The angle obtained by pointing the telescope of a transit at an object, reading the angle on the vertical circle, then reversing the instrument to put the vertical circle on the other side of the observer, redirecting the telescope at the object, and reading the new angle on the vertical circle. The difference of the two readings is the double zenith angle and is twice the zenith angle. Double zenith angles are used in trigonometric levelling and in astronomy because they are little affected by inclination of the vertical axis of the instrument used. Only certain types of transit, however, allow double zenith angles to be measured.