Soma (Sanskrit: सोम) connotes the Moon as well as a medicinal deity in post-Vedic Hindu mythology. In Puranic mythology, Soma is a moon deity, but the name is sometimes also used to refer to Vishnu, Shiva (as Somanatha), Yama and Kubera. In some Indian texts, Soma is the name of an Apsara; alternatively it is the name of any medicinal concoction, or rice-water gruel, or heaven and sky, as well as the name of certain places of pilgrimage.
The earliest use of Soma to refer to the moon is a subject of scholarly debate, with some scholars suggesting that the reference to moon as Soma is to be found in the Vedas, while other scholars suggest that such usage emerged only in the post-Vedic literature. The Hindu texts state that the moon is lit and nourished by the sun, and that it is moon where the divine nectar of immortality resides.
Soma's iconography varies in Hindu texts. The most common is one where he is a white colored deity, holding a mace in his hand, riding a chariot with three wheels and three or more white horses (up to ten).
Soma is the root of the word Somavara or Monday in the Hindu calendar. The word "Monday" in the Greco-Roman and other Indo-European calendars is also dedicated to the Moon. Soma is part of the Navagraha in Hindu zodiac system. The zodiac and naming system of Hindu astrology, one that includes Moon as Soma, likely developed in the centuries after the arrival of Greek astrology with Alexander the Great, their zodiac signs being nearly identical.
Soma was presumed to be a planet in Hindu astronomical texts. It is often discussed in various Sanskrit astronomical texts, such as the 5th century Aryabhatiya by Aryabhatta, the 6th century Romaka by Latadeva and Panca Siddhantika by Varahamihira, the 7th century Khandakhadyaka by Brahmagupta and the 8th century Sisyadhivrddida by Lalla. Other texts such as Surya Siddhanta dated to have been complete sometime between the 5th century and 10th century present their chapters on various planets with deity mythologies. However, they show that the Hindu scholars were aware of elliptical orbits, and the texts include sophisticated formulae to calculate its past and future positions: