...@pacbell.net wrote:
> Gary writes:
>
> It is interesting to compare Wright's almanac with the modern one. The
> sun's declination today, December 7, 2007 at noon in England is 22�
> 35.7' south. If we were using Wright's almanac for the same day, which
> would be November 27th in the column for 1607 (found on page 118), we
> get his value of 22� 38' south which is close enough for government
> work. In comparing Wright's almanac with the modern one you must
> subtract 10 days from his tabulation since England did not change to
> the Gregorian calendar until September 25, 1752 while most Catholic
> countries changed on October 5, 1582. (This caused me much confusion
> when I first looked at his almanac almost thirty years ago since I
> assumed that Wright was also using the Gregorian calendar since he
> published 17 years after that calendar went into effect.)
>
> Also, comparing his value for the declination of the sun at the summer
> solstice of 1597, he gives it as 23� 30' north while a modern almanac
> program gives it as 23� 29.6' north, a difference within the limit of
> precision of Wright's table!
>
> Remember that the letter "s" was printed in 1599 with a character that
> looks like the modern letter "f" and that the letters "u" and "v" are
> often interchanged.
>


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