Jyotisha (Sanskrit: ज्योतिष, IAST: Jyotiṣa), now the term for traditional Hindu astrology, historically was the branch of knowledge dedicated to the observation of astronomical bodies in order to keep the right time for the Vedic sacrifices.[1][2] It is one of the six Vedangas, or ancillary science connected with the Vedas that developed .[1][2] This field of study was concerned with fixing the days and hours of Vedic rituals.[2][3]

Hindu astrology from the 3rd century BCE was greatly influenced by Greek tradition,[4][5][6][2]. But the concept of Vedanga predates Greek contact, and there have also been later independent developments. Hindu astrology as it stands today is inherently a study of karma[7], which gives it a very different foundation compared to Greek astrology. In addition to this, the predictive techniques such as Dashas (planetary and sign-based time periods), Vargas (harmonic divisions of the horoscope) are not that evolved in Greek astrology.

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.[1][2] It aimed to keep time, maintain calendar, and predict auspicious times for Vedic rituals.[1][2][8]

According to David Pingree, the field of timekeeping in Jyotisha may have been "derived from Mesopotamia during the Achaemenid period",[9] but Yukio Ohashi considers this proposal as "definitely wrong".[10] Ohashi states that this Vedanga field developed from actual astronomical studies in ancient India.[4] Other scholars dismiss various arguments of Pingree and K. S. Shukla points out a controversy by showing Pingree’s incorrect amendations to the manuscript of the Yavanajātaka, which Pingree believed to be highly corrupted.[11]

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.[12]

Timekeeping as well as the nature of solar and lunar movements are mentioned in Vedic texts.[13] 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.[14][15]

[The current year] minus one,
multiplied by twelve,
multiplied by two,
added to the elapsed [half months of current year],
increased by two for every sixty [in the sun],
is the quantity of half-months (syzygies).

The ancient extant text on Jyotisha is the Vedanga-Jyotisha, which exists in two editions, one linked to Rigveda and other to Yajurveda.[10] The Rigveda version consists of 36 verses, while the Yajurveda recension has 43 verses of which 29 verses are borrowed from the Rigveda.[17][18] The Rigveda version is variously attributed to sage Lagadha, and sometimes to sage Shuci.[18] The Yajurveda version credits no particular sage, has survived into the modern era with a commentary of Somakara, and is the more studied version.[18]

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.[19] 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.[19] The text is notable for presenting very large integers, such as 4.32 billion years as the lifetime of the current universe.[20]

The ancient Hindu texts on Jyotisha only discuss time keeping, and never mention astrology or prophecy.[21] These ancient texts predominantly cover astronomy, but at a rudimentary level.[3] Technical horoscopes and astrology ideas in India came from Greece and developed in the early centuries of the 1st millennium CE.[22][4][5] Later medieval era texts such as the Yavana-jataka and the Siddhanta texts are more astrology-related.[23]

The field of Jyotisha deals with ascertaining time, particularly forecasting auspicious day and time for Vedic rituals.[2] The field of Vedanga structured time into Yuga which was a 5-year interval,[16] divided into multiple lunisolar intervals such as 60 solar months, 61 savana months, 62 synodic months and 67 sidereal months.[10] A Vedic Yuga had 1,860 tithis (तिथि, dates), and it defined a savana-day (civil day) from one sunrise to another.[13]

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.[24] 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,[4] 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.[13] 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.[25] Further, adds Ohashi, the Mesopotamian formula is different from the 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.[26] According to Asko Parpola, the Jyotisha and luni-solar calendar discoveries in ancient India, and similar discoveries in China in "great likelihood result from convergent parallel development", and not from diffusion from Mesopotamia.[27]

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.[28][29] 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.[28][30] Water clock and sun dials are mentioned in many ancient Hindu texts such as the Arthashastra.[31][32] 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.[33]

The Jyotisha texts present mathematical formulae to predict the length of day time, sun rise and moon cycles.[13][34] For example,

Water clock
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.