Hindu temples in Kerala regularly undergo detailed and public astrological scrutiny. While this procedure tells a lot to the public as well as to the social scientist, the specific question addressed here is: how do the astrologers' statementsacquire the authority to be accepted as truth? One important aspect has to do with Sanskrit authoritative treatises. But success in establishing an accepted truth relies also heavily on rhetorical skills, and on social consensus. Proofs and arguments cannot prevent the possibility of later contestation, a possibility all the more real with present day's democratization of astrological knowledge and the multiplication of experts from non-traditional social backgrounds.