Around 2500 BC many extant texts were written by sages such Agastya and Bhrigu. Each sign was divided in three more strata called “charna” similar to decanates of Western astrology. The Nirayana, or sidereal zodiac, is an imaginary belt of 360 degrees, which, like the Sāyana, or tropical zodiac, is divided into 12 equal parts. Each twelfth part (of 30 degrees) is called a sign or rāśi (Sanskrit: ‘part’). Vedic (Jyotiṣa) and Western zodiacs differ in the method of measurement. While synchronically, the two systems are identical, Jyotiṣa uses primarily the sidereal zodiac (in which stars are considered to be the fixed background against which the motion of the planets is measured), whereas most Western astrology uses the tropical zodiac (the motion of the planets is measured against the position of the Sun on the Spring equinox). This difference becomes noticeable over time. After two millennia, as a result of the precession of the equinoxes, the origin of the ecliptic longitude has shifted by about 22 degrees. As a result, the placement of planets in the Jyotiṣa system is consistent with the actual zodiac, while in western astrology the planets fall into the following sign, as compared to their placement in the sidereal zodiac, about two thirds of the time.