shoreline— The intersection of the land with the water surface. The shoreUne sliown on 

 charts represents the line of contact between the land and a selected water elevation. In 

 areas affected by tidal fluctuations, this line of contact is usually the mean high water 

 line. In confined coastal waters of diminished tidal influence, tiie mean water level line 

 may be used. 



sidereal day— The time of the rotation of the Earth witli respect to the vernal equinox. It 

 equals approximately 0.99727 of a mean solar day. Because of tlie precession of the 

 equinoxes, the sidereal day thus defined is slightly less than the period of rotation with 

 respect to the fixed stars, but the difference is less than the hundredtli part of a second. 



sidereal month— Average period of the revolution of tlie Moon around the Earth with 

 respect to a fixed star, equal to 27.321661 mean solar days. 



sidereal time— This is usually defined by astronomers as the hour angle of the vernal 

 equinox. The sidereal day is the interval between two successive upper transits of the 

 vernal equinox. Note that when applied to the month and year, tlie term "sidereal" has 

 reference to motion with respect to the fixed stars; the term "tropical" is used for 

 motion with respect to the vernal equinox. Because of the precession of the equinox 

 there is a slight difference. 



sidereal year— Average period of the revolution of the Earth around the Sun with respect to 

 a fixed star. Its length is approximately 365.2564 mean solar days. 



slack water— The state of a tidal current when its speed is near zero, especially the moment 

 when a reversing current changes direction and its speed is zero. The term is also applied 

 to tlie entire period of low speed near the time of turning of tlie current when it is too 

 weak to be of any practical importance in navigation. The relation of the time of slack 

 water to the tidal phases varies in different localities. For standing tide waves, slack water 

 occurs near the times of high and low water; for progressive tide waves, slack water 

 occurs midway between high and low water. 



solar day— The period of the rotation of the Earth vfith respect to the Sun. The mean solar 

 day is the time of the rotation with respect to the mean Sun. The solar day commencing 

 at midnight is called a civil or calendar day, but if tlie day is reckoned from noon it is 

 known as an astronomical day because of its former use in astronomical calculations. 



solar tide— (1) The part of the tide that is due to the tide-producing force of tlie Sun. 

 (2) The observed tide in areas where the solar tide is dominant. This condition provides 

 for phase repetition at about the same time each solar day. 



solar time— Time measured by the hour angle of the Sun. It is called apparent time when 

 referred to the actual Sun and mean time when referred to the mean Sun. It is also 

 classified as local, standard, or Greenwich according to whether it is reckoned from the 

 local, standard, or Greenwich meridian. 



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