The distinguishing characteristic of the sextant is the arrangement of mirrors, which enables the observer to sight at two different objects simultaneously, and thus to measure an angle at a single observation.  While other instruments must be held stationary during the time between the backsight and the foresight, the sextant may be held in the hand, since the two sights are taken at the same time, and horizontal, vertical, or inclined angles may be measured very accurately even though the instrument is in motion.