​MERIDIAN CIRCLE, or TRANSIT CIRCLE, an instrument used in observatories in modern times to combine the functions of a transit instrument and of the old mural circle. A vertical circle is carried on the axis of the transit instrument and revolves with it, its divisions being read by micrometer microscopes mounted solidly on one of the piers. In this way both co-ordinates of the position of a heavenly body, its right ascension and declination are determined at the same meridian passage—a great saving of time over the old method with the instruments.