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John wrote, about the longitudes quoted by Wright-

"There are some issues that leave me scratching my head. "

I was just thinking of the longitudes of Hereford, Marseilles, and Toledo, 
as referred to by Roger of Hereford; not asking for all the others to be 
dug out.

There was indeed a great variety of meridian lines that were used as the 
starting-place for measuring longitude, over the years.

Although it doesn't relate to your mediaeval period of interest, not 
starting until 1500, Art Jonkers, who wrote "The Earth's Magnetism in the 
days of Sail", has produced an informative paper on that topic, to be found 
at-
http://earth.uni-muenster.de/~jonkers/PARAMER.pdf
It's titled "Parallel meridians: Diffusion and change in early-modern 
oceanic reckoning", and dates from 2005.

Even as late as 1767, such an experienced navigator as James Cook was not 
noting his longitudes as from Greenwich, in his North Atlantic crossings. 
They were always from the point from which he took his "departure" (which 
doesn't imply that he departed from there), such as St Agnes' light, in the 
Scillies, or St.John's, Newfoundland. Or, sometimes, to his next headland, 
such as Cape Race, St Agnes, or very occasionally to the nearest point of 
land, Cape Clear in SW Ireland, though there was no intention of going near 
it. Never from Greenwich, much to my surprise.

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" 
To: 
Sent: Saturday, March 20, 2010 2:17 PM
Subject: [NavList] Re: AW: Early lunars


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  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" 
> To: 
> 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  
> 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" 
> > To: 
> > 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 K�berer <
> > 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
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
> >
> >
>
>
>
>
>

   

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