Petosiris to Nechepso is a letter describing an ancient divination

Divination

Divination is the attempt to gain insight into a question or situation by way of an occultic standardized process or ritual...

 technique using numerology

Numerology

Numerology is any study of the purported mystical relationship between a count or measurement and life. It has many systems and traditions and beliefs...

 and a diagram. It is likely to be a pseudepigraph.
Petosiris

Petosiris

Petosiris, called Ankhefenkhons, was the high priest of Thoth at Hermopolis and held various priestly degrees in the service of Sakhmet, Khnum, Amen-Re and Hathor. The son of Sishu and Nefer-renpet, he lived in the second half of the 4th century BCE under Persian rule...

 and Nechepso are considered to be the founders of astrology

Astrology

Astrology consists of a number of belief systems which hold that there is a relationship between astronomical phenomena and events in the human world...

 in some traditions. One translation of this letter into Latin is attributed to Saint Bede, and can be found in Cotton Tiberius

Bede, Ecclesiastical History (British Library, MS Cotton Tiberius C. II)

British Library, MS Cotton Tiberius C. II, or the Tiberius Bede, is an 8th century illuminated manuscript of Bede's Historia ecclesiastica gentis Anglorum. It is one of only four surviving 8th century manuscripts of Bede. As such it is on the closest texts to Bede's autograph. The manuscript has...

.
The technique is known by several names, including the Petosiris Circle,
the Sphere of Apuleius

Apuleius

Apuleius was a Latin prose writer. He was a Berber, from Madaurus . He studied Platonist philosophy in Athens; travelled to Italy, Asia Minor and Egypt; and was an initiate in several cults or mysteries. The most famous incident in his life was when he was accused of using magic to gain the...

, Columcille's Circle, and Democritus

Democritus

Democritus was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated an atomic theory for the cosmos....

's Sphere. The attribution of ancient authors is a typical practice of Neoplatonism and Gnosticism

Neoplatonism and Gnosticism

Neoplatonism is the modern term for a school of Hellenistic philosophy that took shape in the 3rd century, based on the teachings of Plato and some of his early followers. Neoplatonism took definitive shape with the philosopher Plotinus, who claimed to have received his teachings from Ammonius...

, and the technique may arise from this tradition. Examples of the figure are known from Anglo-Saxon manuscripts.

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

Lunar month

In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

 to find the remainder, which is (mod 29) or (mod 30) in modular arithmetic

Modular arithmetic

In mathematics, modular arithmetic is a system of arithmetic for integers, where numbers "wrap around" after they reach a certain value—the modulus....

. The number is then found on the diagram, to determine the prognosis

Prognosis

Prognosis is a medical term to describe the likely outcome of an illness.When applied to large statistical populations, prognostic estimates can be very accurate: for example the statement "45% of patients with severe septic shock will die within 28 days" can be made with some confidence, because...

.

Manuscript illustrations

• Page from St. John's College, Cambridge http://www.joh.cam.ac.uk/library/special_collections/manuscripts/medieval_manuscripts/medman/A/Web%20images/B15f53v.htm