A simple form of micrometer, consisting of two non-parallel wires, webs, or bars, usually in the form of a cross, in the field of view of an eyepiece. The eyepiece is oriented so that the wires cross the field diagonally, and timings are made as stars cross the wires as the Earth rotates. The interval between successive crossings of the wires depends on a star's position in the field of view. A star's position can be calculated by comparison with those of stars of known position.
Subjects: Astronomy and Astrophysics.