An instrument used in astronomy for measuring angular distances and relative positions on the sky. Micrometers are used particularly for measuring the orbital motions of double stars, but can also be used to measure the locations of features on planets. There are many types of micrometer, but all introduce standard reference points into the field of view of an eyepiece. These references may be illuminated reticles or wires, although the wires may be spider thread and are referred to as webs. The webs are located in the same plane as the field stop, so that they are always in focus. The reticle may be fixed, as in the cross-wire micrometer, in which case the daily movement of the sky is used for reference; or there may be a movable portion, as in the filar micrometer, with scales which should be calibrated against the sky’s daily motion. Other types of micrometer provide movable artificial stars or double images of the stars in the field of view.