California Current—A North Pacific Ocean current setting southeastward along the west coast of the United States and Baja California.

Callippic cycle—A period of four Metonic cycles equal to 76 Julian years, or 27,759 days. Devised by Callippus, a Greek astronomer, about 350 B.C., as a suggested improvement on the Metonic cycle for a period in which new and full Moon would recur on the same day of the year. Taking the length of the synodical month as 29.530,588 days, there are 940 lunations in the Callippic cycle, with about 0.25 day remaining.

Canary Current—A North Atlantic Ocean current setting southward off the west coast of Portugal and along the northwest coast of Africa.

celestial sphere—An imaginary sphere of infinite radius concentric with the Earth on which all celestial bodies except the Earth are imagined to be projected.

centibar—The unit of pressure equal to 1 metric ton (1000 kilograms) per meter per second per second. See decibar.

chart datum—The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-water elevation, and its depression below mean sea level is represented by the symbol Z;. Since 1980, chart datum has been implemented to mean lower low water for all marine waters of the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands. See datum and National Tidal Datum Convention of 1980.

Charybdis—Same as Galofaro.

chlorinity (Cl)—The total amount in grams of chlorine, bromine, and iodine contained in one kilogram of seawater, assuming the bromine and iodine to be replaced by chlorine. The number giving the chlorinity in grams per kilogram of a seawater sample is identical with the number giving the mass in grams of atomic weight silver just necessary to precipitate the halogens in 0.328,523,3 kilogram of the seawater sample. S(‰) = 1.806,55 x Cl(‰) where S(‰) is the salinity in parts per thousand. See salinity.

civil day—A mean solar day commencing at midnight.

civil time—Time in which the day begins at midnight as distinguished from the former astronomical time in which the day began at noon.

classification—See type of tide.

Coast and Geodetic Survey—A former name of the National Ocean Service. The organization was known as: Survey of the Coast from its founding in 1807 to 1836, Coast Survey from 1836 to 1878, Coast and Geodetic Survey from 1878 to 1970, and National Ocean Survey from 1970 to 1982. In 1982 it was named National Ocean Service. From 1965 to 1970, the Coast and Geodetic Survey was a component of the Environmental Science Services Administration (ESSA). The National Ocean Survey was a component of the National Oceanic and Atmospheric Administration (NOAA). NOAA became the successor to ESSA in 1970. The National Ocean Service is a component of NOAA, U.S. Department of Commerce.

coast line—The low water datum line for purposes of the Submerged Lands Act (Public Law 31). See shoreline.

coastal boundary—The mean high water line (MHWL) or mean higher high water line (MHHWL) when tidal lines are used as the coastal boundary. Also, lines used as boundaries inland of and measured from (or points thereon) the MHWL or MHHWL. See marine boundary.

coastal zone (legal definition for coastal zone management) —The term coastal zone means the coastal waters (including the lands therein and thereunder) and the adjacent shorelands (including the waters therein and thereunder), strongly influenced by each and in proximity to the shorelines of the several coastal states, and includes islands, transitional and intertidal areas, salt marshes, wetlands, and beaches. The zone extends, in Great Lakes waters, to the international boundary between the United States and Canada and in other areas seaward to the outer limit of the United States territorial sea. The zone extends inland from the shorelines only to the extent necessary to control shorelands, the uses of which have a direct and significant impact on the coastal waters. Excluded from the coastal zone are lands the use of which is by law subject solely to the discretion of or which is held in trust by the Federal Government, its officers, or agents.

coastline—Same as shoreline. See coast line.

cocurrent line—A line on a map or chart passing through places having the same current hour.

comparison of simultaneous observations—A reduction process in which a short series of tide or tidal current observations at any place is compared with simultaneous observations at a control station where tidal or tidal current constants have previously been determined from a long series of observations. The observations are typically high and low tides and monthly means. For tides, it is usually used to adjust constants from a subordinate station to the equivalent value that would be obtained from a l9-year series. See first reduction, standard method, modified-range ratio method, and direct method.

compass direction—Direction as indicated by compass without any correction for compass error. The direction indicated by a compass may differ by a considerable amount from true or magnetic direction.

compass error—The angular difference between a compass direction and the corresponding true direction. The compass error combines the effects of deviation and variation.

component—(1) Same as constituent. (2) That part of a tidal current velocity which, by resolution into orthogonal vectors, is found to flow in a specified direction.

compound tide—A harmonic tidal (or tidal current) constituent with a speed equal to the sum or difference of the speeds of two or more elementary constituents. The presence of compound tides is usually attributed to shallow water conditions.

constants, current—See current constants.

constants, harmonic—See harmonic constants.

constants, tidal —See tidal constants.

constituent day—The time of the rotation of the Earth with respect to a fictitious celestial body representing one of the periodic elements in the tidal forces. It approximates in length the lunar or solar day and corresponds to the period of a diurnal constituent or twice the period of a semidiurnal constituent. The term is not applicable to the long-period constituents.

constituent hour—One twenty-fourth part of a constituent day.

control current station—A current station at which continuous velocity observations have been made over a minimum period of 29 days. Its purpose is to provide data for computing accepted values of the harmonic and nonharmonic constants essential to tidal current predictions and circulatory studies. The data series from this station serves as the control for the reduction of relatively short series from subordinate current stations through the method of comparison of simultaneous observations. See current station and subordinate current station (1).

control station—See primary control tide station, secondary control tide station, and control current station.

corange line—A line passing through places of equal tidal range.

corrected current—A relatively short series of current observations from a subordinate station to which a factor is applied to adjust the current to a more representative value based on a relatively long series from a nearby control station. See current and total current.

cotidal hour—The average interval between the Moon's transit over the meridian of Greenwich and the time of the following high water at any place. This interval may be expressed either in solar or lunar time. When expressed in solar time, it is the same as the Greenwich high water interval. When expressed in lunar time, it is equal to the Greenwich high water interval multiplied by the factor 0.966.

cotidal line —A line on a chart or map passing through places having the same tidal hour.

countercurrent—A current usually setting in a direction opposite to that of a main current. See Equatorial Countercurrent.

crest—The highest point in a propagating wave. See high water and tidal wave.

current—Generally, a horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the Sun, Moon, and Earth and are part of the same general movement of the sea that is manifested in the vertical rise and fall, called tide. Tidal currents are periodic with a net velocity of zero over the particular tidal cycle. See tidal wave. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. Current, however, is also the British equivalent of our nontidal current. See total current.

current constants—Tidal current relations that remain practically constant for any particular locality. Current constants are classified as harmonic and nonharmonic. The harmonic constants consist of the amplitudes and epochs of the harmonic constituents, and the nonharmonic constants include the velocities and intervals derived directly from the current observations.

current curve—A graphic representation of the flow of the current. In the reversing type of tidal current, the curve is referred to rectangular coordinates with time represented by the abscissa and the speed of the current by the ordinate, the flood speeds being considered as positive and the ebb speeds as negative. In general, the current curve for a  reversing tidal current approximates a cosine curve.

current diagram—A graphic table published in the Tidal Current Tables showing the speeds of the flood and ebb currents and the times of slacks and strengths over a considerable stretch of the channel of a tidal waterway, the times being referred to tide or tidal current phases at some reference station.

current difference—Difference between the time of slack water (or minimum current) or strength of current in any locality and the time of the corresponding phase of the tidal current at a reference station for which predictions are given in the Tidal Current Tables.

current direction—Same as set.

current ellipse—A graphic representation of a rotary current in which the velocity of the current at different hours of the tidal cycle is represented by radius vectors and vectoral angles. A line joining the extremities of the radius vectors will form a curve roughly approximating an ellipse. The cycle is completed in one-half tidal day or in a whole tidal day, according to whether the tidal current is of the semidiurnal or the diurnal type. A current of the mixed type will give a curve of two unequal loops each tidal day.

current hour—The mean interval between the transit of the Moon over the meridian of Greenwich and the time of strength of flood, modified by the times of slack water (or minimum current) and strength of ebb. In computing the mean current hour, an average is obtained of the intervals for the following phases: flood strength, slack (or minimum) before flood increased by 3.10 hours (one-fourth of tidal cycle), slack (or minimum) after flood decreased by 3.10 hours, and ebb strength increased or decreased by 6.21 hours (one-half of tidal cycle). Before taking the average, the four phases are made comparable by the addition or rejection of such multiples of 12.42 hours as may be necessary. The current hour is usually expressed in solar time, but if lunar time is desired, the solar hour should be multiplied by the factor 0.966.

current line—A graduated line attached to a current pole formerly used in measuring the velocity of the current. The line was marked in such a manner that the speed of the current, expressed in knots and tenths, was indicated directly by the length of line carried out by the current pole in a specified interval of time. When marked for a 60-second run, the principal divisions for whole knots were spaced at 101.33 feet and the subdivisions for tenths of knots were spaced at 10.13 feet. The current line was also known as a  log line.

current meter—An instrument for measuring the speed and direction or just the speed of a current. The measurements are Eulerian when the meter is fixed or moored at a specific location. Current meters can be mechanical, electric, electromagnetic, acoustic, or any combination thereof.

current pole—A pole used in observing the velocity of the current. The pole formerly used by the Coast and Geodetic Survey was about 3 inches in diameter and 15 feet long, and was weighted at one end to float upright with the top about 1 foot out of water. Shorter poles were used when necessary for shallow water. In use, the pole was attached to the current line but separated from the graduated portion by an ungraded section of approximately 100 feet, known as the stray line. As the pole was carried out from an observing vessel by the current, the amount of line passing from the vessel during a specific time interval indicated the speed of the current. The set was obtained from a relative bearing from the vessel to the pole. The bearing was then related to the ship's compass and converted to true. See pelorus.

current station—The geographic location at which current observations are conducted. Also, the facilities used to make current observations. These may include a buoy, ground tackle, current meters, recording mechanism, and radio transmitter. See control current station and subordinate current station (1).