Chandra

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.[12] 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.[3]

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).[18]

Soma as the Moon-deity is also found in Buddhism,[19] and Jainism.[20]

Soma is the root of the word Somavara or Monday in the Hindu calendar.[11] The word "Monday" in the Greco-Roman and other Indo-European calendars is also dedicated to the Moon.[21] Soma is part of the Navagraha in Hindu zodiac system. The role and importance of the Navagraha developed over time with various influences. Deifying the moon and its astrological significance occurred as early as the Vedic period and was recorded in the Vedas. The earliest work of astrology recorded in India is the Vedanga Jyotisha which began to be compiled in the 14th century BCE. It was possibly based on works from the Indus Valley Civilization as well as various foreign influences. Babylonian astrology which was the first to develop astrology and the calendar, and was adopted by multiple civilizations including India. The moon and various classical planets were referenced in the Atharvaveda around 1000 BCE.

The Navagraha was furthered by additional contributions from Western Asia, including Zoroastrian and Hellenistic influences. The Yavanajataka, or 'Science of the Yavanas', was written by the Indo-Greek named "Yavanesvara" ("Lord of the Greeks") under the rule of the Western Kshatrapa king Rudrakarman I. The Yavanajataka written in 120 CE is often attributed to standardizing Indian astrology. The Navagraha would further develop and culminate in the Shaka era with the Saka, or Scythian, people. Additionally the contributions by the Saka people would be the basis of the Indian national calendar, which is also called the Saka calendar.

The Hindu calendar is a Lunisolar calendar which records both lunar and solar cycles. Like the Navagraha, it was developed with the successive contributions of various works.

Soma was presumed to be a planet in Hindu astronomical texts.[22] 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.[23] 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.[23] 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:[24]

The longitude of Moon = ${\displaystyle \left(m-{\frac {P*Rsin(m-a)}{360}}\right)}$

– Surya Siddhanta II.39.43[24]

where m is the Moon's mean longitude, a is the longitude at apogee, P is epicycle of apsis, R=3438'.