What do the semicircular canals detect?

The Bony Semicircular Canals (canales semicirculares ossei).--The bony semicircular canals are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule. They are unequal in length, compressed from side to side, and each describes the greater pa of a circle. Each measures about 0.8 mm. in diameter, and presents a dilatation at one end, called the ampulla, which measures more than twice the diameter of the tube. They open into the vestibule by five orifices, one of the apeures being common to two of the canals. Ref: Anatomy of the Human Body Henry Gray.