15 Eunomia is a large asteroid in the middle asteroid belt. It is the largest of the stony (S-type) asteroids, with a mean diameter of about 270 kilometres (170 mi). It is between the eight and twelfth largest main belt asteroid, containing about 1% of its mass. It was discovered on 29 July 1851 by astronomer Annibale de Gasparis, who chose to name the asteroid after the Greek goddess Eunomia. It is elongated and irregular in shape, and rotates once every six hours and five minutes.