KNOWLEDGE & SCIENTIFIC NEWS. 



I December, 1905. 



the introduction of the transit-circle. '■= This is evi- 

 denced by the fact that its illustrious author, who prob- 

 ably constructed it in 1468, shortly after the completion 

 of the Cathedral, obtained by its means a verv accurate 

 value for the obliquity of the ecliptic as then existing ; 

 his result of 23° 30' being- a more exact figure than the 

 23° 28' found by Furbach and his brilliant pupil Regio- 

 montanus. \\'hether, however, the gnomon was erected 

 by Toscanelli with a view to determining the variation 

 in obliquity, as maintained by Leonardo Ximenes and 

 others, is a question to which Celoria' considers no 

 positive answer can now safely be given. The supposi- 

 tion in itself would involve nothing unre.-Lsonable, for 

 this graduaJ variation in the obliquitv- of the ecliptic, 

 which the modern examination of ancient monuments, 



as well as other researche,s, lead us to conclude was 

 suspected even among some of the ancients, was cer- 

 tainly known in Toscanelli 's day, and it is, therefore, 

 improbable that this remarkable man, who was the 

 author of the map used by Columbus on the voyagt; 

 which resulted in the discovery of America, should not 

 have borne this matter in mind when constructing his 

 great gnomon. 



Nevertheless, the mean annual diminution in obliquity 

 being something like o".468, the chief factor — time- 

 in so delicate an investigation as this would neces- 

 sarily be, carried out with the means then obtainable, 

 would be ;dl but absent. Here, again, the Kgyptian 

 sun-god might assert his superior claim to be heard in 

 matters astronomical, for the great temple of Amen-Ra 



• Introduced about 1600 by the Danish astronomer, Olaus 

 Romer, who 6rst measured the velocity of light. 



t Sulle oiservaxi'ini, etc., fatti da Paolo Dal I'ozzo TouantW , 

 Rome, 1894. 



having stood for over 50 centuries, its solstitial orienta- 

 tion now shows a deviation of something like one 

 degree ; whereas with the Florence Duomo, which can 

 only boast an existence of a little over four centuries, 

 the observed variation, being comparatively small, 

 would be a maticr of considerable difficulty to accu- 

 ratcJy determine, even supposing the gnomon never to 

 ha\e l>cen displaced from its position, which, as we 

 shall presently see, is unfortunately not the case. 



The task which Toscanelli had priniarih- set himself, 

 howe\cr, was undoubtedly the correction of the .\lphon- 

 sine Tables, which were then in operation, but which 

 gave a very inadequate representation of the true solar 

 motion, more especially as regards the exact length of 

 the tropical year. To correct this error, he knew that 

 it would be necessary to institute regular ot>servations 

 of the sun's motion, and it is for this reason, in all 

 probability, that he undertook the construction of the 

 gnomon. This ehiborate care to determine the exact 

 moment of the summer solstice in the Florence Cathe- 

 dral is~not without its significance in view of the annual 

 illumination ol the dome on Midsummer Night, or the 

 feast of St. John the Baptist ; and the S/. /a/in's fires, 

 kindled in tormer times in celebration of the summer 

 solstice, now find their analogue in the displav of fire- 

 works, which, to the modern Florentine, forms the 

 chief attraction to the festivities annually observed in 

 honour of his patron-saint, San Giovanni. 



.As it is, many valuable facts relating to the history 

 of the gnomon have unfortunatclv been lost, for the 

 origin.'il inscription on the marble disc marking the 

 solstiti.'il point, which Toscanelli caused to be let into 

 the pavement of the north transept of the Cathedral, 

 was all but obliterated even in the lime of I,c<inardo 

 Ximenes, whose curious description,'" published at 

 l^'lorence in 1757, still forms the classic work on this 

 subject. Surrounding Toscanelli 's solstitial disc, and 

 placed eccentrically to it, is a larger circle of a different 

 kind of marble, on which is inscribed the date MDX 

 I'RIDIK ID I\'XII (i_nh June, 1510), the day on 

 which the sinnmer solstice fell in that year, owing to 

 the displacement of nine days produced by the Julian 

 Calendar which was then in force, the Gregorian Re- 

 form not licing introduced until o\cr 70 years later. t 

 This larger circle is thought to be the work cither of a 

 nephew of Tf>scanelli, or, more probably, of a certain 

 Antonio Dulciati (who is said to have written on the 

 reform of the Calendar), and is intended to mark the 

 position of the round patch of light which the sim's 

 ravs, passing through the circular orifice of the gno- 

 mon in the lantern, loniied on the lloor of Ihe Cathedral 

 at the time of the summer solstice. 



As a matter of fact, both the large and the small 

 circle lie somewhat to the west of the true meridian, 

 thus anticipating the exact time of mid-day by about 

 a minute and a half, and it was partly this error which 

 induced Leonardo Ximenes to draw the meridian line 

 which extends for over 30 feet along the pavement. 

 Ximenes, indeed, whf> was the founder of the Ximcnian 

 Observatorv in Florence — the institution which still 

 retains the curatorship of the gnomon — not only made 

 important researches in its histon.% but hiinself insti- 

 tuted a series of very careful experiments with this huge 

 sundial. Besides tracing a true meridian, and causing 



' Dil veuhio e nuovo Gnomone fiorenlino. (It is interesting to note 

 that Florio in his quaint Italian-KnKlish Dictionary, A IVorlde 0/ 

 Wordii, defines the Italian gnomone as " the know-man or gnowman of 

 a diall. the shadow whereof poinlelli out the howers.") 



t Viz., in 1582. The Reform was not adopted in England until 

 1752 



