Tides

Tides

Ocean beaches are sometimes covered by water, and sometimes they are not.  They are covered and uncovered in regular daily cycles by the slow rise and fall of the ocean, called tides.  The upper and lower edges of a beach are determined by the high- tide mark and the low-tide mark.

Centuries ago, people realized that the cycle of tidal movement is linked to the motion of the Moon.  The largest tidal movements, called spring tides, occur when Earth, the Moon, and the Sun are in a line.  At these times, the tides are extra high and extra low.  The smallest tidal movements, called neap tides, occur when the Sun and the Moon are at right angles to each other.  On these days, there is very little difference in depth between high and low tides.  The difference in level between a high tide and a low tide is called the tidal range.

The link between Earth, the Moon, the Sun, and tides is gravity.  Gravity is the force of attraction between two masses.  Tidal movements result mainly from the pull of the Moon’s gravity on the ocean.  The Sun is much farther from Earth than the Moon is.  The Sun has less than half as much influence on the tides as the Moon does, despite the Sun’s much greater size.

During spring tides, the Sun adds its gravitational pull to the Moon's during the new moon and pulls in the opposite direction during the full moon, producing a large tidal range.  During neap tides, the Sun and Moon pull in different directions.

If you look at the water moving up a beach as the tide rises, you may think that the volume of the ocean is increasing.  However, the bulge of water that produces a high tide along one coastline draws water away from the other side of the ocean, just like water sloshing from one end of a bath tub to the other.

This movement of water causes a low tide along the opposite coastline.  As Earth turns on its axis, different locations on Earth’s surface face the Moon.  The result is a sequence of high and low tides that follow each other around the world.  On many of Earth’s shorelines, tides rise and fall about twice a day.