low water line— The intersection of the land with the water surface at an elevation of low 

 water. 



lower hi^ water (LHW)-The lower of the two high waters of any tidal day. 



lower low water (LLW)— The lower of the two low waters of any tidal day. 



lower low water datum (LLWD)— An approximation of mean lower low water that has heen 

 adopted as a standard reference for a limited area, and is retained for an indefinite period 

 although it may differ sUghtly from a better determination of mean lower low water 

 from a subsequent series of observations. Used primarily for river and harbor engineering. 

 Columbia River lower low water datum is an example. 



lunar cycle— An ambiguous expression which has been applied to various cycles associated 

 with the Moon's rotation. See lunation, metonic cycle, node cycle, and synodical month. 



lunar day— The time of the rotation of the Earth with respect to the Moon, or the interval 

 between two successive upper transits of the Moon over the meridian of a place. The 

 mean lunar day is approximately 24.84 solar hours in length, or 1.035 times as great as 

 the mean solar day. 



lunar month— Same as synodical month. 



lunar nodes— The points where the plane of the Moon's orbit intersects tlie ecUptic. The 

 point where the Moon crosses in going from south to north is called the ascending node 

 and tlie point where the crossing is from north to south is called the descending node. 

 References are usually made to the ascending node, which for brevity may be called the 

 node. 



lunar tide— That part of the tide on the Earth due solely to the Moon as distinguished from 

 that part due to the Sun. 



lunar time— Time based upon the rotation of the Earth relative to the Moon. See lunar day. 



lunation— Same as synodical month 



L2— Smaller lunar eUiptic semidiurnal constituent. This constituent, with N2, modulates 

 the amplitude and frequency of M2 for the effect of variation in the Moon's orbital 

 speed due to its elliptical orbit. 



mean high water (MHW)— A tidal datum. The arithmetic mean of the high water heights 

 observed over a specific 19-year metonic cycle (the National Tidal Datum Epoch). See 

 epoch (2). For stations with shorter series, simultaneous observational comparisons are 

 made with a primary control tide station in order to derive the equivalent of a 19-year 



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