The Ocean 



Fig. 3. Steinhauer star projection to show the distribution of oceans and continents. 



three major oceans (with and without adjacent seas) as well as for the marginal and 

 mediterranean seas (Kossinna, 1921; Landolt-Bornstein, 1952, article by Dietrich, 

 p. 460). 



3. Sea -level and its Variations. Chart Datum 



The surface of the ocean which forms the boundary between the ocean and the 

 atmosphere is in a physical sense a free boundary that may assume different forms at 

 different times under the influence of various internal and external forces. This bound- 

 ary surface is called the "sea-level". If the Earth was covered entirely by a homogeneous 

 ocean unaffected by atmospheric phenomena such as winds and atmospheric pressure 

 or the tidal forces of the sun and the moon, then there would be only a single force 

 acting on the sea : gravity. In the equilibrium state there can be no component of the 

 force of gravity along the surface of the sea and the direction of the force of gravity 

 must be perpendicular to the surface. This "ideal" sea-level is thus a geopotential 

 surface or a gravitational equipotential surface. If minor variations in the force of 

 gravity due to the irregular distribution of the mass of the outer crust of the Earth are 

 disregarded, the ideal sea-level will coincide with the surface of a rotational ellipsoid. 

 Even if the sea does not cover the entire Earth, the ideal sea-level will correspond to 

 the surface of this rotational ellipsoid. When the small irregularities in gravitational 

 force due to the irregular mass distribution of the Earth crust are taken into account. 



