r^i THE / 



Lhoreooraphv 



OFTHtTIDtS 7 



By Carlo B. Burgess 

 jAris 



istotle once flung 

 himself into the ocean in a 

 desperate attempt to master 

 the mystery of its ebb and 

 flow. In my own quest to 

 understand the tides, I merely 

 held hands with an enlight- 

 ened co-worker as we spun 

 each other around the room 

 in merry-go-round fashion — 

 I, the Earth; she, the moon. 



Of course, I had a lot 

 more scientific information at 

 the outset than did the 

 heralded Greek philosopher, 

 who thought the seas were 

 produced by the Earth 

 sweating. I already knew that 

 the moon, the sun, gravity and 

 centrifugal force were key 

 players in the daily rise and fall of the 

 sea that we call tides. I just needed to 

 sharpen the image in my mind with a 

 hands-on dance of the orbs. 



When we speak of the tide, we 

 often describe it as "going out" or 

 "coming in." From the fairly flat 

 vantage point of a beach, it would seem 

 so. Two times daily in North Carolina, 

 the edge of the ocean tags the upper 

 beach. And twice again it shrinks 

 toward the horizon. 



But take a global look at this 

 phenomenon, and you realize that the 

 surface of the seas actually lifts and falls 

 in response to the gravitational pull of 

 the moon and sun combined with the 

 Earth's own movement. 



The moon's gravitational pull 

 exerts the strongest influence on the 

 tides. Though much smaller than the 

 sun, the moon is closer to our planet. 

 Imagine that the Earth's surface 



Adapted from Marine Biology 



On the open sea, the changing tides 



are barely discernible. 

 But where the edges lay at shorelines, 

 the change in water level — 

 or tidal range — is marked. 

 In North Carolina, the variance 

 is only a few feet; 

 but along the funnel-shaped 

 Bay of Fundy in Canada, 

 the tide may vary as much as 

 50 feet from low to high. 



was enveloped completely by water. At 

 any given time, the water would "bulge" 

 at opposite sides of the Earth. One tidal 

 bulge would appear on the side closest 

 the orbiting moon, which draws the 

 surface water toward it as it passes. 

 Another bulge of water would 



appear on the opposite side 

 of the Earth as a result of 

 centrifugal force. To be 

 precise, the moon doesn't 

 circle the Earth. The two 

 bodies are both orbiting a 

 central point of mass as would 

 a merry-go-round. So at the 

 same time gravity pulls water 

 toward the moon, the outward- 

 moving centrifugal force 

 pushes a bulge of water away 

 from the Earth on the opposite 

 side. In other words, the water 

 is flung outward, producing a 

 mirror high tide. 



Meanwhile, the Earth is 

 spinning on its own axis, 

 completing one rotation in 24 

 hours. This means that each 

 point on Earth rotates through 

 a tidal bulge twice a day. 

 Ideally, each spot on Earth would 

 experience two high tides and two low 

 tides daily. In reality, of course, the 

 continents divide that hypothetical 

 envelope of water into many oceans, each 

 with coastlines and bottoms of various 

 shapes and depths. So tides behave 

 differently worldwide. Generally, the 

 East Coast of North America, Europe 

 and Africa all experience two of each 

 every day. 



If the Earth and moon were always 

 in fixed locations, high tide and low tide 

 would recur every 12 hours. But the 

 moon is actually moving slightly faster 

 than the Earth. Therefore, a complete 

 tidal cycle requires one "lunar day," or 24 

 hours and 50 minutes. That means that 

 successive low and high tides will be 

 separated by about 12 hours and 25 

 minutes. If the tide rises at 6 p.m. at 

 Atlantic Beach, for instance, it will be 

 high again around 6:25 the next morning. 



18 MARCH /APRIL 1995 



