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The Nautical Dictionary
The Nautical Dictionary
The Nautical Dictionary
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The Nautical Dictionary

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The digital nautical dictionary for the 21st century.

10,000 nautical words. The old & the new. A treasure trove.

Thousands of links to online sources. 

LanguageEnglish
PublisherVbooks
Release dateMar 3, 2021
ISBN9798201291815
The Nautical Dictionary
Author

Alan Phillips

ABOUT THE AUTHORTwice Around-the-World +Alan Phillips did his first circumnavigation in a boat called “Scorpio “starting in 1980. His next circumnavigation commenced in 1992 onboard “Wallaby Creek “. This voyage lasted 15 years.He has also done a two year passage to China via Solomon Islands, Micronesia, Philippines etc.His is Australian and has circumnavigated Australia and done numerous passages out into the Pacific Ocean from Australia, including seven times into Papua New Guinea and six times across the China Sea.After 25 years and 100,000 nautical miles of ocean cruising he is one of the most experience cruising sailors on earth.Along the way he has developed his own philosophies and has views that exhibit original thinking.OTHER THINGS HE HAS DONESailed my small boat thru the very eye of a tropical cycloneand lived to tell.Hit a reef, wrecked my boat and was castaway alone on Orchilla Isle in the Caribbean to live like Robinson Crusoe.and lived to tellLooked into the barrel of a PLO gun with the terrorists screaming at me in some languageand lived to tellLooked into the barrel of an Israel machine gun while being screamed at in some languageand lived to tellHad a beer in the Beirut yacht club while the Israelis rocketed and bombed with everything they had and lived.Sailed up to and anchored at Pirate Island in the Southern Philippines and shared a bottle of rum with the piratesand lived to tellGot caught smuggling in Panama and then had to drink the customs officer under the table. After which we altered the paperwork and were friendsand stayed out of jailEscaped from the immigration police in Indonesia by stealing a speed boat and pulling Wallaby Creek thru the reefDrilled out my own tooth with tools from the tool box.Circumnavigated Australia and twice around the world.Found a Polynesian tribe that is still living an uncivilized life and has never been visited by a tourist.Been becalmed for 3 weeks under the tropical sun and at the end had counted out our last meals.Found a cave in PNG with dozens of human skeletons from people who entered the cave in order to die.Fathered a beautiful baby with a young Swiss girl and married her even though she is 25 years youngerand also younger then 2 of my other children.Survived three shark attacks where the sharks ripped my spear and fish from my hands. I continue to spearfish at every opportunity.Spent months alone at sea in a leaky boat with no motor and no electrical or electronic thing on the boat.Left a crewman with a primitive tribe in PNG and returned a year later to look for him. Received a message in a bottle.Crossed the jungles of the Western Province of PNG where no white man had been for 20 years.Did a seven-week single-handed passage from Panama to Tahiti. And didn’t end up much crazier.

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    The Nautical Dictionary - Alan Phillips

    THE NAUTICAL DICTIONARY

    VBooks

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    10,000 nautical words. Old & new.

    A

    "Abel Brown": A sea shanty (song) about a young sailor trying to sleep with a maiden.

    A.M. . Abbreviation for Ante Meridian; before noon in zone time.

    Abaft the beam: Further aft than the beam: a relative bearing of greater than 90 degrees from the bow: two points abaft the port beam.  Any direction between broad on the beam and astern.

    abaft. , adv. In a direction farther aft in a ship than a specified reference position, such as abaft the mast. See also ABAFT THE BEAM, AFT, ASTERN. Toward the stern, relative to some object (abaft the fore hatch).

    abampere. , n. The unit of current in the centimeter gram-second electromagnetic system. The abampere is 10 amperes.

    Abandon ship!: An imperative to leave the vessel immediately, usually in the face of some imminent danger. It is an order issued by the Master or a delegated person in command. It is usually the last resort after all other mitigating actions have failed.

    abeam. , adv. In a line approximately at right angle to the ship’s keel opposite the waist or middle part of a ship. See also BROAD ONTHE BEAM.

    abeam. , adv. In a line approximately at right angle to the ship’s keel opposite the waist or middle part of a ship. See also BROAD ON THE BEAM. On the beam, a relative bearing at right angles to the centerline of the ship's keel.

    aberration constant. . See CONSTANT OF ABERRATION.

    aberration. , n. 1. The apparent displacement of a celestial body in the direction of motion of the earth in its orbit caused by the motion of the earth combined with the finite velocity of light. When, in addition to the combined effect of the velocity of light and the motion of the earth, account is taken of the motion of the celestial body in space during the interval that the light is traveling to the earth from the luminous body, as in the case of planets, the phenomenon is termed planetary aberration. The aberration due to the rotation of the earth on its axis is termed diurnal aberration or daily aberration. The aberration due to the revolution of the earth about the sun is termed annual aberration. The aberration due to the motion of the center of mass of the solar system in space is termed secular aberration but is not taken into account in practical astronomy. See also CONSTANT OF ABERRATION. 2. The convergence to different foci, by a lens or mirror, of parallel rays of light. In a single lens having spherical surfaces, aberration may be caused by differences in the focal lengths of the various parts of the lens: rays passing through the outer part of the lens come to a focus nearer the lens than do rays passing through its central part. This is termed spherical aberration and, being due to the faulty figure of the lens, is eliminated by correcting that figure. A lens so corrected is called an aplanatic lens. Aberration may also result from differences in the wavelengths of light of different colors: light of the shorter wavelengths (violet end of the spectrum) comes to a focus nearer the lens than light of the longer wavelengths (red end of the spectrum). This is termed chromatic aberration, and is practically eliminated over a moderate range of wavelengths by using a composite lens, called an achromatic lens, composed of parts having different dispersive powers.

    ablation. All processes by which snow, ice or water in any form are lost from a glacier, floating ice or snow cover. These include melting, evaporation, calving, wind erosion and avalanches. Also used to express the quantities lost by these processes.

    abnormal magnetic variation. Designation applied to any anomalous value of the magnetic variation of which the cause is unknown. See also local magnetic anomaly.

    aboard. In the sense used in pilotage and ship handling means near. eg To keep the E shore aboard. Close aboard means Very near. See also borrow.

    Aboard: On or in a vessel (see also close aboard).

    Above board: On or above the deck, in plain view, not hiding anything.

    above. Uptide or upstream of a position.

    Above-water hull: The hull section of a vessel above the waterline, the visible part of a ship. Also, topsides.

    above-water. A shoal, rock or other feature is termed above-water if it is visible at any state of the tide. See also awash, dries, below-water.

    abrasion. , n. Rubbing or wearing away, or the result of such action.

    abroholos. , n. A squall frequent from May through August between Cabo de Sao Tome and Cabo Frio on the coast of Brazil.

    abrupt. , adv. Steep, precipitous. See also BOLD.

    abscissa. , n. The horizontal coordinate of a set of rectangular coordinates. Also used in a similar sense in connection with oblique coordinates.

    Absentee pennant: Special pennant flown to indicate absence of commanding officer, admiral, his chief of staff, or officer whose flag is flying (division, squadron, or flotilla commander).

    absolute accuracy. . The ability of a navigation or positioning system to define an exact location in relation to a coordinate system.

    Absolute bearing: The bearing of an object in relation to north. Either true bearing, using the geographical or true north, or magnetic bearing, using magnetic north. See also bearing and relative bearing.

    absolute gain. . See ISOTROPIC GAIN (of an antenna).

    absolute humidity. . The mass of water vapor per unit volume of air.

    absolute motion. . Motion relative to a fixed point. If the earth were stationary in space, any change in the position of another body, relative to the earth, would be due only to the motion of that body. This would be absolute motion, or motion relative to a fixed point. Actual motion is motion of an object relative to the earth.

    absolute temperature. . Temperature measured from absolute zero which is zero on the Kelvin scale, 273.16°C on the Celsius scale, and 459.69°F on the Fahrenheit scale. The sizes of the Kelvin and Celsius degree are equal. The size of a degree on the Fahrenheit scale equals that on the Rankine scale.

    absolute value. . The value of a real number without regard to sign. Thus, the absolute value of +8 or -8 is |8|. Vertical lines on each side of a number indicate that its absolute value is intended.

    absolute zero. . The theoretical temperature at which molecular motion ceases, 459.69°F or -273.16°C.

    absolute.  Pertaining to measurement relative to a universal constant or natural datum.

    absorption. . The process by which radiant energy is absorbed and converted to other forms of energy. See ATTENUATION.

    abyss. , n. A very deep area of the ocean. The term is used to refer to a particular deep part of the ocean, or to any part below 300 fathoms.

    abyssal gap. A narrow break, in a ridge or rise, or separating two abyssal plains.

    abyssal hills. A tract of small elevations on the sea floor.

    abyssal or abysmal. Relating to the greatest depths of the ocean (literally, without bottom).

    abyssal plain.  A flat, gently sloping or nearly level region at abyssal depths.

    accelerate. , v., t. To move or cause to move with increasing velocity.

    acceleration error. . The error resulting from change in velocity (either speed or direction); specifically, deflection of the apparent vertical, as indicated by an artificial horizon, due to acceleration. Also called BUBBLE ACCELERATION ERROR when applied to an instrument using a bubble as an artificial horizon.

    acceleration. , n. 1. The rate of change of velocity. 2. The act or process of accelerating, or the state of being accelerated. Negative acceleration is called DECELERATION.

    accelerometer. , n. A device used to measure the accelerations of a craft, resulting from the craft’s acceleration with respect to the earth, acceleration of gravity, and Coriolis acceleration.

    accidental error. . See RANDOM ERROR. An error of accidental nature. (Not to be confused with MISTAKE.)

    Accommodation ladder: A portable flight of steps down a ship's side.

    accretion or deposition. The depositing of material on the bottom or the coast by water movement; the opposite to erosion.

    accretion. , n. Accumulation of material on the surface of an object.

    accumulation. All processes by which snow, ice or water in any form are added to a glacier, floating ice or snow cover. These include direct precipitation of snow, ice or rain, condensation of ice from vapour, and transport of snow and ice to a glacier. Also used to express the quantities added by these processes.

    accuracy. , n. 1. In navigation, a measure of the difference between the position indicated by measurement and the true position. Some expressions of accuracy are defined in terms of probability. 2. A measure of how close the outcome of a series of observations or measurements approaches the true value of a desired quantity. The degree of exactness with which the true value of the quantity is determined from observations is limited by the presence of both systematic and random errors. Accuracy should not be confused with PRECISION, which is a measure of the repeatability of the observations. Observations may be of high precision due to the quality of the observing instrument, the skill of the observer and the resulting small random errors, but inaccurate due to the presence of large systematic errors. Accuracy implies precision, but precision does not imply accuracy. See also ERROR, RADIAL ERROR, ABSOLUTE ACCURACY, PREDICTABLE ACCURACY, RELATIVE ACCURACY, REPEATABLE ACCURACY.

    achromatic lens. . See under ABERRATION, definition 2. a lens that transmits light without separating it into constituent colors

    aclinal. , adj. Without dip; horizontal. Without magnetic dip.

    aclinic line. . The magnetic equator; the line on the surface of the earth connecting all points of zero magnetic dip.

    acoustic depth finder. . See ECHO SOUNDER.

    acoustic navigation. . See SONIC NAVIGATION.

    acoustic sounding. . See ECHO SOUNDING.

    acoustics. , n. 1. That branch of physics dealing with sound. 2. The sound characteristics of a room, auditorium, etc., which determine its quality with respect to distinct hearing.

    acquisition. , n. The selection of those targets or satellites requiring a tracking procedure and the initiation of their tracking.

    acre. , n. A unit of area equal to 43,560 square feet.

    across-the-scope echo. . See CLASSIFICATION OF RADAR ECHOES.

    Act of Pardon, Act of Grace: A letter from a state or power authorising action by a privateer. Also see Letter of marque.

    active satellite. . 1. An artificial satellite which transmits an electromagnetic signal. A satellite with the capability to transmit, repeat, or retransmit electromagnetic information, as contrasted with PASSIVE SATELLITE. 2. As defined by International Telecommunications Union (ITU), an earth satellite carrying a station intended to transmit or re transmit radio communication signals.

    active tracking system. . A satellite tracking system which operates by transmission of signals to and receipt of responses from the satellite.

    actual motion. . Motion of an object relative to the earth. See also MOTION.

    acute angle. . An angle less than 90°.

    additional secondary phase factor correction. . A correction in addition to the secondary phase factor correction for the additional time (or phase delay) for transmission of a low frequency signal over a composite land-water path when the signal transit time is based on the free-space velocity.

    ADF reversal. . The swinging of the needle on the direction indicator of an automatic direction finder through 180°, indicating that the station to which the direction finder is tuned has been passed.

    adiabatic. , adj. Referring to a thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, compression causes warming, expansion causes cooling.

    adjacent angles. . Two angles having a common vertex and lying at opposite ends of a common side.

    adjustment. , n. The determination and application of corrections to observations, for the purpose of reducing errors or removing internal inconsistencies in derived results.

    Admiral: Senior naval officer of Flag rank. In ascending order of seniority, Rear Admiral, Vice Admiral, Admiral and Admiral of the Fleet (Royal Navy). Derivation Arabic, from Amir al-Bahr (Ruler of the sea).

    Admiralty law: Body of law that deals with maritime cases. In the UK administered by the Probate, Divorce and Admiralty Division of the High Court of Justice or supreme court.

    admiralty. . Pertaining to the body of law that governs maritime affairs. A high naval authority in charge of a state's Navy or a major territorial component. In the Royal Navy (UK) the Board of Admiralty, executing the office of the Lord High Admiral, promulgates Naval law in the form of Queen's (or King's) Regulations and Admiralty Instructions.

    adrift. , adj. & adv. Afloat and unattached to the shore or the sea bottom, and without propulsive power. See also UNDERWAY. Afloat and unattached in any way to the shore or seabed, but not under way. It implies that a vessel is not under control and therefore goes where the wind and current take her (loose from moorings, or out of place). Also refers to any gear not fastened down or put away properly. It can also be used to mean absent without leave.

    Advance note: A note for one month's wages issued to sailors on their signing a ship's articles.

    advance. , n. 1. The distance a vessel moves in its initial direction from the point where the rudder is started over until the heading has changed 90°. 2. The distance a vessel moves in the initial direction for heading changes of less than 90°. See also TRANSFER.

    advance. , v., t. & i. To move forward, as to move a line of position forward, parallel to itself, along a course line to obtain a line of position at a later time. The opposite is RETIRE.

    advanced line of position. . A line of position which has been moved forward along the course line to allow for the run since the line was established. The opposite is RETIRED LINE OF POSITION.

    advection fog. . A type of fog caused by the advection of moist air over a cold surface, and the consequent cooling of that air to below its dew point. SEA FOG is a very common advection fog that is caused by moist air in transport over a cold body of water.

    advection. , n. Transport of atmospheric properties solely by mass motion of the atmosphere. WIND refers to air motion, while ADVECTION refers more specifically to the transfer of any property of the atmosphere (temperature, humidity, etc.) from one area to another.

    aero light. . Short for AERONAUTICAL LIGHT.

    aeromarine light. . A marine light having part of its beam deflected to an angle of 10° to 15° above the horizon for use by aircraft.

    aeromarine radiobeacon. . A radiobeacon established for use by both mariners and airmen.

    aeronautical beacon. . A visual aid to navigation, displaying flashes of white or colored light or both, used to indicate the location of airports, landmarks, and certain points of the Federal airways in mountainous terrain and to mark hazards.

    aeronautical light. . A luminous or lighted aid to navigation intended primarily for air navigation. Often shortened to AERO LIGHT.

    aeronautical radiobeacon. A radiobeacon primarily for the use of aircraft. Usually abbreviated to aero radiobeacon.

    aeronautical. , adj. Of or pertaining to the operation or navigation of aircraft.

    aestival. , adj. Pertaining to summer. The corresponding adjectives for fall, winter, and spring are autumnal, hibernal and vernal.

    affluent. , n. A stream flowing into a larger stream or lake; a tributary.

    afloat. , adj. & adv. Floating on the water; water-borne. See also SURFACED, UNCOVERED, AGROUND, ASHORE. Of a vessel which is floating freely (not aground or sunk). More generally of vessels in service (the company has 10 ships afloat).

    aft. , adv. Near, toward, or at the stern of a craft. See also ABAFT, ASTERN.

    afterglow. , n. 1. The slowly decaying luminescence of the screen of the cathode-ray tube after excitation by an electron beam has ceased. See also PERSISTENCE. 2. A broad, high arch of radiance or glow seen occasionally in the western sky above the highest clouds in deepening twilight, caused by the scattering effect of very fine particles of dust suspended in the upper atmosphere.

    Afternoon watch: The 1200–1600 watch.

    age of diurnal inequality. . The time interval between the maximum semimonthly north or south declination of the moon and the maximum effect of the declination upon the range of tide or the speed of the tidal current; this effect is manifested chiefly by an increase in the height or speed difference between the two high (low) waters or flood (ebb) currents during the day. The tides occurring at this time are called TROPIC TIDES. Also called DIURNAL AGE.

    age of phase inequality. . The time interval between new or full moon and the maximum effect of these phases upon the range of tide or the speed of the tidal current. Also called AGE OF TIDE.

    age of the moon. . The elapsed time, usually expressed in days, since the last new moon. See also PHASES OF THE MOON.

    age of the Moon. The interval in days and decimals of a day since the last New Moon.

    age of tide. . See AGE OF PHASE INEQUALITY. age of the tide. Old term for the lag between the time of new or full Moon and the time of maximum spring tidal range.

    AGENCIES. US GOVERNMENT.

    aged ridge. A ridge which has undergone considerable weathering. These ridges are best described as undulations.

    Ageton. . n. 1. A divided triangle method of sight reduction in which a perpendicular is dropped from the GP of the body to the meridian of the observer. 2. Rear Admiral Arthur A. Ageton, USN, inventor of the Ageton method.

    agger. , n. See DOUBLE TIDE.

    agonic line. . A line joining points of no magnetic variation, a special case of an isogonic line.

    agravic. , adj. Of or pertaining to a condition of no gravitation.

    aground. , adj. & adv. Resting or lodged on the bottom. on or touching the ground or bottom (usually involuntarily).

    Agulhas Current. . A generally southwestward flowing ocean current of the Indian Ocean, one of the swiftest ocean currents. To the south of latitude 30°S the Agulhas Current is a well-defined and narrow current that extends less than 100 km from the coast of South Africa. To the south of South Africa the greatest volume of its water bends sharply to the south and then toward the east, thus returning to the Indian Ocean.

    ahead reach. . The distance traveled by a vessel proceeding ahead at full power from the time the engines are reversed until she is at full stop.

    ahead. , adv. Bearing approximately 000° relative. The term is often used loosely for DEAD AHEAD or bearing exactly 000° relative. The opposite is ASTERN. Forward of the bow.

    Ahoy: A cry to draw attention. Term used to hail a boat or a ship, as Boat ahoy!

    ahull. . The condition of a vessel making no way in a storm, allowing wind  and sea to determine the position of the ship. Sailing vessels lying ahull lash the helm alee, and may carry storm sails. 1. When the boat is lying broadside to the sea. 2. To ride out a storm with no sails and helm held to leeward.

    aid to navigation. . A device or structure external to a craft, designed to assist in determination of position, to define a safe course, or to warn of dangers or obstructions. If the information is transmitted by light waves, the device is called a visual aid to navigation; if by sound waves, an audible aid to navigation; if by radio waves; a radio aid to navigation. Any aid to navigation using electronic equipment, whether or not radio waves are involved, may be called an electronic aid to navigation. Compare with NAVIGATIONAL AID, meaning an instrument, device, chart, method, etc., intended to assist in the navigation of a craft. (ATON) Any device external to a vessel or aircraft specifically intended to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation. aid to navigation. A device or system external to the vessel that is designed and operated to enhance the safe and efficient navigation of vessels and/or vessel traffic. Examples include buoys, beacons, lights, radio beacons, leading marks, radio position fixing systems. See also navigation aid.

    Aid to Navigation: (ATON) Any device external to a vessel or aircraft specifically intended to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation.

    air almanac. . 1. A periodical publication of astronomical data designed primarily for air navigation, but often used in marine navigation. See also ALMANAC FOR COMPUTERS. 2. Air Almanac, a joint publication of the U.S. Naval Observatory and H. M. Nautical Almanac Office, Royal Greenwich Observatory, designed primarily for air navigation. In general the information is similar to that of the Nautical Almanac, but is given to a precision of 1' of arc and 1s of time, at intervals of 10m (values for the sun and Aries are given to a precision of 0.1').

    air defense identification zone (ADIZ). . Airspace of defined dimensions within which the ready identification location, and control of aircraft are required.

    air draught. The height of the highest point of the vessel above the water−line.

    air mass. . An extensive body of air with fairly uniform (horizontal) physical properties, especially temperature and humidity. In its incipient stage the properties of the air mass are determined by the characteristics of the region in which it forms. It is a cold or warm air mass if it is colder or warmer than the surrounding air.

    air temperature correction. . A correction due to nonstandard air temperature, particularly the sextant altitude correction due to changes in refraction caused by difference between the actual temperature and the standard temperature used in the computation of the refraction table. The Nautical Almanac refraction table is based upon an air temperature of 50°F (10°C) at the surface of the earth. Refraction is greater at lower temperatures, and less at higher temperatures. The correction for air temperature varies with the temperature of the air and the altitude of the celestial body, and applies to all celestial bodies, regardless of the method of observation. It is not applied in normal navigation.

    air. , n. 1. The mixture of gases comprising the earth’s atmosphere. It is composed of about 78% nitrogen, 21% oxygen, 1% other gases, and a variable amount of impurities such as water vapor, suspended dust particles, smoke, etc. See also ATMOSPHERE. 2. Wind of force 1 (1-3 knots or 1-3 miles per hour) on the Beaufort wind scale, called LIGHT AIR.

    air-mass classification. . Air masses are classified according to their source regions. Four such regions are generally recognized- (1) equatorial (E), the doldrum area between the north and south trades; (2) tropical (T), the trade wind and lower temperate regions, (3) polar (P), the higher temperate latitudes; and (4) Arctic or Antarctic (A), the north or south polar regions of ice and snow. This classification is a general indication of relative temperature, as well as latitude of origin. Air masses are further classified as maritime (m) or continental (c), depending upon whether they form over water or land. This classification is an indication of the relative moisture content of the air mass. A third classification sometimes applied to tropical and polar air masses indicates whether the air mass is warm (w) or cold (k) relative to the underlying surface. The w and k classifications are primarily indications of stability, cold air being more stable.

    Alaska Current. . A North Pacific Ocean current flowing counterclockwise in the Gulf of Alaska. It is the northward flowing division of the Aleutian Current.

    albedo. , n. The ratio of radiant energy reflected to that received by a surface, usually expressed as a percentage; reflectivity. The term generally refers to energy within a specific frequency range, as the visible spectrum. Its most frequent application in navigation is to the light reflected by a celestial body.

    alert time calculations. . Computations of times and-altitudes of available satellite passes in a given period of time at a given location, based on orbital data transmitted from satellite memory. Sometimes called ALERT.

    Aleutian Current. . An eastward flowing North Pacific Ocean current which lies north of the North Pacific Current. As it approaches the coast of North America it divides to form the northward-flowing ALASKA CURRENT, and the southward-flowing CALIFORNIA CURRENT. Also called SUBARCTIC CURRENT.

    alga. (pl. algae), n. A plant of simple structure which grows chiefly in water, such as the various forms of seaweed. It ranges in size from a microscopic plant, large numbers of which sometimes cause discoloration of water, to the giant kelp which may extend for more than 600 feet in length. The Red Sea owes its name to red algae, as does the red tide.

    algorithm. . A defined procedure or routine used for solving a specific mathematical problem.

    alidade. , n. The part of an optical measuring instrument comprising the optical system, indicator, vernier, etc. In modern practice the term is used principally in connection with a bearing circle fitted with a telescope to facilitate observation of bearings. Also called TELESCOPIC ALIDADE.

    align. , v., t. To place objects in line.

    alignment. , n. 1. The placing of objects in a line. 2. The process of orienting the measuring axes of the inertial components of inertial navigation equipment with respect to the coordinate system in which the equipment is to be used.

    All hands: Entire ship's company, both officers and enlisted personnel.

    All night in: Having no night watches.

    Allard’s law. . A formula relating the illuminance produced on a normal surface at a given distance from a point source of light, the intensity of the light, and the degree of transparency of the atmosphere, assumed to be uniform. See OMNIDIRECTIONAL LIGHT.

    all-weather. , adj. Designed or equipped to perform by day or night under any weather conditions.

    almanac. , n. A periodical publication of ephemeral astronomical data. If information is given in a form and to a precision suitable for marine navigation, it is called a nautical almanac. See also nautical almanac; if designed primarily for air navigation, it is called an air almanac. See also EPHEMERIS, ASTRONOMICAL ALMANAC.

    almucantar staff. . An ancient instrument formerly used for amplitude observations

    .

    almucantar. , n. A small circle on the celestial sphere paralleled to the horizon. Also called CIRCLE OF EQUAL ALTITUDE, PARALLEL OF ALTITUDE.

    alnico. , n. An alloy composed principally of aluminum, nickel, cobalt, and iron; used for permanent magnets.

    aloft. . Up in the rigging of a ship. In the rigging of a sailing ship. Above the ship's uppermost solid structure; overhead or high above.

    alongshore current. . See LONGSHORE CURRENT.

    alongside. A ship is alongside when side by side with a wharf, wall, jetty, or another ship.

    alphanumeric grid. . See ATLAS GRID.

    alphanumeric. . Referring to a set of computer characters consisting of alphabetic and numeric symbols.

    alternate blanking. . See under DUAL-RATE BLANKING.

    alternating current. . An electric current that continually changes in magnitude and periodically reverses polarity.

    alternating fixed and flashing light. . A fixed light varied at regular intervals by a single flash of greater luminous intensity, with color variations in either the fixed light or flash, or both. See ALTERNATING LIGHT.

    alternating fixed and group flashing light. . A fixed light varied at regular intervals by a group of two or more flashes of greater luminous intensity, with color variations in either the fixed light or flashes or both.

    alternating flashing light. . A light showing a single flash with color variations at regular intervals, the duration of light being shorter than that of darkness. See also FLASHING LIGHT.

    alternating group flashing light. . A group flashing light which shows periodic color change.

    alternating group occulting light. . A group occulting light which shows periodic color change.

    alternating light. . A light showing different colors alternately.

    alternating occulting light. . A light totally eclipsed at regular intervals, the duration of light always being longer than the duration of darkness, which shows periodic color change. See also ALTERNATING LIGHT.

    alternating. . Referring to periodic changes in color of a lighted aid to navigation.

    altitude azimuth. . An azimuth determined by solution of the navigational triangle with altitude, declination, and latitude given. A time azimuth is computed with meridian angle, declination, and latitude given. A time and altitude azimuth is computed with meridian angle, declination, and altitude given.

    altitude circle. . See PARALLEL OF ALTITUDE.

    altitude difference. . 1. See ALTITUDE INTERCEPT. 2. The change in the altitude of a celestial body occurring with change in declination, latitude, or hour angle, for example the first difference between successive tabulations of altitude in a latitude column of Pub. No. 229, Sight Reduction Tables for Marine Navigation.

    altitude intercept method. . See ST. HILAIRE METHOD.

    altitude intercept. . The difference in minutes of arc between the computed and the observed altitude (corrected sextant altitude), or between precomputed and sextant altitudes. It is labeled T (toward) or A (away) as the observed (or sextant) altitude is greater or smaller than the computed (or precomputed) altitude. Also called ALTITUDE DIFFERENCE, INTERCEPT.

    altitude of the apogee. . As defined by the International Telecommunication Union (ITU), the altitude of the apogee above a specified reference surface serving to represent the surface of the earth.

    altitude of the perigee. . As defined by the International Telecommunication Union (ITU), the altitude of the perigee above a specified reference surface serving to represent the surface of the earth.

    altitude tints. . See HYPSOMETRIC TINTING.

    altitude tints. . See HYPSOMETRIC TINTING.

    altitude. , n. Angular distance above the horizon; the arc of a vertical circle between the horizon and a point on the celestial sphere, measured upward from the horizon. Angular distance below the horizon is called negative altitude or depression. Altitude indicated by a sextant is called sextant altitude. Sextant altitude corrected only for inaccuracies in the reading (instrument, index, and personal errors, as applicable) and inaccuracies in the reference level (principally dip) is called apparent or rectified altitude. After all corrections are applied, it is called corrected sextant altitude or observed altitude. An altitude taken directly from a table, before interpolation, is called tabulated altitude. After interpolation, or if determined by calculation, mechanical device, or graphics, it is called computed altitude. If the altitude of a celestial body is computed before observation, and sextant altitude corrections are applied with reversed sign, the result is called precomputed altitude. The difference between computed and observed altitudes (corrected sextant altitudes), or between precomputed and sextant altitudes, is called altitude intercept or altitude difference. An altitude determined by inexact means, as by estimation or star finder, is called an approximate altitude. The altitude of a celestial body on the celestial meridian is called meridian altitude. The expression exmeridian altitude is applied to the altitude of a celestial body near the celestial meridian, to which a correction is to be applied to determine the meridian altitude. A parallel of altitude is a circle of the celestial sphere parallel to the horizon, connecting all points of equal altitude. See also EQUAL ALTITUDES.

    alto-. . A prefix used in cloud classification to indicate the middle level. See also CIRRO-.

    altocumulus. , n. Clouds within the middle level (mean height 6,500- 20,000 ft.) composed of flattened globular masses, the smallest elements of the regularly arranged layers being fairly thin, with or without shading. These elements are arranged in groups, in lines, or waves, following one or two directions, and are sometimes so close together that their edges join. See also CLOUD CLASSIFICATION.

    altostratus. , n. A sheet of gray or bluish cloud within the middle level (mean height 6,500-20,000 ft.). Sometimes the sheet is composed of a compact mass of dark, thick, gray clouds of fibrous structure; at other times the sheet is thin and through it the sun or moon can be seen dimly. See also CLOUD CLASSIFICATION.

    ambient temperature. . The temperature of the air or other medium surrounding an object. See also FREE-AIR TEMPERATURE.

    ambiguity. , n. In navigation, the condition obtained when a given set of observations defines more than one point, direction, line of position, or surface of position.

    ambiguous. , adj. Having two or more possible meanings or values. American Ephemeris and Nautical Almanac. . See ASTRONOMICAL ALMANAC.

    American Ephemeris and Nautical Almanac. . See ASTRONOMICAL ALMANAC.

    American Practical Navigator, The. . A navigational text and reference book published by the National Imagery and Mapping Agency (NIMA); originally by Nathaniel Bowditch (1773-1838). Popularly called BOWDITCH.

    Amidships (or midships): In the middle portion of ship, along the line of the keel. adv. At, near, or toward the middle of a ship.

    ampere per meter. . The derived unit of magnetic field strength in the International System of Units.

    ampere. , n. The base unit of electric current in the International System of Units; it is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length.

    amphidrome. A point in the sea where the tide has no amplitude. Co-tidal lines radiate from an amphidromic point and co-range lines encircle it.

    amphidromic point. . Point on a tidal chart where the cotidal lines meet.

    amphidromic region. . An area surrounding a no-tide point from which the radiating cotidal lines progress through all hours of the tidal cycle.

    amplification. , n. 1. An increase in signal magnitude from one point to another, or the process causing this increase. 2. Of a transducer, the scalar ratio of the signal output to the signal input.

    amplifier. , n. A device which enables an input signal to control power from a source independent of the signal and thus be capable of delivering an output which is greater than the input signal.

    amplitude compass. . A compass intended primarily for measuring amplitude. It is graduated from 0° at east and west to 90° at north and south. Seldom used on modern vessels.

    amplitude distortion. . Distortion occurring in an amplifier or other device when the output amplitude is not a linear function of the input amplitude.

    amplitude modulation. . The process of changing the amplitude of a carrier wave in accordance with the variations of a modulating wave. See also MODULATION.

    amplitude. , n. 1. Angular distance of a celestial body north or south of the prime vertical circle; the arc of the horizon or the angle at the zenith between the prime vertical circle and a vertical circle through the celestial body measured north or south from the prime vertical to the vertical circle. The term is customarily used only with reference to bodies whose centers are on the celestial horizon, and is prefixed E orW, as the body is rising or setting, respectively; and suffixed N or S to agree with the declination. The prefix indicates the origin and the suffix the direction of measurement. Amplitude is designated as true, magnetic, compass, or grid as the reference direction is true, magnetic, compass, or grid east or west, respectively. 2. The maximum value of the displacement of a wave, or other periodic phenomenon, from the zero position. 3. One-half the range of a constituent tide. By analogy, it may be applied also to the maximum speed of a constituent current.

    Amver System. . Operated by the U.S. Coast Guard, the Amver System is a maritime mutual-assistance program that aids coordination of search and rescue efforts by maintaining a worldwide computerized DR plot of participating vessels.

    anabatic wind. . Any wind blowing up an incline. A KATABATIC WIND blows down an incline.

    analemma. , n. A graduated scale of the declination of the sun and the equation of time for each day of the year located in the Torrid Zone on the terrestrial globe.

    analog computer. . A computer in which quantities are represented by physical variables. Problem parameters are translated into equivalent mechanical or electrical circuits as an analog for the physical phenomenon being investigated without the use of a machine language. An analog computer measures continuously; a digital computer counts discretely. See DIGITAL.

    analog. , adj. Referring to the processing and/or transfer of information via physical means such as waves, fluids, or mechanical devices.

    Anchor ball: Round black shape hoisted in the forepart of a vessel to show that it is anchored.

    anchor buoy. . A buoy marking the position of an anchor on the bottom, usually painted green for the starboard anchor and red for the port anchor, and secured to the crown of the anchor by a buoy rope. A small buoy secured by a light line to anchor to indicate position of anchor on bottom.

    anchor buoy. Small buoy occasionally used to mark the position of the anchor when on the bottom; usually painted green (starboard) or red (port), and secured to the crown of the anchor by a buoy rope.

    Anchor chain or anchor cable: Chain connecting the ship to the anchor.

    Anchor detail: Group of men who handle ground tackle when the ship is anchoring or getting underway.

    Anchor home: The term for when the anchor is secured for sea. Typically rests just outside the hawse pipe on the outer side of the hull, at the bow of a vessel.

    anchor ice. . Submerged ice attached or anchored to the bottom, irrespective of the nature of its formation.

    anchor ice. Submerged ice attached or anchored to the bottom, irrespective of the nature of its formation.

    Anchor light: White light displayed by a ship at anchor. Two such lights are displayed by a ship over 150 feet (46 m) in length.  A light shown from a vessel or aircraft to indicate its position when riding at anchor. Also called RIDING LIGHT.

    Anchor rode: The anchor line, rope or cable connecting the anchor chain to the vessel. Also Rode.

    Anchor sentinel: A separate weight on a separate line which is loosely attached to the anchor rode so that it can slide down it easily. It is made fast at a distance slightly longer than the draft of the boat. It is used to prevent the anchor rode from becoming fouled on the keel or other underwater structures when the boat is resting at anchor and moving randomly during slack tide. Also called a kellet.

    Anchor watch: The crewmen assigned to take care of the ship while anchored or moored, charged with such duties as making sure that the anchor is holding and the vessel is not drifting. Most marine GPS units have an Anchor Watch alarm capability.

    anchor. , n. A device used to secure a ship to the sea floor. Anchor: An object designed to prevent or slow the drift of a ship, attached to the ship by a line or chain; typically a metal, hook-like or plough-like object designed to grip the bottom under the body of water (but also see sea anchor).

    anchor. , v,t. To use the anchor to secure a ship to the sea floor. If more than one anchor is used the ship is moored.

    Anchor: An object designed to prevent or slow the drift of a ship, attached to the ship by a line or chain; typically a metal, hook-like or plough-like object designed to grip the bottom under the body of water (but also see sea anchor).

    anchorage buoy. . A buoy which marks the limits of an anchorage, not to be confused with a MOORING BUOY.

    anchorage chart. . A nautical chart showing prescribed or recommended anchorages.

    anchorage mark. . A navigation mark which indicates an anchorage area or defines its limits.

    anchorage. , n. An area where vessels may anchor, either because of suitability or designation.

    anchorage. Water area which is suitable and of depth neither too deep nor too shallow, nor in a situation too exposed, for vessels to ride in safety. An area set apart for vessels to anchor, such as:

    Anchorage: A suitable place for a ship to anchor. Area of a port or harbor. An area where vessels may anchor, either because of suitability or designation.

    Anchor's aweigh: Said of an anchor when just clear of the bottom.

    Andrew: Traditional lower-deck slang term for the Royal Navy.

    anemometer. , n. An instrument for measuring the speed of the wind. Some instruments also indicate the direction from which it is blowing. See also VANE, definition l; WIND INDICATOR.

    aneroid barometer. . An instrument which determines atmospheric pressure by the effect of such pressure on a thin-metal cylinder from which the air has been partly exhausted. See also MERCURIAL BAROMETER.

    angel. . A radar echo caused by a physical phenomenon which cannot be seen.

    angle of cut. . The smaller angular difference of two bearings or lines of position.

    angle of cut. The lesser angle between two position lines.

    angle of depression. . The angle in a vertical plane between the horizontal and a descending line. Also called DEPRESSION ANGLE. See ANGLE OF ELEVATION.

    angle of deviation. . The angle through which a ray is bent by refraction.

    angle of elevation. . The angle in a vertical plane between the horizontal and an ascending line, as from an observer to an object. A negative angle of elevation is usually called an ANGLE OF DEPRESSION. Also called ELEVATION ANGLE.

    angle of incidence. . The angle between the line of motion of a ray of radiant energy and the perpendicular to a surface, at the point of impingement. This angle is numerically equal to the ANGLE OF REFLECTION.

    angle of reflection. . The angle between the line of motion of a ray of reflected radiant energy and the perpendicular to a surface, at the point of reflection. This angle is numerically equal to the ANGLE OF INCIDENCE.

    angle of refraction. . The angle between a refracted ray and the perpendicular to the refracting surface.

    angle of roll. . The angle between the transverse axis of a craft and the horizontal. Also called ROLL ANGLE.

    angle of uncertainty. . The horizontal angle of the region of indefinite characteristic near the boundaries of a sector of a sector light. Also called ARC OF UNCERTAINTY.

    angle. , n. The inclination to each other of two intersecting lines, measured by the arc of a circle intercepted between the two lines forming the angle, the center of the circle being the point of intersection. An acute angle is less than 90°; a right angle, 90° an obtuse angle, more than 90° but less than 180°- a straight angle 180°; a reflex angle, more than 180° but less than 360°; a perigon, 360°. Any angle not a multiple of 90 is an oblique angle. If the sum of two angles is 90°, they are complementary angles; if 180°, supplementary angles; if 360°, explementary angles. Two adjacent angles have a common vertex and lie on opposite sides of a common side. A dihedral angle is the angle between two intersecting planes. A spherical angle is the angle between two intersecting great circles.

    angstrom. , n. A unit of length, used especially in expressing the length of light waves, equal to one ten-thousandth of a micron or one hundred millionth of a centimeter.

    angular distance. . 1. The angular difference between two directions, numerically equal to the angle between two lines extending in the given directions. 2. The arc of the great circle joining two points, expressed in angular units. 3. Distance between two points, expressed in angular units of a specified frequency. It is equal to the number of waves between the points multiplied by 2π if expressed in radians, or multiplied by 360° if measured in degrees.

    angular distortion. . Distortion in a map projection because of non-conformity.

    angular momentum. . The quantity obtained by multiplying the moment of inertia of a body by its angular speed.

    angular rate of the earth’s rotation. . Time rate of change of angular displacement of the earth relative to the fixed stars equal to 0.729211 X l0-4 radian per second.

    angular rate. . See ANGULAR SPEED.

    angular resolution. . See BEARING RESOLUTION.

    angular speed. . Change of direction per unit time. Also called ANGULAR RATE. See also LINEAR SPEED.

    angular. , adj. Of or pertaining to an angle or angles.

    anneal. , v., t. To heat to a high temperature and then allow to cool slowly, for the purpose of softening, making less brittle, or removing permanent magnetism. When Flinders bars or quadrantal correctors acquire permanent magnetism which decreases their effectiveness as compass correctors, they are annealed.

    annotation. , n. Any marking on illustrative material for the purpose of clarification such as numbers, letters, symbols, and signs.

    annual aberration. . See under ABERRATION, definition 1.

    annual inequality. . Seasonal variation in water level or tidal current speed, more or less periodic due chiefly to meteorological causes.

    annual parallax. . See HELIOCENTRIC PARALLAX.

    annual. , adj. Of or pertaining to a year; yearly.

    annular eclipse. . An eclipse in which a thin ring of the source of light appears around the obscuring body. Annular solar eclipses occur, but never annular lunar eclipses.

    annular. , adj. Ring-shaped.

    annulus. , n. A ring-shaped band.

    anode. , n. 1. A positive electrode; the plate of a vacuum tube; the electrode of an electron tube through which a principal stream of electrons leaves the inter-electrode space. 2. The positive electrode of an electrochemical device, such as a primary or secondary cell, toward which the negative ions are drawn. See also CATHODE.

    anomalistic month. . The average period of revolution of the moon from perigee to perigee, a period of 27 days, 13 hours, 18 minutes, and 33.2 seconds in 1900. The secular variation does not exceed a few hundredths of a second per century.

    anomalistic period. The interval between two successive passes of a satellite through perigee. Also called PERIGEE-TO-PERIGEE PERIOD RADIAL PERIOD. See also ORBITAL PERIOD.

    anomalistic year. . The period of one revolution of the earth around the sun, from perihelion to perihelion, averaging 365 days, 6 hours, 13 minutes, 53.0 seconds in 1900, and increasing at the rate of 0.26 second per century.

    anomalistic. , adj. Pertaining to the periodic return of the moon to its perigee, or of the earth to its perihelion.

    anomaly. , n. 1. Departure from the strict characteristics of the type, pattern, scheme, etc. 2. An angle used in the mathematical description of the orbit of one body about another. It is the angle between the radius vector of the body and the line of apsides and is measured from pericenter in the direction of motion. When the radius vector is from the center of the primary to the orbiting body, the angle is called true anomaly. When the radius vector is from the center of the primary to a fictitious body moving with a uniform angular velocity in such a way that its period is equal to that of the actual body, the angle is called mean anomaly. When the radius vector is from the center of the elliptical orbit to the point of intersection of the circle defined by the semimajor axis with the line perpendicular to the semimajor axis and passing through the orbiting body, the angle is called eccentric anomaly or eccentric angle. 3. Departure of the local mean value of a meteorological element from the mean value for the latitude. See also MAGNETIC ANOMALY.

    antarctic air. . A type of air whose characteristics are developed in an Antarctic region. Antarctic air appears to be colder at the surface in all seasons, and at all levels in fall and winter, than ARCTIC AIR.

    Antarctic Circle. . The parallel of latitude at about 66° 33'S, marking the northern limit of the south Frigid Zone. This latitude is the complement of the sun’s greatest southerly declination, and marks the approximate northern limit at which the sun becomes circumpolar. The actual limit is extended somewhat by the combined effect of refraction, semidiameter of the sun, parallax, and the height of the observer’s eye above the surface of the earth. A similar circle marking the southern limit of the north Frigid Zone is called ARCTIC or NORTH POLAR CIRCLE. Also called SOUTH POLAR CIRCLE.

    Antarctic Circumpolar Current. . See WEST WIND DRIFT.

    antarctic front. . The semi-permanent, semi-continuous front between the Antarctic air of the Antarctic Continent and the polar air of the southern oceans; generally comparable to the arctic front of the Northern Hemisphere.

    antarctic whiteout. . The obliteration of contrast between surface features in the Antarctic when a covering of snow obscuring all landmarks is accompanied by an overcast sky, resulting in an absence of shadows and an unrelieved expanse of white, the earth and sky blending so that the horizon is not distinguishable. A similar occurrence in the Arctic is called ARCTIC WHITEOUT.

    antarctic. , adj. referring to the Antarctic region.

    Antarctic. , n. The region within the Antarctic Circle, or, loosely, the extreme southern regions of the earth.

    ante meridian (AM). . Before noon, or the period of time between midnight (0000) and noon (1200). The period between noon and midnight is called POST MERIDIAN.

    antenna array. . A combination of antennas with suitable spacing and with all elements excited to make the radiated fields from the individual elements add in the desired direction, i.e., to obtain directional characteristics.

    antenna assembly. . The complete equipment associated with an antenna, including, in addition to the antenna, the base, switches, lead-in wires, revolving mechanism, etc.

    antenna bearing. . The generated bearing of the antenna of a radar set, as delivered to the indicator.

    antenna coupler. . 1. A radio-frequency transformer used to connect an antenna to a transmission line or to connect a transmission line to a radio receiver. 2. A radio-frequency transformer, link circuit, or tuned line used to transfer radio-frequency energy from the final plate-tank circuit of a transmitter to the transmitter to the transmission line feeding the antenna.

    antenna directivity diagram. . See DIRECTIVITY DIAGRAM.

    antenna directivity diagram. . See DIRECTIVITY DIAGRAM.

    antenna effect. . A spurious effect, in a loop antenna, resulting from the capacitance of the loop to ground.

    antenna feed. . The component of an antenna of mirror or lens type that radiates, or receives energy from, the mirror or lens. See also HORN ANTENNA.

    antenna radiation pattern. . See RADIATION PATTERN.

    antenna. , n. A structure or device used to collect or radiate electromagnetic waves.

    anthelion. , n. A rare kind of halo, which appears as a bright spot at the same altitude as the sun and 180° from it in azimuth. See also PARHELION.

    anti-clutter gain control. . See SENSITIVITY TIME CONTROL.

    anti-clutter rain. . See FAST TIME CONSTANT CIRCUIT.

    anti-clutter sea. . See SENSITIVITY TIME CONTROL.

    anticorona. , n. A diffraction phenomenon very similar to but complementary to the corona, appearing at a point directly opposite to the sun or moon from the observer. Also called BROKEN BOW, GLORY.

    anticrepuscular arch. . See ANTITWILIGHT.

    anti-crepuscular rays. . Extensions of crepuscular rays, converging toward a point 180° from the sun.

    anticyclone. , n. An approximately circular portion of the atmosphere, having relatively high atmospheric pressure and winds which blow clockwise around the center in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. An anticyclone is characterized by good weather. Also called HIGH. See also CYCLONE.

    anticyclonic winds. . The winds associated with a high pressure area and constituting part of an anticyclone.

    Antilles Current. . This current originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current. It flows along the northern side of the Greater Antilles. The Antilles Current eventually joins the Florida Current (north of Grand Bahama Island) to form the Gulf Stream.

    antilogarithm. , n. The number corresponding to a given logarithm. Also called INVERSE LOGARITHM.

    antinode. , n. Either of the two points on an orbit where a line in the orbit plane, perpendicular to the line of nodes, and passing through the focus, intersects the orbit.

    antipodal effects. . See as LONG PATH INTERFERENCE under MULTIPATH ERROR.

    antipodal effects. . See as LONG PATH INTERFERENCE under MULTIPATH ERROR.

    antipode. , n. Anything exactly opposite to something else. Particularly, that point on the earth 180° from a given place.

    Anti-rolling tanks: A pair of fluid-filled, usually water, tanks mounted on opposite sides of a ship below the waterline. Fluid would be pumped between them in an attempt to dampen the amount of roll.

    antisolar point. . The point on the celestial sphere 180° from the sun.

    anti-TR tube. . See TR TUBE.

    antitrades. , n., pl. The prevailing western winds which blow over and in the opposite direction to the trade winds. Also called COUNTERTRADES.

    antitwilight. , n. The pink or purplish zone of illumination bordering the shadow of the earth in the dark part of the sky opposite the sun after sunset or before sunrise. Also called ANTI CREPUSCULAR ARCH.

    anvil cloud. . Heavy cumulus or cumulonimbus having an anvil-like upper part.

    apastron. , n. The point of the orbit of one member of a double star system at which the stars are farthest apart. That point at which they are nearest together is called PERIASTRON.

    aperiodic compass. . Literally a compass without a period, or a compass that, after being deflected, returns by one direct movement to its proper reading without oscillation. Also called DEADBEAT COMPASS.

    aperiodic. , adj. Without a period; of irregular occurrence.

    aperture antenna. . An antenna in which the beam width is determined by the dimensions of a horn, lens, or reflector.

    aperture ratio. . The ratio of the diameter of the objective to the focal

    aperture. , n. 1. An opening; particularly, the opening in the front of a camera through which light rays pass when a picture is taken. 2. The diameter of the objective of a telescope or other optical instrument, usually expressed in inches, but sometimes as the angle between lines from the principal focus to opposite ends of a diameter of the objective. 3. Of a directional antenna, that portion of nearby plane surface that is perpendicular to the direction of maximum radiation and through which the major part of the radiation passes.

    apex. , n. The highest point of something, as of a cone or triangle, or the maximum latitude (vertex) of a great circle.

    aphelion. , n. That point in the elliptical orbit of a body about the sun farthest from the sun. That point nearest the sun is called PERIHELION.

    aphelion. The point in the orbit of a planet which is farthest from the Sun. See also perihelion.

    aphylactic map projection. . A map projection which is neither conformal nor equal area. Also called ARBITRARY MAP PROJECTION.

    aplanatic lens. . See under ABERRATION, definition 2.

    apoapsis. , n. See APOCENTER.

    apoapsis. , n. See APOCENTER.

    apocenter. , n. In an elliptical orbit, the point in the orbit which is the farthest distance from the focus, where the attracting mass is located. The apocenter is at one end of the major axis of the orbital ellipse. The opposite is PERICENTER, PERIFOCUS, PERIAPSIS. Also called APOAPSIS, APOFOCUS.

    apofocus. , n. See APOCENTER.

    apogean range. . The average semidiurnal range of the tide occurring at the time of apogean tides. It is smaller than the mean range, where the type of tide is either semidiurnal or mixed, and is of no practical significance where the type of tide is diurnal.

    apogean tidal currents. . Tidal currents of decreased speed occurring monthly as the result of the moon being at apogee (farthest from the earth).

    apogean tides. . Tides of decreased range occurring monthly as the result of the moon being at apogee (farthest from the earth).

    apogean tides. . Tides of decreased range occurring monthly as the result of the moon being at apogee (farthest from the earth).

    apogee. , n. That orbital point of a non-circular orbit farthest from the center of attraction. Opposite is PERIGEE. See APOCENTER, PERICENTER.

    apogee. The point in the orbit of the Moon which is farthest from the Earth. See also perigee.

    apparent altitude. . Sextant altitude corrected for inaccuracies in the reading (instrument, index, and personal errors) and inaccuracies in the reference level (principally dip or Coriolis/acceleration), but not for other errors. Apparent altitude is used in obtaining a more accurate refraction correction than would be obtained with an uncorrected sextant altitude. Also called RECTIFIED ALTITUDE. See also OBSERVED ALTITUDE, SEXTANT ALTITUDE.

    apparent horizon. . See VISIBLE HORIZON.

    apparent motion. . Motion relative to a specified or implied reference point which may itself be in motion. The expression usually refers to movement of celestial bodies as observed from the earth. Usually called RELATIVE MOVEMENT when applied to the motion of one vessel relative to that of another. Also called RELATIVE MOTION.

    apparent noon. . Twelve o’clock apparent time, or the instant the apparent sun is over the upper branch of the meridian. Apparent noon may be either local or Greenwich depending upon the reference meridian. High noon is local apparent noon.

    apparent place. . The position on the celestial sphere at which a celestial body would be seen if the effects of refraction, diurnal aberration, and geocentric parallax were removed; the position at which the object would actually be seen from the center of the earth. Also called APPARENT POSITION.

    apparent position. . See APPARENT PLACE.

    apparent precession. . Apparent change in the direction of the axis of rotation of a spinning body, such as a gyroscope, due to rotation of the earth. As a result of gyroscopic inertia or rigidity in space, to an observer on the rotating earth a gyroscope appears to turn or precess.

    apparent secular trend. . The non-periodic tendency of sea level to rise, fall and/or remain stationary with time. Technically, it is frequently defined as the slope of a least-squares line of regression through a relatively long series of yearly mean sea level values. The word apparent is used since it is often not possible to know whether a trend is truly non periodic or merely a segment of a very long oscillation.

    apparent shoreline. . A line drawn on the chart in lieu of the mean high water line or the mean water level line in areas where either may be obscured by marsh, mangrove, cypress, or other marine vegetation. This line represents the intersection of the appropriate datum with the outer limits of vegetation and appears to the navigator as the shoreline.

    apparent

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