NEW YORK SHELL CLUB NOTES No. 253 June-July-Aug. 1979 Page 7 
WHAT THE TIDE TABLES DON'T TELL YOU 
Robert J. L. Wagner 
Today, tides are printed in handy booklets. These give one fairly 
ace knowledge of tidal predictions along the shorelines of the 
But tides are affected by many factors besides the simple, widely 
known influence of the moon and sun. A change in barometric pressure 
of one inch can permit the sea to rise one foot. There are also wind 
tides. Maybe the tables for an area say it's supposed to be low 
tide, but Mother Nature can't read tables and her winds can roll up 
enough water so that a puzzled observer thinks somebody goofed. 
So tidal knowledge isn't linked to the tables alone. It consists of 
understanding the tides, what generally controls them and how. and why 
they vary. Ever hear of the "Harris theory?" 
Years ago Dr. Harris, a scientist with the U.S.C. & G.S. (United 
States Coast and Geodetic Survey --Ed.), explored masses of data 
and came up with the Harris "standing wave" theory of tidal flow. 
He said that tides are formed, not on a global basis, but by moon 
oscillation of vast water masses in ocean basins, and that such 
'regional' tides can flow almost independently of each other. For 
example, the tide in the N.E. portion of the Gulf of Mexico is a 
"tropic tide." There is only one high and one low tide each day. 
Yet on Florida's opposite coast and along almost the entire Atlantic 
Seaboard, tides are semidiurnal, or two highs and two lows each day. 
Throughout the West Indies, tides are "mixed." When the moon is 
near the equator, tides are semidiurnal (2 highs and lows daily). 
At the time of maximum declination, tides are "tropic" with a single 
high and low daily. 
Ranges also vary considerably. There is almost no tidal flow in the 
eastern half of Florida Bay. But on the east coast of U.S., each 
major indentation in the coast line has a higher range than parts of 
the coast extending further out to sea. Tides range about 3 feet. 
along lower Florida, 8 feet in the area where Georgia recedes in- 
ward, 5 feet where the New York - New Jersey coast inches inward, 
and gain fantastic ranges above New England, where a basin is formed 
ending in the Bay of Fundy where tidal ranges approach 50 feet. 
Because the normal tides are caused by the moon and because the moon 
rises about 50 minutes later each day, each high tide, every day, is 
not quite one hour later than the high tide the day before. That's 
a normal semidiurnal tide. Yet in Tahiti the two high tides occur 
exactly at noon and midnight; the two low tides exactly at dusk and 
dawn. There's no 50 minute delay, and there never has been. 
There is another complication in the tides. When the sun and moon 
are in line (as times of both new and full moon), tidal ranges are 
greater. These are called "spring" tides. It has nothing to do with 
the season of the year; they occur at all times of the year -- when- 
ever there is a new moon or full moon. 
When the sun and the moon are not in line, 
These are called "neap" tides. Ranges are a 
moon is closest to the earth (is perigee). 
I don't mean to knock tide tables. They are potentially invaluable 
tidal ranges are smaller. 
lso greater when the 






