Why did a Jewish scientist create the Rules for the Astrolabe?
Jewish scientists were the surviving remnant of the great Judeo-Arab tradition of mathematics and astronomy. From the ninth until the thirteenth century, all of the great achievements in science were carried out in Muslim world. Knowledge of the circulation of blood in the body, the development of trigonometry, lists of pharmaceuticals all were compiled in the Muslim world. Many of the great advances in science and medicine (including the astrolabe) were developed on the Iberian peninsula (Spain and Portugal).
Muslim scientists invited the cooperation of people from other religions. But in scientific matters, Christians showed less interest than Jews. Hence the scientific tradition in Spain and Portugal--the development of the astrolabe for measuring the height of stars, the development of trigonometry, and the accurate solar calendar--all emerged in Jewish and Muslim communities in Spain and Portugal during Europe's Middle Ages.
The Christian calendar is based on the motion of the earth around the sun; while the Islamic calendar is based on the motion of the moon.
Finally, the Hebrew calendar combines both, in that its years are linked to the motion of the earth around the sun, and its months are linked to the motion of the moon.
Muslim astronomers in Spain had created an accurate solar calendar in the 1200s while European Christians still relied upon an inaccurate Roman version (called the Julian calendar). Not until 1582 did Christian Europe have an accurate solar calendar. To catch up to the sun's real year Pope Gregory XIII had to drop 10 entire days from the year!
Scientifically accurate latitude could only have developed with the right answer to the question of which century year is a leap year. It could only have happened in 1400 in a European countries having a significant number of people with technical expertise in Arabic and Hebrew astronomy. It was no accident that an astronomer from the Judeo-Arab scientific tradition created the first scientific means of determining latitude. For more on this see Ceremonies of Possession in Europe's Conquest of the New World (chap. 4).
The reason Christian Europe had the wrong calendar is the same reason why computer scientists and programmers the world over had tthe "Y2K" problem.