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Several problems with Gerard of Cremona's notion , taking John's 
translation.

" When the moon is on the meridian, if you compare her position with that 
given in the lunar tables for some other locality, you may determine the 
difference in longitude between the place where you are and that for which 
the lunar tables were constructed by noting by noting the differences in 
the position of the moon as actually observed and recorded in the tables. 
It will not be necessary to wait for an eclipse."


How do you determine the moment when the Moon is on the meridian? To do 
that, one might establish some sort of suspended line, exactly to the South 
of a peep where the observer's eye is to be placed. (That was what James 
did, in an island off Hudson Bay, in 1632, but it's not easy: he then 
measured the simultaneous altitude of a star, to give his local time.)

Then what? You then know the Moon's position; on the meridian. So what do 
you compare it with? With where it's predicted to be, somewhere else, at 
that same moment, perhaps? But the chronometer, to establish that same 
moment, hasn't yet been invented. So all that can be done is to compare the 
Moon's position, somehow, with that of some star. Effectively, measure a 
lunar distance. And each arc-minute of that difference corresponds to about 
30 minutes of longitude. Gerard's ambiguous wording hides the reality of 
the problem; the need for high precision of such an observation, to produce 
an imprecise answer.

Gerard suggests that the predictions of tables for other places are used 
for comparison. Around 1500, I doubt if anyone was predicting Moon 
positions to better than a degree, which would itself give rise to a 
30-degree error in longitude. That could be got around by using hindsight, 
instead of a prediction, if a corresponding observation of that same Moon 
passage had been made at the other station. And now there are two sets of 
observational errors to combine.

So I doubt whether Gerard followed his own precepts. He would have been 
better waiting to observe an eclipse of the Moon.

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 3:05 PM
Subject: [NavList] Re: AW: Re: AW: Early lunars


Gerard of Cremona is quoted in Wright as:

"Luna existente in medio coeli si oequaveris eam per tabulas alicuius
regionis:  scies longitudinem inter regiones per differentiam locorum 
lunae:
 et no oportebit te expectare eclipsim."

Something like " When the moon is on the meridian, if you compare her
position with that given in the lunar tables for some other locality, you
may determine the difference in longitude between the place where you are
and that for which the lunar tables were constructed by noting by noting 
the
differences in the position of the moon as actually observed and recorded 
in
the tables.   It will not be necessary to wait for an eclipse."

But this is a quotation of Gerard in a text from 1478.     Wright's
translation, not mine.

On Sat, Mar 20, 2010 at 10:47 AM, Wolfgang K�berer <
koeberer@navigationsgeschichte.de> wrote:

> According to Maddison ("Medieval scientific instruments and the 
> development
> of navigational instruments in the XVth and XVIth centuries", 30) the
> nocturnal was first described by Ram�n Llull by the end of the XIIIth
> century, but this is based on an edition of 1721 and I would not place 
> too
> much faith in it.
> Hester Higton ("Sundials at Greenwich", 387) also states that nocturnals
> were in use "by at least the tenth century"; she does not give any
> reference, though.
> Finally G�nther Oestmann has written an article in the SIS-Bulletin (No.
> 69,
> 5 - 9) on the history of the nocturnal and says that there must have been
> forerunners of the instrument that early and that - in contrast to the
> astrolabe - manuscripts on this type of instrument are rare; only 18
> manuscripts describing the use and manufacture from the 15th and 16th
> century are known to him.
>
> Wolfgang
>
>
>
>
>
>

   

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