Vedic ritual is time-bound in a way that makes astronomy a practical necessity rather than an optional science: a sacrifice performed on the wrong day, at the wrong phase of the moon or the wrong solar station, was held to fail regardless of how correctly its other elements were carried out. The Vedanga Jyotisha exists to solve that problem. It is one of the six Vedangas, the ancillary disciplines, alongside phonetics, ritual procedure, grammar, etymology and metrics, that support the correct performance of Vedic rites, and its particular assignment is the calendar.
Its central device is a five year cycle, a yuga, built to reconcile the lunar month against the solar year by tracking the sun and moon's movement against the nakshatras, the twenty seven or twenty eight lunar mansions the moon passes through in a sidereal month. The text gives rules for intercalating an extra month to keep lunar and solar reckoning aligned, for finding the solstices, and for correlating tithis, the lunar days used to schedule ritual, against the solar calendar the agricultural and sacrificial year actually followed.
Two closely related recensions survive, one attached to the Rigveda and one to the Yajurveda, differing mainly in verse count and minor detail rather than in their underlying method, which suggests the calendrical system itself circulated somewhat independently of either specific Vedic school before being versified into each.