Mesha is the first rashi, the Vedic sign corresponding to Aries and named for the ram. In jyotisha it is ruled by Mars (Mangala), and it is the sign of the Sun's exaltation, where the Sun is held to reach its greatest strength. Its quality is movable (chara), the cardinal mode that begins each season, and its element is fire (agni tattva). Jyotisha reads Mesha through Mars's forward drive: initiative, courage, competition, and the willingness to act first. A key point about the Vedic signs is that they are sidereal, fixed to the stars, so a chart cast in jyotisha places planets in signs offset by roughly 24 degrees from the tropical Western zodiac used elsewhere on this site. The rulerships and exaltations, however, are shared inheritances of the same Hellenistic-and-earlier tradition, and Mars ruling the ram and the Sun exalted in it are common to both systems.
The twelve equal thirty-degree signs themselves are not of Indian origin: they were defined in Babylonian celestial divination by roughly the fifth or fourth century BCE, and no twelve-sign zodiac appears anywhere in the older Vedic corpus. The scheme reached Sanskrit only later, through the Yavanajataka, a Sanskrit versification of a Greek astrological text datable to roughly the second through fourth centuries CE, and Mesha itself is a direct translation of this transmission, rendering the Greek sign-name krios, ram, rather than naming any distinctly Indian constellation. The rulership, exaltation and elemental scheme jyotisha now states for Mesha belong to a still later classical layer, standardly codified in compilations such as the Brihat Parashara Hora Shastra of roughly the seventh or eighth century CE, pseudepigraphically ascribed to the sage Parashara.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with The Yavanajataka of SphujidhvajaDavid Pingree, Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree