What we know, and what we don't know about Vettius Valens; and why attempts to fill in the blanks with speculation may be obscuring our understanding of when and where he lived and compiled his work.
Vettius Valens is typically introduced in biographies as being a younger contemporary of Ptolemy (c.100-170 CE) since analysis of his charts suggests he was most actively involved with astrology between 152 and 162, with continued involvement up to 188. His flourishing period therefore crosses the reigns of the Roman emperors Antonius Pius (138-161) and Marcus Aurelius (161-180).
Valens' nine-part Anthologies (i.e., 'Collections') is the most comprehensive account of astrological theory and practice to have survived from antiquity. Only the eight-part Mathesis of Firmicus Maternus comes close to its length, although Firmicus makes reference to only one example nativity, whereas Valens takes every opportunity to illustrate his teachings with horoscopes he has worked on or has knowledge of. Valens does not identify the names of his subjects, provide horoscope diagrams, or even give dates for most of the example charts he describes, but research by Otto Neugebauer in the mid-1950s proved each set of planetary positions to be an authentic horoscope that can be dated to the first or second century.
In his identification of the oldest chart to "about sunrise" on 15 December 37 CE, Neugebauer failed to realise (but others quickly noted) that this was the horoscope of the Roman Emperor Nero, whose birth was recorded by the historian Suetonius in 121 CE: "Nero was born at Antium, on 15 December 37 A.D. ... The Sun was rising". Valens uses the chart as a typical example of how his techniques identify crisis periods, which in this chart occurs within the 31st year (68 CE, when Nero committed suicide). It is this rich supply of ancient chart-data that has made Valens' work of great significance to scholars of classical history, calendar systems, and astronomy; as well as students of ancient astrological techniques. Its value was underlined by Otto Neugebauer and Henry Van Hoesen in their 1959 work Greek Horoscopes, which says in regard to the "importance of the Anthology of Vettius Valens":
With its about 130 (partial or complete) horoscopes it contains twice as many examples of Greek horoscopes as all papyri combined. Without Vettius Valens (whose examples range from A.D. 37 to 188) we should have only five examples of "literary" horoscopes before A.D. 380.