N—Rate of change (as of January 1, 1900) in mean longitude of the Moon's node. N = – 0.002,206,41° per solar hour.

National Geodetic Vertical Datum of 1929 [NGVD 1929]—A fixed reference adopted as a standard geodetic datum for elevations determined by leveling. The datum was derived for surveys from a general adjustment of the first-order leveling nets of both the United States and Canada. In the adjustment, mean sea level was held fixed as observed at 21 tide stations in the United States and 5 in Canada. The year indicates the time of the general adjustment. A synonym for Sea-level Datum of 1929. The geodetic datum is fixed and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because the geodetic datum represents a best fit over a broad area, the relationship between the geodetic datum and local mean sea level is not consistent from one location to another in either time or space. For this reason, the National Geodetic Vertical Datum should not be confused with mean sea level. See North American Vertical Datum of 1988 (NAVD 1988).

National Spatial Reference System (NSRS)—A consistent national coordinate system that defines latitude, longitude, height, scale, gravity, and orientation throughout the nation, and how these values change with time. The NSRS is developed and maintained by the National Geodetic Survey using advanced geodetic, photogrammetric, and remote sensing techniques.

National Tidal Datum Convention of 1980—Effective November 28, 1980, the Convention: (1) establishes one uniform, continuous tidal datum system for all marine waters of the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands, for the first time in history;

(2) provides a tidal datum system independent of computations based on type of tide;

(3) lowers chart datum from mean low water to mean lower low water along the Atlantic coast of the United States;

(4) updates the National Tidal Datum Epoch from 1941 through 1959, to 1960 through 1978;

(5) changes the name Gulf Coast Low Water Datum to mean lower low water;

(6) introduces the tidal datum of mean higher high water in areas of predominantly diurnal tides; and

(7) lowers mean high water in areas of predominantly diurnal tides. See chart  datum.

National Tidal Datum Epoch—The specific l9-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present National Tidal Datum Epoch is 1960 through 1978. It is reviewed annually for possible revision and must be actively considered for revision every 25 years.

National Water Level Observation Network (NWLON)—The network of tide and water level stations operated by the National Ocean Service along the marine and Great Lakes coasts and islands of the United States. The NWLON is composed of the primary and secondary control tide stations of the National Ocean Service. This Network provides the basic tidal datums for coastal and marine boundaries and for chart datum of the United States. Tide observations at a secondary control tide station or tertiary tide station are reduced to equivalent l9-year tidal datums through comparison of simultaneous observations with a primary control tide station. In addition to hydrography, nautical charting, and delineation of coastal and marine boundaries, the Network is used for coastal processes and tectonic studies, tsunami and storm surge warnings, and climate monitoring.

The National Water Level Observation Network also includes stations operated throughout the Great Lakes Basin. The network supports regulation, navigation and charting, river and harbor improvement, power generation, various scientific activities, and the adjustment for vertical movement of the Earth's crust in the Great Lakes Basin.

neap range—See neap tides.

neap tides or tidal currents—Tides of decreased range or tidal currents of decreased speed occurring semimonthly as the result of the Moon being in quadrature. The neap range (Np) of the tide is the average range occurring at the time of neap 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. The average height of the high waters of the neap tide is called neap high water or high water neaps (MHWN) and the average height of the corresponding low waters is called neap low water or low water neaps (MLWN).

Next Generation Water Level Measurement System (NGWLMS)—A fully integrated system encompassing new technology sensors and recording equipment, multiple data transmission options, and an integrated data processing, analysis, and dissemination subsystem.

nodal line—A line in an oscillating body of water along which there is a minimum or zero rise and fall of the tide.

nodal point—The zero tide point in an amphidromic region.

node—See lunar nodes.

node cycle—Period of approximately 18.61 Julian years required for the regression of the Moon's nodes to complete a circuit of 360° of longitude. It is accompanied by a corresponding cycle of changing inclination of the Moon's orbit relative to the plane of the Earth's Equator, with resulting inequalities in the rise and fall of the tide and speed of the tidal current.

node factor (f)—A factor depending upon the longitude of the Moon's node which, when applied to the mean coefficient of a tidal constituent, will adapt the same to a particular year for which predictions are to be made.

nodical month—Average period of the revolution of the Moon around the Earth with respect to the Moon's ascending node. It is approximately 27.212,220 days in length.

nonharmonic constants—Tidal constants such as lunitidal intervals, ranges, and inequalities which may be derived directly from high and low water observations without regard to the harmonic constituents of the tide. Also applicable to tidal currents.

nontidal current—See current.

normal tide—A nontechnical term synonymous with tide; i.e., the rise and fall of the ocean due to the gravitational interactions of the Sun, Moon, and Earth alone. Use of this term is discouraged.

North American Vertical Datum of 1988 [NAVD 1988]—A fixed reference for elevations determined by geodetic leveling. The datum was derived from a general adjustment of the first-order terrestrial leveling nets of the United States, Canada, and Mexico. In the adjustment, only the height of the primary tidal bench mark, referenced to the International Great Lakes Datum of 1985 (IGLD 1985) local mean sea level height value, at Father Point, Rimouski, Quebec, Canada was held fixed, thus providing minimum constraint. NAVD 1988 and IGLD 1985 are identical. However, NAVD 1988 bench mark values are given in Helmert orthometric height units while IGLD 1985 values are in dynamic heights. See International Great Lakes Datum of 1985, National Geodetic Vertical Datum of 1929, and geopotential difference. North Atlantic Current—A North Atlantic Ocean current setting northeastward from southeast of the Grand Banks at about latitude 40° north, longitude 50° west, to the British Isles. A segment of the Gulf Stream System, the North Atlantic Current continues the flow of the Gulf Stream to the Norwegian and Canary Currents.

North Cape Current—An Arctic Ocean current setting eastward off the north coast of Scandinavia in the Barrents Sea.

North Equatorial Current—A current setting westward in the North Atlantic and North Pacific Oceans and in the Indian Ocean from about October to July. It occurs immediately north of the Equatorial Counter Current.

North Pacific Current—A North Pacific Ocean current setting eastward from about 160° east to somewhat beyond about 150° west. It continues the flow of the Kuroshio Extension, sending branches to the south.

Speed = 2T – 3s + 4h – p = 28.512,583,1° per solar hour.