Ceres is the only object in the asteroid belt rounded by its own gravity, although Vesta and perhaps other asteroids were so in the past. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, peaking once at opposition every 15 to 16 months, which is its synodic period. Thus even at its brightest, it is too dim to be seen by the naked eye, except under extremely dark skies. Ceres has been classified as a C-type asteroid and, due to the presence of clay minerals, as a G-type asteroid.