14   Tide

The moon and earth are in a whirling, pirouetting dance around the sun.
Together they tour the sun once every year, at the same time whirling
around each other once every 28 days. The moon also turns around once
every 28 days so that she always shows the same face to her dancing part-
ner, the earth. The prima donna earth doesn’t return the compliment; she
pirouettes once every day. This dance is held together by the force of grav-
ity: every bit of the earth, moon, and sun is pulled towards every other
bit of earth, moon, and sun. The sum of all these forces is almost exactly
what’s required to keep the whirling dance on course. But there are very
slight imbalances between the gravitational forces and the forces required
to maintain the dance. It is these imbalances that give rise to the tides.

The imbalances associated with the whirling of the moon and earth
around each other are about three times as big as the imbalances associated
with the earth’s slower dance around the sun, so the size of the tides varies
with the phase of the moon, as the moon and sun pass in and out of
alignment. At full moon and new moon (when the moon and sun are in
line with each other) the imbalances reinforce each other, and the resulting
big tides are called spring tides. (Spring tides are not “tides that occur at
spring-time;” spring tides happen every two weeks like clockwork.) At
the intervening half moons, the imbalances partly cancel and the tides are
smaller; these smaller tides are called neap tides. Spring tides have roughly
twice the amplitude of neap tides: the spring high tides are twice as high
above mean sea level as neap high tides, the spring low tides are twice as
low as neap low tides, and the tidal currents are twice as big at springs as
at neaps.

Why are there two high tides and two low tides per day? Well, if
the earth were a perfect sphere, a smooth billiard ball covered by oceans,
the tidal effect of the earth-moon whirling would be to deform the wa-
ter slightly towards and away from the moon, making the water slightly
rugby-ball shaped (figure 14.1). Someone living on the equator of this
billiard-ball earth, spinning round once per day within the water cocoon,
would notice the water level going up and down twice per day: up once
as he passed under the nose of the rugby-ball, and up a second time as he
passed under its tail. This cartoon explanation is some way from reality.
In reality, the earth is not smooth, and it is not uniformly covered by water
(as you may have noticed). Two humps of water cannot whoosh round
the earth once per day because the continents get in the way. The true
behaviour of the tides is thus more complicated. In a large body of water
such as the Atlantic Ocean, tidal crests and troughs form but, unable to
whoosh round the earth, they do the next best thing: they whoosh around
the perimeter of the Ocean. In the North Atlantic there are two crests and
two troughs, all circling the Atlantic in an anticlockwise direction once a

Figure 14.1. An ocean covering a billiard-ball earth. We’re looking down on the North pole, and the moon is 60 cm off the page to the right. The earth spins once per day inside a rugby-ball-shaped shell of water. The oceans are stretched towards and away from the moon because the gravitational forces supplied by the moon don’t perfectly match the required centripetal force to keep the earth and moon whirling around their common centre of gravity.
Someone standing on the equator (rotating as indicated by the arrow) will experience two high waters and two low waters per day.