Petosiris to Nechepso is a letter describing an ancient divination technique using numerology and a diagram.
[1] Petosiris and Nechepso are considered to be the founders of astrology in some traditions.
[2] One translation of this letter into Latin is attributed to Saint Bede,[3] and can be found in British Library, MS Cotton Tiberius C. II.
The attribution of ancient authors is a typical practice of Neoplatonism and Gnosticism, and the technique may arise from this tradition.
[5] The technique involves calculating the numerical value of a patient's name, then dividing by 30 or 29, a number derived from the lunar month to find the remainder, which is (mod 29) or (mod 30) in modular arithmetic.