Perigee is a type of apsis&colon; an extreme point in an object's orbit. Apogee is a type of apsis&colon; an extreme point in an object's orbit. The apsis was decorated with frescoes by Pietro Cavallini in the 13th century. A choir in Gothic style was built on the foundation of a Romanesque apsis. The left nave has a small altar dedicated to St Sebastian in its apsis. In 1908, frescos were discovered in the late Gothic apsis vaults. The chancel and the finely rounded apsis are part of the original structure. This contains the explanation of the motion of the apsis.