The Astronomica, also known as the Astronomicon, is a Latin hexameter didactic poem written by Manilius during the reign of Caesar Augustus or Tiberius. The five-book work describes celestial phenomena, explaining the zodiac and astrology. The poem espouses a Stoic, deterministic understanding of a universe overseen by a god and governed by reason. The astrological systems of houses, linking human affairs with the circuit of the zodiac, have evolved over the centuries, but they make their first appearance in Astronomica. Marcus Manilius (1st century AD) was a Roman poet and astrologer. The author of Astronomica is neither quoted nor mentioned by any ancient writer. Even his name is uncertain, but it was probably Marcus Manilius; in the earlier books the author is anonymous, the later give Manilius, Manlius, Mallius.