diopter

…contains a description of the diopter, a surveying instrument used for the same purposes as the modern theodolite. The treatise also contains applications of the diopter to measuring celestial distances and describes a method for finding the distance between Alexandria and Rome from the difference between local times at which…

…that were improved into the diopter, or dioptra (an early theodolite), and were described in the 1st century ad by Heron of Alexandria.