A modern surveyor will have an alidade in their toolbox in order to measure angles, but this is a centuries-old device that was originally used in conjunction with an astrolabe for early astronomy and celestial navigation. The word “alidade” came into English from French and Spanish, but its etymological root is the Arabic “al-‘iḍāda,” meaning “the revolving radius.” Both the ancient and modern versions are sighting devices that involve lining up with a designated object to determine the distance, height, or angle of elevation, but the difference is that modern versions tend to be digital or electronic.