Re: Armillary Dial

[heiner thiessen]

Thank you all for a most enlightening tour through the centuries. Assuming then that the 'modern' pole style dial  with equal hours was an (Arabic?) innovation and perhaps an improvement over the nodus based  dial of antiquity,  what would have been the pre-requisite astronomical knowledge of the period to actually arrive at this new 'cutting edge' shadow clock with a gnomon pointing to the celestial north pole? Or was it perhaps initially merely a matter of trial and error until someone had pointed the gnomon in such a way that it read the hours consistently throughout the year? Best wishes Heiner Thiessen 51N   1W   ----- Original Message ----- From: Gianni Ferrari To: LISTA INGLESE Sent: Monday, July 18, 2005 4:56 PM Subject: Re: Armillary Dial Often, when I have to answer to a questions as that of Heiner Thiessen  “when did sundials with gnomons parallel to the planet's axis   first appear?“ I have a doubt on what to answer because there are two different possible answers depending on we refer to sundials in which the polar axis is fundamental and necessary for their working (sundials whose operation would not be possible without a polar axis) and some ancient sundials in which we have the presence of a polar axis that doesn't have an essential function, but that was placed  for a different reason: geometrical, geographical, aesthetics, etc.    I think that the classical sundials with polar style,  that mark the astronomic hours  (24 equal hours in the day) and in which the shadow of the whole style marks the time, belong to the first kind and are  the 'modern'  dials  of  Thiessen’s  question .   For this reason I think that a sundial with  temporary or seasonal hours cannot belong to this type.  In them the hours are  necessarily marked by the shadow of a point or nodus and a possible polar (or horizontal, vertical, etc.) style  is not useful to the operation.    I am therefore of the same opinion clearly expressed by Frans W. Maes in his last message  (7/14).    Certainly the Greek and Roman knew the astronomical hours and probably some Alexandrian astronomer understood the possibility to make sundials with a polar style, but  it doesn't exist, to my knowledge, any written evidence and any sure archaeological find that bear witness to the invention and to the construction of this kind of sundials before 1300-1400 AD   The only ancient sundials that I  know in which "perhaps" a polar gnomon was present, (without a specific function), are only 2 Roman sundials with seasonal hours.    The first one is that described in BSS-Bullettin - Vol. 14(iii) - September 2002 - pag. 111 (I thank here Karlheinz Schaldach for his prompt and kind information).  It is a Roman sundial of the 2nd  century AD, found in Israel, engraved on the Northern  face of a block of marble on a plane parallel to the celestial equator. On this plane we find  the temporary hour lines and three circular daily  lines, the larger one corresponding to the summer Solstice.  The presence in this sundial of a polar style is however doubtful because the modern reconstruction of the clock, shown in a photo, is wrong and can lay itself open to misinterpretation or to debatable conclusions.  In this reconstruction the authors have placed a very thin polar style with an exaggerated length, at least 4 times the real one : the value of the length found from simple calculation is only 4.5 cm.about. Probably in this clock, as in other Roman sundials, the nodus was the vertex of a gnomon having the shape of a pointed prism, with its axis perpendicular to the plane of the sundial and therefore necessarily parallel to the polar axis for geometrical reasons (and not for functional ones) and it didn't have the shape of a thin and long style as in the reconstruction.    The second Roman sundial, found in Pompei, with a pseudo polar gnomon has been described by Nicola Severino in a CD of recent publication: also in this case the gnomon, one of the few original found till now, has the shape of a short pointed prism (few cm), whose vertex was used to mark the temporary hours, and with its axis having a direction roughly toward the Norterh celestial pole . In the CD of Severino, devoted only to the Roman sundials, there are several  original photos very detailed.    Finally on the sundials of the ancient Islam. The only refeence that the sundials with polar style were invented in the tenth century   is found in 4 lines of a paper delivered by Prof. D. King (the most great expert of Arabic astronomy) at a Congress in Bucarest in 1981 and republished by Variorum in 1987 (Islamic Astronomical Instruments pag.11 - lines 11-14)  This reference is not repeated in any of the following numerous writings of the Prof. King.  To my knowledge one of the first Islamic sundials with polar gnomon is the one  made  in the year 773H (1371-1372 AD) from the astronomer Ibn al Shatir in  Damascus.    Regards Gianni Ferrari 44° 39' N      10° 55' E Mailto : [EMAIL PROTECTED]