Dodwell: The Obliquity of the Ecliptic

 

CHAPTER 7

MEDIAEVAL AND MODERN OBSERVATIONS OF THE OBLIQUITY OF THE ECLIPTIC

 

These are taken partly from Wendelin’s Memoir, (Solis Obliquitas), and partly from such sources as the Encyclopaedia Britannica, 9th Edition, article “Astronomy,”  Hutton’s Mathematical and Philosophical Dictionary, vol. 1, p. 455, article “Ecliptic,” list of Obliquities contained in Ball’s Astronomy, page 354, E. Bernard’s list of Obliquity determinations, previously mentioned, and other source to which reference will be made.

MEDIAEVAL OBSERVATIONS OF THE OBLIQUITY FROM 1020 A.D. TO 1600 A.D.

Early in the 11th century, the development of astronomical research spread from the East into Europe, and was particularly encouraged in Spain, both amongst the Moors and the Spanish Princes, particularly Alphonso X of Castile, who employed several notable astronomers at Toledo, where he prepared the Alphonsian Tables.  Amongst these astronomers was Al Zarkali (Arzachel), who determined the Obliquity of the Ecliptic at Toledo in 1070 A.D.  His solstitial observations, together with some others mentioned, according to Wendelin’s statement, are recorded as follows:

Arzachel the Moor, about our year 1070, noted that the declination of the sun did not exceed 23° 34’, and Almaeon, who followed him in the year 1140, observed 23° 33 ½ ‘, and then Prophatius Judaeus, in the year 1328