The ninth house is the ninth of the twelve houses of the horoscope. It governs higher learning, philosophy, religion and belief, law, and long journeys to distant places, the reach of the mind and life beyond the near and familiar, toward meaning and the far horizon. It is a cadent house, held to be of lesser force, associated with the mutable phase of its quadrant, yet the older tradition regarded it favourably as the house of God and of wisdom. These are the interpretive conventions of the astrological tradition for this division of the chart, in which the ninth house describes the search for understanding and the crossing of great distances, physical and mental; it stands opposite the third house of everyday communication and short journeys, the axis of the near mind against the far across the wheel.
The Hellenistic astrologers who fixed the twelve-place scheme, already traditional in Vettius Valens around 175 CE and systematised two centuries later by Paulus Alexandrinus, called the ninth house Theos, God, and held that the Sun has his joy there, one of seven places across the wheel where a planet was said to act with special ease. The name explains why the ninth carried such consistent weight for religion and higher meaning long before Jupiter's rulership of Sagittarius became the usual modern explanation: it was named directly for the divine, and the Sun's joy there, the luminary of will, purpose, and identity, ties the house's philosophical reach to the same solar principle that governs the self in the first house, the far horizon read as an extension of who a person fundamentally is.
Sources A History of Western AstrologyNicholas Campion with Introduction to Astrology (Eisagogika)Paulus Alexandrinus