Precision instrument used in mechanical operations, astronomy, and geodesy to measure very small linear values (distances, lengths, and internal and external diameters). It can be fitted on optical instruments such as microscopes and telescopes to determine the dimension or position of the objects observed. Galileo (1564-1642) was the first to realize its usefulness for taking precise angular measurements of celestial bodies through a telescope. When applied to the eyepiece or the objective of a microscope, it is called an eyepiece micrometer or objective micrometer.