When the tide is in, a beach may be covered with water. When the tide
goes out, tiny pools of water, often with a few sea creatures in them,
are left among the rocks.
Tide in, tide out
“Get all the cargo aboard. We’ll sail with the tide.”
You’ve probably heard sea captains say this in movies or on television.
But what do they mean? What is the tide? What are the high and low
tides that sailors speak of?
If you visit a place by the sea, you’ll find that the water creeps
slowly up onto the land, then sinks slowly back again. This usually
happens twice a day. This regular, daily movement of the sea is what we
call tide.
In early morning, an ocean beach may be dry, silvery-gray sand. You can
have a picnic on it, and build sand castles. But, about six hours later,
the whole beach will be covered with water. The sea has risen, and we
say the tide is \”high” or \”in.” Then, about six hours after that, the
beach can be seen again. It will be wet, and littered with driftwood and
other things left behind as the water went back down. Now the tide is
\”low,” or \”out.”
This rise and fall of the sea is caused by the turning of the earth and
the tug of gravity from the moon and sun.
The moon’s gravity tugs at the earth. It can’t make the earth move much,
because the earth is too big and heavy. But it does make the water move
a bit. As the earth turns, the pull of the moon’s gravity is strongest
on the part of the earth that’s facing the moon. The pull causes the sea
at that place to bulge upward, making a high tide. Then, as that part of
earth turns away from the moon, the sea sinks back down. It is low tide.
When the tide is high on one side of the earth, it is also high on the
exact opposite side. Suppose it is six o’clock in the morning. The part
of the sea you are looking at faces the moon and the tide is high. The
earth turns completely around once every twenty- four hours. This means
that, twelve hours later, at six o’clock at night, the earth will have
turned halfway around. The part of the sea that you are looking at is
now on the side of the earth farthest from the moon. And the tide will
again be high.
