How do the
Beach season is right around the corner. When I
think of the beach, I think of the ocean, the waves, and the sand. Then
I started wondering more about the ocean tides and how they happen.
The word “tides” is a generic term used to describe
the alternating rise and fall in sea level with respect to the land,
produced by the gravitational attraction of the Moon and the Sun to the
Earth’s surface. Most people associate tides specifically with the
ocean, but the entire planet is affected by this gravitational force. To
a much smaller extent, tides also occur in large lakes, the atmosphere,
and with the Earth.
Lunar tides are created by the gravitational
attraction between the Earth and the Moon. The Moon tries to pull at
anything on the Earth to bring it closer, but the Earth is able to hold
onto everything except the water. Each day, there are two high tides and
two low tides. The ocean is constantly moving from high tide to low
tide, and then back to high tide. There is about 12 hours and 25 minutes
between the two high tides.
The gravitational attraction of the Moon causes the
oceans to bulge out in the direction of the Moon. Another bulge occurs
on the opposite side, since the Earth is also being pulled toward the
Moon. Ocean levels fluctuate daily as the Sun, Moon and Earth interact.
As the Moon travels around the Earth and as they, together, travel
around the Sun, the combined gravitational forces cause the world’s
oceans to rise and fall.
Tides also vary in frequency. In some locations,
tides are diurnal--one high tide and one low tide every day. In other
areas, tides are semidiurnal--two high and two low points. By observing
tidal patterns in an area and keeping track of the movements of the
Earth, Moon and Sun, it is possible to create tide charts, predictions
which list the time and height of various tides. Tide charts are
extremely important for navigation, especially in areas with extreme
There are other types of tides that do not occur
each day, such as spring tides, neap tides, and a Proxigean spring tide.
When the Moon is full or new, the gravitational pull of the Moon and Sun
are combined. At these times, the high tides are extremely high and the
low tides are very low. This is the spring tide, which has nothing to do
with the season. During this type of tide, the Sun, Moon and Earth are
in alignment. The gravitational force of the Sun and Moon contribute to
the tides. Spring tides happen about once every two weeks.
During the Moon’s quarter phases, the Sun and Moon
are at right angles to each other. This causes the bulges to cancel each
other out. When this occurs, the difference between a high tide and a
low tide are much smaller. This is called a neap tide. These tides occur
when the gravitational forces of the Moon and Sun are perpendicular to
one another with respect to the Earth. Neap tides also occur about once
every two weeks.
The last type of tide is called the proxigean
spring tide. It is an unusually high tide. It is rare, and can only
occur when the Moon is both unusually close to the Earth, called the
proxigee, and during the New Moon phase. The proxigean spring tide
occurs, at most, once every one and a half years.