Astrología matemática en el «Llibre de les nativitats» de Bartomeu de Tresbens

This is an attempt to analyse the mathematical techniques used by Bartomeu de Tresbens in the Llibre de les nativitats. The book offers no information about his procedure for dividing the houses of the horoscope, but it explains how to trace a planet or a significant point of the ecliptic (tasyīr / “atazir”) in order to determine the amount of time until a particular event will take place. There is also a complete explanation of the astrological technique of casting the rays and, in both cases (tasyīr and projection of rays), Tresbens gives numerical examples that allow us to conject that he is using the tables of oblique ascensions for the beginning of the fifth climate preserved in the Toledan Tables. Finally, the author gives some details about the nativity horoscope of his son and this paper tries to date the event which, probably, took place in March 1364. As Tresbens states that he was forty-seven years old when his son was born, I suggest that he was probably born in 1317.