Ceres (sîrˈēz) [key], in astronomy, a dwarf planet, the first asteroid to be discovered. It was found on Jan. 1, 1801, by G. Piazzi. He took three distinct observations; on the basis of these the mathematician Gauss calculated Ceres' orbit with such accuracy that it was found one year later within 0.5° of the predicted position. Ceres is the largest and most massive of the asteroids; it has a diameter of c.590 mi (949 km) and a mass 1/100,000 that of the earth. Its orbit has a semimajor axis of 2.78 astronomical units and a period of 1,681 days. From its discovery until the 1850s, when many additional asteroids began to be identified, Ceres was regarded by many astronomers as a planet. In 2006 the reconsideration by astronomers of Pluto's status as a planet also led to the reclassification of Ceres as a dwarf planet.
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