Letter to Judah Barzillai of Barcelona

It is said that the first science of the sciences of the stars is the investigation of the form of the earth and the heavens and their shapes; and to give proofs and evidence that they [the heavens] are circular with a spherical shape, and the earth stands suspended in the middle of the firmament, and the firmament surrounds and orbits it from east to west; and to investigate the form of the planets and their positions in the firmaments, layered one below the next; and to know the form of their paths from east to west and their position in the north and south of the celestial equator and of the wheel of the zodiac, and also their distance from the earth; and to calculate all their motions in order to know for every single planet the location of its position within the zodiac, and whether it can be seen north or south of the wheel of the zodiac, and if it is close to the earth or far, and whether forward or retrograde in its movement. Everything depends on this, and I need not elaborate. [ . . . ]

The second science [astrology], which depends on this science [astronomy], is knowledge of the rule and influence given to the planets and the zodiac in their orbits around the earth; and which influence is given to which star in its position within each constellation; and how this influence changes as the planets move through the firmament to north and south, and closer and farther from the earth, as they move forward or retrograde, exerting their influence over the earth and everything on it, every single person in his birth and the events of his life, as it [this science] has been passed down and verified by the sages of this art, who either received it from the mouths of the ancients by way of tradition or the spirit of God or learned it through their own investigations. [ . . . ]

Our sages praised these two sciences, learned their ways and made use of their knowledge, and obligated anyone able to calculate the seasons and constellations to do so, in order to fulfill that which is written: For it is your wisdom and your understanding in the sight of the nations (Deuteronomy 4:6).

And you cannot say that “calculate the seasons” refers to [calculating] the four seasons of the year, for this is an easy matter requiring little calculation or exceptional wisdom, and all the nations of the world can do so. [ . . . ]

It would have sufficed them to say merely “to calculate seasons,” but since they said “seasons and constellations,” we know that “seasons” refers to the calculations of the movements of the stars and the constellations, the calculations of their [astrological] decrees as seen on earth, for such do we call the predictions of the stars and the decrees of the constellations, saying mazal tov [a good constellation] over a good one and mazal ra‘ [a bad constellation] over its opposite.

And I can bring a proof from the verse, For it is your wisdom and your understanding in the sight of the nations, for calculations of the movements of the stars is not the wisdom of Israel in the sight of the nations, since most of the nations exceed Israel in this, but the wisdom of Israel and their understanding in the sight of the nations is that their righteousness annuls the decrees of the stars, while the decrees of the stars over the nations endure and are not annulled unless the Holy One removes the power of the star and weakens it. But no star is weakened for Israel, because no star rules over them. And if a non-Jewish sage should calculate the fates of two people with the same births and with similar horoscopes, one a Jew and one a non-Jew, he will find that the Jew is saved from all the harsh decrees decreed at his birth, while the wicked non-Jew will suffer all of them, and the sages of the nations will wonder at this and be astounded, and see the greatness of Israel. [ . . . ]

It is thus incumbent upon all who know how to calculate the seasons and the constellations to do so, and if one sees a good prediction, one should not remain confident in it but rather take care lest one sin, and if one sees a prediction that is not good, one should atone and beseech God, and perhaps He will see fit to annul the decree. You will find that it is incumbent upon one who knows this art to use it in order to strengthen one’s faith and fear of heaven, and one should increase one’s prayers and almsgiving at all times.