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An instrument consisting of two bars mounted perpendicular to each other in the focal plane of a telescope, and inclined to the east-west path of stars by 45 ; used to measure differences in right ascension and declination of celestial objects.



Related Terms

FILAR MICROMETER

An instrument used to measure small distances in the field of an eyepiece by using two parallel wires, one of which is fixed while the other is moved at right angles to its length by means of an accurately cut screw. Also known as bifilar micrometer.

NYQUIST CONTOUR

A directed closed path in the complex frequency plane used in constructing a Nyquist diagram, which runs upward, parallel to the whole length of the imaginary axis at an infinitesimal distance to the right of it, and returns from jn to jn along a semicircle of infinite radius in the right half-plane.

GRATICULE

  1. The network of lines representing parallels and meridians on a map, chart, or plotting sheet. A fictitious graticule represents fictitious parallels and fictitious meridians. 2. A scale at the focal plane of an optical instrument to aid in the measurement of objects.

RETICLE

A system of lines, wires, etc., placed in the focal plane of an optical instrument to serve as a reference. A cross hair is a hair, thread, or wire constituting part of a reticle.

COUNTER/FREQUENCY METER

An instrument that contains a frequency standard and can be used to measure the number of events or the number of cycles of a periodic quantity that occurs in a specified time, or the time between two events.

EXTENSOMETER

  1. A strainometer that measures the change in distance between two reference points separated 60 90 feet (20 30 meters) or more; used in studies of displacements due to seismic activities. 2. An instrument designed to measure minute deformations of small objects subjected to stress.

GEOIDAL HORIZON

The circle of the celestial sphere formed by the inter- section of the celestial sphere and a plane through a point on the sea level surface of the earth, and perpendicular to the zenith-nadir line.

TELESCOPE

An optical instrument used as an aid in viewing or photo- graphing distant objects, particularly celestial objects. A reflecting telescope collects light by means of a concave mirror; a refracting telescope by means of a lens or system of lenses. A Cassegrainian telescope is a reflecting telescope in which the immergent light is reflected from the main mirror onto a secondary mirror, where it is reflected through a hole in the main mirror to an eyepiece; a Newtonian telescope is a reflecting telescope in which the immergent beam is reflected from the main mirror onto a small plane mirror, and from there to an eyepiece at the side of the telescope.

ERGOMETER

An instrument with a remeasure tactile sensibility by determining the distance by which two points pressed against cording device used to measure work performed.

CELESTIAL HORIZON

That circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through the center of the earth and perpendicular to the zenith-nadir line. Also called RATIONAL HORIZON.

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