Jyotisha (Sanskrit: ज्योतिष, IAST: Jyotiṣa) is the science of tracking and predicting the movements of astronomical bodies in order to keep time. It refers to one of the six ancient Vedangas, or ancillary science connected with the Vedas – the scriptures of Hinduism. This field of study was concerned with fixing the days and hours of Vedic rituals.
The term Jyotisha also refers to Hindu astrology, a field that likely developed in the centuries after the arrival of Greek astrology with Alexander the Great, their zodiac signs being nearly identical.
Jyotisha, states Monier-Williams, is rooted in the word Jyotish which means light, such as that of sun or moon or heavenly body. The term Jyotisha includes the study of astronomy, astrology and the science of timekeeping using the movements of astronomical bodies. It aimed to keep time, maintain calendar, and predict auspicious times for Vedic rituals.
David Pingree has proposed that the field of timekeeping in Jyotisha may have been "derived from Mesopotamia during the Achaemenid period", but Yukio Ohashi considers this proposal as "definitely wrong". Ohashi states that this Vedanga field developed from actual astronomical studies in ancient India.
The texts of Vedic Jyotisha sciences were translated into the Chinese language in the 2nd and 3rd centuries CE, and the Rigvedic passages on astronomy are found in the works of Zhu Jiangyan and Zhi Qian.
Timekeeping as well as the nature of solar and moon movements are mentioned in Vedic texts. For example, Kaushitaki Brahmana chapter 19.3 mentions the shift in the relative location of the sun towards north for 6 months, and south for 6 months.
The ancient extant text on Jyotisha is the Vedanga-Jyotisha, which exists in two editions, one linked to Rigveda and other to Yajurveda. The Rigveda version consists of 36 verses, while the Yajurveda recension has 43 verses of which 29 verses are borrowed from the Rigveda. The Rigveda version is variously attributed to sage Lagadha, and sometimes to sage Shuci. The Yajurveda version credits no particular sage, has survived into the modern era with a commentary of Somakara, and is the more studied version.
The Jyotisha text Brahma-siddhanta, probably composed in the 5th century CE, discusses how to use the movement of planets, sun and moon to keep time and calendar. This text also lists trigonometry and mathematical formulae to support its theory of orbits, predict planetary positions and calculate relative mean positions of celestial nodes and apsides. The text is notable for presenting very large integers, such as 4.32 billion years as the lifetime of the current universe.
The ancient Hindu texts on Jyotisha only discuss time keeping, and never mention astrology or prophecy. These ancient texts predominantly cover astronomy, but at a rudimentary level. Technical horoscopes and astrology ideas in India came from Greece, states Nicholas Campion, and developed in the early centuries of the 1st millennium CE. Later medieval era texts such as the Yavana-jataka and the Siddhanta texts are more astrology-related.
The field of Jyotisha deals with ascertaining time, particularly forecasting auspicious day and time for Vedic rituals. The field of Vedanga structured time into Yuga which was a 5-year interval, divided into multiple lunisolar intervals such as 60 solar months, 61 savana months, 62 synodic months and 67 sidereal months. A Vedic Yuga had 1,860 tithis (तिथि, dates), and it defined a savana-day (civil day) from one sunrise to another.
The Rigvedic version of Jyotisha may be a later insertion into the Veda, states David Pingree, possibly between 513 and 326 BCE, when Indus valley was occupied by the Achaemenid from Mesopotamia. The mathematics and devices for time keeping mentioned in these ancient Sanskrit texts, proposes Pingree, such as the water clock may also have arrived in India from Mesopotamia. However, Yukio Ohashi considers this proposal as incorrect, suggesting instead that the Vedic timekeeping efforts, for forecasting appropriate time for rituals, must have begun much earlier and the influence may have flowed from India to Mesopotamia. Ohashi states that it is incorrect to assume that the number of civil days in a year equal 365 in both Hindu and Egyptian–Persian year. Further, adds Ohashi, the Mesopotamian formula is different than Indian formula for calculating time, each can only work for their respective latitude, and either would make major errors in predicting time and calendar in the other region.
Kim Plofker states that while a flow of timekeeping ideas from either side is plausible, each may have instead developed independently, because the loan-words typically seen when ideas migrate are missing on both sides as far as words for various time intervals and techniques. Further, adds Plofker, and other scholars, that the discussion of time keeping concepts are found in the Sanskrit verses of the Shatapatha Brahmana, a 2nd millennium BCE text. Water clock and sun dials are mentioned in many ancient Hindu texts such as the Arthashastra. Some integration of Mesopotamian and Indian Jyotisha-based systems may have occurred in a roundabout way, states Plofker, after the arrival of Greek astrology ideas in India.
The Jyotisha texts present mathematical formulae to predict the length of day time, sun rise and moon cycles. For example,The length of daytime =
A prastha of water [is] the increase in day, [and] decrease in night in the [sun's] northern motion; vice versa in the southern. [There is] a six-muhurta [difference] in a half year.
— Yajurveda Jyotisha-vedanga 8, Translator: Kim Plofker