When the Moon, Earth, and Sun fall in a straight
line, which we call syzygy (siz-eh-gee),
we notice the greatest difference between high and low tide water
levels. These spring tides occur twice
each month, during the full and new Moon. If the Moon is at perigee,
the closest it approaches Earth in its orbit, the tides are especially
high and low.
When the Sun and Moon form a right angle, as
when we see a half moon, their pulls fight each other and we notice
a smaller difference between high and low tides. These are called
Factors such as the path the Moon takes around the Earth, our
planet's tilt, even the water's depth, and the ocean floor affect
tides. Therefore, not all coasts experience two high and two
low tides each day.
Semi-diurnal tides occur
twice a day. This means a body of water with semi-diurnal tides,
like the Atlantic Ocean, will have two high tides and two low
tides in one day. Diurnal tides occur
once a day. A body of water with diurnal tides, like the Gulf
has only one high tide and one low tide in a 25-hour period.
Some bodies of water, including parts of the Pacific Basin, have mixed
tides, where a single low tide follows two high tides.
The difference in the height of the water surface between the high
and low tides is the tidal range. Tidal
ranges can be measured in inches, like those in Lake Superior, Michigan,
or in feet or yards. In fact, the Bay of Fundy, a V-shaped Canadian
inlet in Nova Scotia, has the greatest tidal range known--up to
50 feet! In areas with large tidal ranges, boats anchored at high
tide are often left stranded on the dry beach at low tide.
As the sea level rises and falls, it generates a tidal
current that flows horizontally. Tidal currents caused
by the dropping water level (as the tide "goes out") are
called ebb currents. The rising tide generates
flood currents. Tidal currents are especially
strong where the ocean is connected to an estuary
or bay, and boats sometimes have to wait for a current in to enter
or leave a harbor.