micrometer

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micrometer 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.

micrometer . 1 Instrument used for measuring extremely small distances. Typical examples are devices used in astronomical telescopes to measure the apparent diameter of celestial objects and similar devices used in microscopes. In both of these devices a fine hair or filament is moved from one extremity to the other of the image of an object and the distance read on a calibrated scale. Another typical micrometer is the micrometer caliper, a device in which an object to be measured is enclosed between two jaws, one fixed and the other movable by means of a fine screw. When the jaws are just touching the object, the distance between the jaws can be read on an associated scale, often to an accuracy of 10 ^-4 (one ten-thousandth) in., or 10 ^-6 (one millionth) m. 2 Unit of linear distance equal to 10 ^-6 (one millionth) m. It was formerly known as a micron.

mi·crom·e·ter¹ / mīˈkrämətər/ (also micrometer caliper) • n. a gauge that measures small distances or thicknesses between its two faces, one of which can be moved away from or toward the other by turning a screw with a fine thread. DERIVATIVES: mi·crom·e·try / -ətrē/ n.

micrometer

micrometer (my-krom-it-er) n. an instrument for making extremely fine measurements of thickness or length.

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