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.
Origins and spreading
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)
Earth
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 Rome in 120, Vanessa Moretti posed as an astrologer for her master Balbus. (PROSE: The Stone Rose)
In San Martino in 1492, Hieronymous served as the Court's official astrologer. (TV: The Masque of Mandragora)
In the 20th century, Adelaide Lessage believed in astrology and used to consult Miss Nethercott every month. The astrologer had warned her about having seen tragedy in her stars. Lord Henry Palmerdale had laughed at the prediction and left Deauville regardless. They were wrecked on Fang Rock where a Rutan scout killed Adelaide. (TV: Horror of Fang Rock)
Nazis Adolf Hitler and Heinrich Himmler believed in astrology. (PROSE: Timewyrm: Exodus) According to the Twelfth Doctor, in 1941, an Austrian "madman" Field Marshall Rommel served under used horoscopes to make his decisions for battle and men were dying all because of "celestial geography". (COMIC: The Instruments of War)
The Doctor
In 1805, the Second Doctor claimed to be an astrologer as part of an attempt to prevent Horatio Nelson's death at the Battle of Trafalgar. (PROSE: H.M.S. TARDIS)
In 1746, the Second Doctor pretended to use astrology to persuade the local bystanders he was a trustworthy physician. (TV: The Highlanders)
Behind the scenes
Though not directly stated, the "madman" mentioned in The Instruments of War is strongly hinted to be the Nazi Fuhrer Adolf Hitler.