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Re: Early lunars
From: John Huth
Date: 2010 Mar 20, 10:17 -0400
From: John Huth
Date: 2010 Mar 20, 10:17 -0400
George -
Yes, Wright does quote longitudes of these locations. But it takes some careful reading to get the gist of the values.
There are some issues that leave me scratching my head. I can quote what's in Wright's article, but there is a caveat.
In looking at the longitudes in addenda to the Toledo and Marseilles tables, along with the Hereford tables, Wright concludes that since Ptolemy's time, there was a shift in the prime meridian from the Canary Islands to a location further west, putting Toledo 28 degrees 30 minutes East of the "new" prime meridian, and cites some text by Francis Bacon to bolster this claim. In some of the tables during that era, Toledo is cited at around 10 or 11 degrees East, which is consistent with the choice of the Canaries as the PM. In ones slightly later, or appended to the Toledo tables, it appears as 28 degrees east.
I entered the data from the Wright's translation of the earlier version of the Toledo tables into an Exel spreadsheet (i.e. ones that have Toledo at 11 degrees E of the prime Meridian).
I then had to do some selective data cutting to obtain numbers that I thought were reliable. I eliminated cities along the Silk Road (probably just dead reckoning), and Baghdad, islands (too large), locations south of the Sahara (Urbs a Nuba, Ghana - again probably from dead reckoning) and concentrated on major cities around the Mediterranean, which seemed like a safer bet. When I took the mean value of the Prime Meridian by fitting against the currently known longitudes, I derived a value of 23 degrees West of Greenwich, with an uncertainty of 1 degree. The biggest outliers were, in fact, Toledo and neighboring cities, and if I accept the updated values for Toledo, I got a better fit.
So, what has me scratching my head is this question "why was the Prime Meridian shifted from Ptolemy's use of the Canaries?"
This would put the longitude of the Prime Meridian at roughly the Cape Verde Islands.
Another source from that period is the Arab geographer is Yaqut ibn Abdallah ur-Rumi (1179-1229). In a translation of the introduction to his geography treatise, he says that the Prime Meridian is located 200 farsakh's west of the coast of the country of the Maghrib. This is a bit difficult to figure, since the "country of the Maghrib" is pretty darn large. But, taking a farsakh to be 3 or 4 miles - it depends on the source, I also get about 23 degrees west of Greenwich for the Prime Meridian used in Yakut's time.
It begs the question as to whether the Arabs had known about the Cape Verde Islands, or had some other reason to shift the prime meridian. I honestly don't know.
Probably the best thing is for me to scan the article and make it available for others to dissect.
Best,
John H.
On Sat, Mar 20, 2010 at 9:27 AM, George Huxtable <george@hux.me.uk> wrote:
That's interesting stuff, from John, and all quite new to me.
Does Wright, or Roger of Hereford, quote any actual numbers for those
longitudes?
In those days, times were, of course, always local times, and predictions
in tables were always those relating to local noon.
The quantity that would be most affected by the longitude of the city, for
which the prediction was made, is the ecliptic longitude of the Moon.
There were (and are) difficulties in comparing times-of-night at which a
particular phase of a Moon eclipse occurred. One relates to the diffuse
edge of the Earth's shadow on the Moon, causing a wide penumbra, and making
the judgment of how far the shadow had got, to be a somewhat individual
matter.
The other is the judgment, and even the definition, of the time-of-night at
which it occurred, in an age without clocks. The monastical hours of the
night, for prayer, were then one-twelfth of the interval between sunset
and sunrise, so the length of an hour varied with season and with latitude.
However, the specified eclipse, on September 12th, was near an equinox, so
those differences were not then great. Time was likely to have been
measured by the burning of a marked candle.. Better would have been a
measured altitude of a star, if they were up to it. I doubt if the
Nocturnal, as an instrument, existed then.
George.
contact George Huxtable, at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Apache Runner" <apacherunner@gmail.com>
To: <NavList@fer3.com>
Sent: Saturday, March 20, 2010 11:52 AM
Subject: [NavList] Re: AW: Early lunars
George -
John Kirtland Wright does not definitively say that some form of lunars
were
employed to find the longitude of major cities, but advances the conjecture
with some evidence, particularly citing Roger of Hereford's observation of
lunar eclipses. I'll just quote the text here from the "Notes on.."
paper:
"Though these various method were known to astronomers and astrologers,
there is nothing to show that any large number of positions were determined
by astronomical means. We know, however, that between the twelfth and
fourteenth centuries, tables were constructed for the meridians of at least
a dozen cities; we have in manuscript such tables for Toledo, Marseilles,
Hereford, London, Toulouse, Cremona, and Novara, and we have clear evidence
that similar tables were in existence in 1232 for Paris, Palermo, Pisa,
Constantinople and Genoa.
In their construction some sort of astronomic observations must have been
made and it is more than likely that the eclipse method of finding
longitudes was given practical application. that this was done in the case
of tables for Hereford, Marseilles, and Toledo is certain, for Roger of
Hereford, who adapted the Toledo Tables to the meridian of his city, tells
us that the time of the eclipse of September 12th, 1178, was observed in
these three cities and that their longitudes in relation to Arin, the world
center, were in that way determined. It will also become evident from
what
follows that the relative positions of several other points in Europe were
known with sufficient accuracy to warrant us in concluding that the
differences in longitudes separating these points had been calculated by
astronomical means."
Pages 84 and 85 in Wright's paper.
I omitted the references in the above. "Arin" was a mythical city that was
used to denote the halfway point from the prime meridian used during that
era. It was a tool to aid the calculations more than anything else,
according to Wright's paper.
The other observations that I'm aware of were two measurements of longitude
carried out by Columbus during two of his voyages. He carried with him
ephemerides constructed in Nuremberg (I forget the name of the astrologer
who constructed them). In both cases he timed the lunar eclipses against
the time of sunset. Unfortunately, the values he derived were quite far
off. This I obtained from Samuel Eliot Morrison's biography of Columbus.
One of the measurements was of St. Ann's Bay in what is now Jamaica,
where,
incidentally, he obtained a latitude using a quadrant on Polaris of 18
degrees, 30 minutes, which is phenomenally good for a quadrant sighting the
North star (having tried this myself in Jamaica). I should add that in
both measurements, he was on solid ground, not on shipboard.
Best,
John H.
On Sat, Mar 20, 2010 at 7:01 AM, George Huxtable <george@hux.me.uk> wrote:
> Thanks for the information, and the offer of a scan.
>
> But there's no urgency, just interest. Isis is available to me, both in
> the
> library of the Museum for the History of Science, in Oxford, and in the
> Bodleian. I'll add it to my long list of things to look at, next visit.
>
> If there's evidence of any real astronomical determination of longitudes,
> before 1500 (or even before the early 1600s), I would be interested to
> learn about it.
>
> Ptolemy's Geographica arrived back in the West some time after 1400. We
> discussed the Marseilles tables on the list a few months back, and as I
> remember, these were stated as dating from 12th century, well before that
> recall of Geographia, and contained many longitudes, some good ones and
> some very bad ones. I have presumed these came from dead-reckoning of
> travellers, in day's journeys, by land or sea, rather than by astronomy.
> Is
> that wrong?
>
> George.
>
> contact George Huxtable, at george@hux.me.uk
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ----- Original Message -----
> From: "Apache Runner" <apacherunner@gmail.com>
> To: <NavList@fer3.com>
> Sent: Saturday, March 20, 2010 9:52 AM
> Subject: [NavList] Re: AW: Early lunars
>
>
> Wolfgang -
>
> Yes on both accounts.
>
> John H.
>
> On Sat, Mar 20, 2010 at 5:30 AM, Wolfgang Köberer <
> koeberer@navigationsgeschichte.de> wrote:
>
> > This is the article, I presume:
> >
> > Wright, John Kirtland
> > Notes on the knowledge of latitudes and longitudes in the Middle Ages.
> > in: Isis, Vol. 5 (1923), 75 - 98.
> >
> > He has also written a book on "The geographical lore of the Time of the
> > Crusades".
> >
> > I can bring along a copy of the article in May, George, or send a scan.
> >
> > Wolfgang
> >
> >
> >
> >
> >
>
>
>
>
>
