Astrology was the belief that the relative position of planets and other celestial bodies could be used to predict the future. In particular, it was used to forecast the futures of individuals, based upon the predominant character traits of people born within certain date ranges. A person who could produce predictions based upon the underlying theories of astrology was called an astrologer.
On Earth, astrology was a common practice in early human history, and various monarchical regimes had "official" or "court" astrologers. In Alexandria, the royal astrologer predicted a planetary conjunction would herald the end of the city. (COMIC: Voyager) In San Martino, Hieronymous served as the Court's official astrologer. (TV: The Masque of Mandragora)
The concept of astrology dated back to the Dark Times and was shared by every species in the universe. (PROSE: Beautiful Chaos) It was therefore not unique to Earth, despite the fact that the human practice of the discipline was entirely based on the Earth's relative position to heavenly bodies in its close proximity. Mr Smith once revealed to Luke Smith that many planets, like Draconia, had a kind of astrology. (TV: Secrets of the Stars) The source of Mr Smith's information may not have been accurate, however, as the Draconian, Salamanca, once told his human captain, Gillian Sherwin, "Draconia has no astrology." (PROSE: The Dark Path)
The Ancient Lights' power derived from astrology and the zodiac, despite the fact that, according to Sarah Jane Smith, astrology contradicted the physics of the universe. It was believed that this was because astrology worked in the Ancient Lights' universe due to its different laws of physics. (TV: Secrets of the Stars)