The longitude at which an observer is assumed to be located for an observation or computation, as the longitude of an assumed position or the longitude used for determining the latitude by meridian altitude. Also called CHOSEN LONGITUDE.

Related Terms

GRID NORTH

1. An arbitrary reference direction used with grid navigation. The direction of the 180th geographical meridian from the north pole is used almost universally as grid north. 2. The northerly or zero direction indicated by the grid datum of directional reference.

GRID PARALLEL

A line parallel to a grid equator, connecting all points of equal grid latitude

HOUR CIRCLE

On the celestial sphere, a great circle through the celestial poles. An hour circle through the zenith is called a celestial meridian Also called CIRCLE OF DECLINATION, CIRCLE OF

HYPERBOLIC NAVIGATION

Radionavigation based on the measurement of the time differences in the reception of signals from several pairs of synchronized transmitters. For each pair of transmitters the isochrones are substantially hyperbolic. The combination of isochrones for two or more pairs of transmitters forms a hyperbolic lattice within which position can be determined according to the measured time differences

HOUR-GLASS EFFECT

A radarscope phenomenon which appears as a constriction or expansion of the display near the center of the plan position indicator, which can be caused by a nonlinear time base or the sweep plot starting on the radar indicator at the same instant as the transmission of the pulse. The phenomenon is most apparent when in narrow rivers or close to shore.

HEELING ADJUSTER

A dip needle with a sliding weight that can be moved along one of its arms to balance magnetic force, used to determine the correct position of a heeling magnet.

HOUR ANGLE

Angular distance west of a celestial meridian or hour circle; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of a celestial meridian or hour circle and the hour circl

MOTION

The act, process, or instance of change of position. Absolute motion is motion relative to a fixed point. Actual motion is motion of an object relative to the earth. Apparent or relative motion is change of position as observed from a reference point which may itself be in motion. Diurnal motion is the apparent daily motion of a celestial body. Direct motion is the apparent motion of a planet eastward among the stars; retrograde motion, the apparent motion westward among the stars. Motion of a celestial body through space is called space motion, which is composed of two components: proper motion, that component perpendicular to the line of sight; and radial motion, that component in the direction of the line of sight. Also called MOVEMENT, especially when used in connection with problems involving the motion of one vessel relative to another.

INDUCED MAGNETISM

The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced mag- netism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes.

GOVERNOR SAFETY STOP

On throttling-type governors, the safety stop is a weighted arm that needs the support of a governor belt. If the belt breaks, the idler arm drops and shuts the steam supply valve to the engine. On Corliss units, the flyballs fall to the lowest position and knock off the safety cams; the cams disengage the catch blocks on the steam intake valves so that no steam is admitted to the engine.

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