The Sun, the Moon, and the Tides x 



By Rear Admiral Leo Otis Colbert 



U. S. Coast and Geodetic Survey (Retired) 



Little or no mention is made in early recorded history of the rise 

 and fall of the sea, which we call the tide. This lack of interest on the 

 part of those writers and historians who lived along the shores of the 

 Mediterranean was due to the small tidal changes which occur there. 

 The earliest reference appears in the writings of Herodotus, the Greek 

 historian, who wrote of the voyages of early navigators. In a descrip- 

 tion of the Arabian coast he mentions an arm of the sea in which 

 "every day the tide ebbs and flows." A century and a half later 

 Pytheas of Massilia wrote of the tidal movement and noted that there 

 was a relationship between the tide and the moon. Pytheas had 

 gained his information because he was one of the few who had ven- 

 tured out of the Straits of Gibraltar into the open ocean. He had 

 sailed to the shores of Britain, where the ebb and flow of the sea is 

 much more noticeable. 



In the first century of the Christian Era the tides are ascribed 

 definitely to the action of the sun and moon. In his Natural History, 

 Pliny describes some of the principal phenomena of the tides, but 

 knowledge of how the sun and moon provided the force necessary to 

 influence the tides was not developed until many centuries later. In 

 1687, Sir Isaac Newton stated that the tides were a necessary conse- 

 quence of the law of gravitation. He simplified the problem by sup- 

 posing the sea to cover the whole earth with a layer of water of 

 considerable depth. By mathematical formula he showed that the 

 relative masses and positions of the sun, moon, and earth could produce 

 a regular movement in the overlying water. 



Under the stated circumstance, the mathematicians assure us that 

 the tides would be uniform. At any place, the time of the moon's 

 passage of the local meridian would mark the time of a high water. 

 Six hours later a low water would occur. The greatest range in the 

 tides would occur at the Equator and the least difference between high 

 and low waters would be at the Poles. The character of the tide at 

 any place would depend upon the distance from the Equator — in 



1 Nineteenth James Arthur lecture, given under the auspices of the Smithsonian Institu- 

 tion on AprU 3, 1952. 



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