Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 111

The Mediterranean is mostly the wrong shape to generate strong
tides. In Homer's day, as now, the typical difference between low and
high tide is about 20 centimetres. On a wave-stirred surface, that is
too small to be easily noticeable, and Homer may not have known of
tides at all, let alone of their connection with the Moon. Charybdis is
an exception. It lies at the site of a narrow, deep channel (the Strait of
Messina) between landmasses and, by a freak of submarine geog-
raphy, the timing of low and high water at either side is offset by sev-
eral hours, so that the water slopes first in one direction along the
Strait, and then in the other. It's not much of a slope in sea level—
about 5 centimetres over 3 kilometres—but this is nevertheless
enough to generate currents of up to 2 metres a second when focused
along this narrow waterway. When those currents begin to reverse as
the slope on the surface of the water see-saws once more, vortices—
whirlpools—are set up. These, particularly when appearing seem-
ingly out of nowhere on a calm summer's day, would have looked
horrifying and mysterious to ancient Greek sailors. 68 The original
Maelström is a similar phenomenon but, with stronger tidal currents
to start with, genuinely more fearsome.
Tides, like wind-formed waves, mainly act to stir the surface and
near-shore waters of the Earth (out in the oceans the tide is small—
usually less than 30 centimetres separates high water from low water).
The effects are usually greatest in estuaries and embayments that fun-
nel and concentrate the tidal forces, but here and there they can also
reach down to several hundred metres—for instance into deep sub-
marine canyons, some of which regularly have their bottoms swept
by tidal currents.
Earth has regular but modest tides, from a small Moon (a little less
than one-eightieth of the mass of the Earth) in a more or less circular
orbit. In the past, it used to have more rapidly repeating, higher tides
from a Moon that was closer (and this can be read, remarkably, by