Chandra, the Moon, is the second graha and, with the Sun, one of the two luminaries jyotisha weighs most heavily. Its presiding deity is Chandra, its gemstone is the pearl, its element is water, and its Vimshottari period is ten years. In jyotisha Chandra signifies the mind (manas), the emotions, the mother, and the public, so central that many Vedic charts are read as much from the Moon's sign as from the ascendant. It is classed as benefic when waxing and full, weaker when waning and dark, since its strength tracks the light it carries. Chandra rules the sign Karka (Cancer) and is exalted in Vrishabha (Taurus). Its condition governs emotional steadiness and the felt quality of life. These are traditional significations rather than statements about the physical Moon, whose light is in fact reflected sunlight, a fact the tradition's waxing-and-waning rule incidentally mirrors.
The word graha itself comes from the Sanskrit root grah-, to seize or grasp, so that the nine grahas of jyotisha are properly the seizers rather than planets in the modern astronomical sense, a name that lets Rahu and Ketu belong to the class while being no physical bodies at all, only the two points where the Moon's own path crosses the ecliptic. Chandra's place among the nine reached Sanskrit astrology from Greek horoscopy by way of the Yavanajataka, a versification of a Greek astrological text datable to roughly the second through fourth centuries CE, though the Moon itself was known and tracked in India long before that transmission; what arrived with the Yavanajataka was the horoscopic apparatus of dignities and exaltations, later systematised in texts such as the Brihat Parashara Hora Shastra, a compilation of roughly the seventh or eighth century CE ascribed to the sage Parashara.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree, The Yavanajataka of SphujidhvajaDavid Pingree