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Introduction: If you have the chance to spend the day at a coastal beach, be careful where you place your towel! The water level will rise or fall over the course of the day. The repeating pattern of rising and falling of ocean water is a tide. Depending on when you arrive at the beach, the water level may be increasing or decreasing. If the tide is coming in, watch out that your towel doesn't wash away!

Daily changes in ocean water levels are caused mainly by differences in the moon's gravitational pull on Earth. However, tidal changes are caused by a combination of forces including the gravitational pull of the moon and sun on Earth as well as the rotation of the Earth. 

The pull of the moon's gravity is strongest on the side of the Earth facing it. The fluid ocean waters are pulled toward the moon, resulting in a bulge of ocean water. But, gravitational force is also pulling on the solid Earth. The moon is pulling less on the side of the Earth facing away from it than the side facing it because it is a greater distance away. This is in part why we see a second bulge of ocean water on the side of Earth facing opposite of the moon. However, inertia, or the tendency for an object to resist change in motion, also works to keep the water moving away from Earth, creating the water bulge. The pull from the sun's gravity on Earth also influences the tides. However, because of the distance of the sun from the Earth, it exerts a weaker pull then the closer moon. The two bulges of ocean water on either side of the Earth are high tides. In the regions between the two high tides, water levels are at the lowest points. These areas experience low tides

The Earth is always revolving around the sun. Plus, the moon is always revolving around the Earth. This means there will be times when the Earth, moon, and sun align. When this happens, the gravitational pull on the oceans is at its strongest. This creates high tides higher than normal and low tides lower than normal. There are known as spring tides. Spring tides occur about twice a month, during the full moon and the new moon.
Alignment of the sun, Earth, and moon during a spring tide.
New moon spring tideFull moon spring tide

What do you think will happen to tide height during a first quarter moon and third quarter moon? Twice a month the moon, Earth, and sun form a right angle relative to Earth's orbit. The gravitational pull from sun and moon no longer aligns. Instead, they work against each other, which results in lower than normal high tides and higher than normal low tides. This is known as a neap tide.

Alignment of the sun, Earth, and moon during a neap tide.
First quarter moon neap tideThird quarter moon neap tide

The difference in water height between high and low tides is the tidal range. The tidal range is greater than average during a spring tide. During a neap tide the tidal range is lower than average. The rise and fall of the ocean level along coastline is a tidal current. A flood tide occurs as the height of the water increases as the tide moves toward land. As the water level decreases as the tide moves away from land, the area experiences an ebb tide.

The moon orbits the Earth once every 24 hours and 50 minutes. Therefore, we may expect each point on Earth to have two high tides and two low tides during this time. These are semidiurnal tides. However, many places have only one high tide and one low tide during this time. These are diurnal tides. The number of high and low tides a location has depends on many factors including lunar position, shapes of coasts, and ocean depth.


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