Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 110

continents, but in some places they are absent (the Mediterranean is
almost entirely tideless, for instance) while elsewhere they are simply
enormous: in the Bay of Fundy, Canada, the difference between high
and low water is a staggering 16 metres, and not far behind is the Sev-
ern Estuary in England with a tidal range of up to 14 metres (yet go
round the UK coastline to, say, Swanage and the tidal range goes down
to a couple of metres).
Where tides exist, though, their mechanism is so predictable that a
connection with the phases of the Moon has been made since the
days of the ancients (who were good practical astronomers). In every-
day understanding, this is often simplified to say that high tides rep-
resent the effect of the pull of the Moon's gravity upon the Earth's
waters. This is only partly true, for there is simultaneously a high tide
too on the side of the Earth opposite to the Moon, as well as on the side
facing it. How so?
This reflects the delicately balanced Earth-Moon system. The Earth
and Moon are held together by gravity, true, but if that was all there
was too it, then the Moon would very soon come crashing down on to
the Earth. The Moon and the Earth are kept apart by another force,
exactly matching the gravitational pull—and that is the centrifugal
force between these two bodies as they whirl rapidly around each
other; without gravity, this force would make the Earth and Moon
instantly fly apart.
It is the difference between the two forces that raises the tides, and the
Earth is in effect rotating beneath an envelope of water that is shaped
into two gigantic bulges. From the point of a human observer the planet
appears stationary, so it appears that a great, slow-moving wave of water
passes by twice a day, as the tide goes in and out. The tides can be
amplified or diminished at any place on Earth depending mostly on the
shape of the sea floor, which can either act to resonate with the tides and
hence make them larger (as in the Bay of Fundy) or dissipate them.