Giuseppe Piazzi discovered the first and largest asteroid, Ceres, on Jan. 1, 1801, while mapping stars. He found it orbiting in an area between Mars and Jupiter. Measuring 583 miles across, this almost spherical asteroid contains about one-fourth of the mass of all known asteroids.
When Piazzi first discovered Ceres, he thought he had found a comet, but the scientific world soon believed it was another planet. After similar, yet smaller, discoveries of this same type of body, scientists began to use the term "asteroid" to define these smaller worlds that revolve around the sun along with the planets. Most of the asteroids discovered are in the same area in which Piazzi first spotted Ceres.