Ocean in Motion: Tides - Characteristics
Tides are the periodic rise and fall of the ocean waters. They
are caused by the gravitational pulls of the Moon and (to a lesser
extent) Sun, as well as the rotation of the Earth.
The Sun and Moon pull on the Earth, the water, even you! But gravitational
attraction depends on distance and mass. For example, you have very
little mass and you're very close to the Earth, so the Sun and Moon
can't just yank you off the planet. The Sun is extremely massive,
but it is an average of 93 million miles (150 million km) from Earth,
compared with about 238,866 miles (384,400 km) from here to the
Moon. And since the Moon is nearly 400 times closer to our planet,
its influence on our oceans is twice as strong as the Sun's.
The key to tides is the varying strength of the Moon's gravitational
pull on different parts of the globe. The Moon pulls most on the
water nearest to it, creating a high tide bulge of water. On the
opposite side of the planet, about 7,926 miles (1,2760 km) away,
the Moon's pull is much weaker and the water is left to form another
high tide bulge. Low tides are found halfway between the highs.
The rotating Earth carries us through these regions of high and
Within a small body of liquid, such as a pond or
bowl of soup, there are no tides because the whole body of water
is the same distance from the Moon, feeling an equal gravitational
The timing of tides is determined by the Earth's rotation and the
Moon's orbit around the Earth. As the Earth rotates once about its
axis in 24 hours, the Moon is moving 1/30th of the way around in
its orbit. It takes a given location on Earth about 50 minutes to
"catch up with" the orbiting Moon, so a particular tide
returns in approximately 24 hours and 50 minutes.