accepted values—Tidal datums and Greenwich high and low water intervals obtained through primary determination or comparison of simultaneous observations with a control tide station in order to derive the equivalent value that would be obtained with a 19-year series.

acoustic Doppler current profiler (ADCP)—A current measuring instrument employing the transmission of high frequency acoustic signals in the water. The current is determined by a Doppler shift in the backscatter echo from plankton, suspended sediment, and bubbles, all assumed to be moving with the mean speed of the water. Time gating circuitry is employed which uses differences in acoustic travel time to divide the water column into range intervals, called bins. The bin determinations allow development of a profile of current speed and direction over most of the water column. The ADCP can be deployed from a moving vessel, tow, buoy, or bottom platform. In the latter configuration, it is nonobtrusive in the water column and thus can be deployed in shipping channels.

ADR gauge—Analog to Digital Recording water level gauge. A float or pressure-actuated water level gauge that records heights at regular time intervals in digital format.

age of diurnal inequality—The time interval between the maximum semimonthly north or south clination of the Moon and the maximum effect of declination upon range of tide or speed of the tidal current. The age may be computed from the harmonic constants by the formula: age of diurnal inequality = 0.911(51° – O1°) hours.

age of Moon—The time elapsed since the preceding new Moon

age of parallax inequality—The time interval between perigee of the Moon and the maximum effect of parallax upon range of tide or speed of the tidal current. This age may be computed from the harmonic constants by the formula:

age of parallax inequality = 1.837(M2° – N2°) hours.

age of phase inequality—The time interval between new or full Moon and the maximum effect of these phases upon range of tide or speed of the tidal current. This age may be computed from the harmonic constants by the formula: age of phase inequality = 0.984(S2° – M2°) hours.

age of tide—Same as age of phase inequality.

agger—Same as double tide.

Agulhas Current—An Indian Ocean current setting southwestward along the southeast coast of Africa.

air acoustic ranging sensor—A pulsed, acoustic ranging device using the air column in a tube as the acoustic sound path. The fundamental measurement is the time it takes for the acoustic signal to travel from a transmitter to the water surface and then back to the receiver. The distance from a reference point to the water surface is derived from the travel time. A calibration point is set at a fixed distance from the acoustic transducer and is used to correct the measured distance using the calibrated sound velocity in the tube. Air temperature sensors are located in the protective well for the purpose of verifying uniformity of temperature for measurements taken by the air acoustic ranging sensor.

Alaska Current—A North Pacific Ocean current setting counterclockwise along the coasts of Canada and Alaska in the Gulf of Alaska.

Alaskan Stream—A North Pacific Ocean current setting westward along the south side of the Aleutian Islands. It is an extension of the Alaska Current.

amphidromic point—A point of zero amplitude of the observed or a constituent tide.

amphidromic region—An area surrounding an amphidromic point from which the radiating cotidal lines progress through all hours of the tidal cycle.

amplitude (H)—One-half the range of a constituent tide. By analogy, it may be applied also to the maximum speed of a constituent current.

analog—A continuous measurement or a continuous graphic display of data. See ADR gauge and marigram.

analysis, harmonic—See harmonic analysis.

analyzer, harmonic—See harmonic analyzer.

angular velocity of the Earth's rotation (S)—Time rate of change of angular displacement relative to the fixed stars. It is equal to 0.729,211 x 10-4 radian/second.

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

anomalistic—Pertaining to the periodic return of the Moon to its perigee or the Earth to its perihelion. The anomalistic month is the average period of the revolution of the Moon around the Earth with respect to lunar perigee, and is approximately 27.554,550 days in length. The anomalistic year is the average period of the revolution of the Earth around the Sun with respect to perihelion, and is approximately 365.259,6 days in length.

anomaly—As applied to astronomy, the anomaly is the angle made at any time by the radius vector of a planet or moon with its line of apsides, the angle being reckoned from perihelion or perigee in the direction of the body's motion. It is called the true anomaly when referred to the actual position of the body, and mean anomaly when referred to a fictitious body moving with a uniform angular velocity equal to the average velocity of the real body and passing perihelion or perigee at the same time.

Antarctic Circumpolar Current—The largest permanent current in the world, setting eastward around the Antarctic Continent south of Cape Horn, Cape of Good Hope, Tasmania, and New Zealand. Through Drake Passage, it transports approximately 200 x 106 m3/s. Same as West Wind Drift.

anticyclonic ring—A meander breaking off from the main oceanic current and spinning in a clockwise direction n the northern hemisphere (counter-clockwise in southern).

Antilles Current—A North Atlantic Ocean current setting northwestward along the northeast coasts of the Bahama Islands.

aphelion—The point in the orbit of the Earth (or other planet, etc.) farthest from the Sun.

apogean tides or tidal currents—Tides of decreased range or currents of decreased speed occurring monthly as the result of the Moon being in apogee. The apogean range (An) of the tide is the average range occurring at the time of apogean tides and is most conveniently computed from the harmonic constants. 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 predominantly diurnal.

apogee—The point in the orbit of the Moon or a man-made satellite farthest from the Earth. The point in the orbit of a satellite farthest from its companion body.

apparent secular trend—The nonperiodic tendency of sea level to rise, fall, or remain stationary with time. Technically, it is frequently defined as the slope of a leastsquares 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 nonperiodic or merely a segment of a very long oscillation (relative to the length of the series).

apparent time—Time based upon the true position of the Sun as distinguished from mean time, which is measured by a fictitious Sun moving at a uniform rate. Apparent time is that shown by the sundial, and its noon is the time when the Sun crosses the meridian. The difference between apparent time and mean time is known as the equation of time. Although quite common many years ago, apparent time is seldom used now.

apsides—The points in the orbit of a planet or moon which are the nearest and farthest from the center of attraction. In the Earth's orbit these are called perihelion and aphelion, and in the Moon's orbit, perigee and apogee. The line passing through the apsides of an orbit is called the line of apsides.

argument—See equilibrium argument.

astres fictifs—Fictitious celestial bodies which are assumed to move in the celestial equator at uniform rates corresponding to the speeds of the several harmonic constituents of the tide producing force. Each astre fictif crosses the meridian at a time corresponding to the maximum of the constituent that it represents. astronomical day—See astronomical time.

astronomical tide—Same as tide.

astronomical time—Time formerly used in astronomical calculations in which the day began at noon rather than midnight. The astronomical day commenced at noon of the civil day of the same date. The hours of the day were numbered consecutively from zero (noon) to 23 (11 a.m. of the following morning). Up to the close of the year 1924, astronomical time was in general use in nautical almanacs. Beginning with the year 1925, the American Ephemeris and Nautical Almanac and similar publications of other countries abandoned the old astronomical time and adopted Greenwich civil (mean) time for the data given in their tables.

augmenting factor—A factor, used in connection with the harmonic analysis of tides or tidal currents by stencils, to allow for the fact that the tabulated hourly heights or speeds used in the summation for any constituent, other than S, do not in general occur on the exact constituent hours to which they are assigned, but may differ from the same  by as much as a half hour.

automatic tide (water level) gauge—An instrument that automatically registers the rise and fall of the water level. In some instruments, the registration is accomplished by recording the heights at regular time intervals in digital format; in others, by a continuous graph of height against time.