Shani, Saturn, is the seventh graha, the great malefic and the slow teacher of the system. Its presiding deity is Yama, lord of death and dharma, its gemstone is blue sapphire, its element is air, and its Vimshottari period is nineteen years. In jyotisha Shani signifies discipline, limitation, delay, labour, longevity, and the hard lessons of time; it rewards patience and punishes shortcuts. It is the natural significator of sorrow, service, and old age. Shani rules two signs, Makara (Capricorn) and Kumbha (Aquarius), and is exalted in Tula (Libra). The tradition follows Saturn's transit called Sade Sati, the seven-and-a-half years it spends around the natal Moon, as a period of trial and maturation. These are jyotisha's significations; the underlying body is the ringed sixth planet, whose slow thirty-year circuit gives Shani its association with the long arc of time.
The Sanskrit root grah-, to seize or grasp, gives the nine grahas their name, defining the class by its grip on a nativity rather than by physical status, a definition wide enough to include Rahu and Ketu, calculated points with no body, alongside a substantial planet like Shani. Saturn's place among the classical seven reached Sanskrit astrology from Greek horoscopy through the Yavanajataka, a versification of a Greek astrological text of roughly the second to fourth centuries CE, which brought the machinery of dignity and exaltation to a body Indian astronomy had already tracked independently for far longer. The discipline and hard-won maturity readings attached to Shani, together with his nineteen-year Vimshottari period, belong to a later layer, credited to compilations such as the Brihat Parashara Hora Shastra, dated to roughly the seventh or eighth century CE.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree, The Yavanajataka of SphujidhvajaDavid Pingree