When both the Sun and Moon are aligned, the effect of each is added together, producing higher than normal tides called spring tides. Spring tides are tides with the greatest tidal range. They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. The material that follows on tides was cut from Ocean Movements section under Coastal Processes.
Tides are the daily rise and fall of sea level at any given place. The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. High tide left and low tide right at Bay of Fundy on the Gulf of Maine. The Bay of Fundy has the greatest tidal ranges on Earth at To understand the tides it is easiest to start with the effect of the Moon on Earth.
As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. This creates a second high tide bulge on the opposite side of Earth from the Moon. Since so much water is pulled into the two high tides, low tides form between the two high tides Figure below.
As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides every day. The tidal range is the difference between the ocean level at high tide and the ocean at low tide Figure below. They are caused by the gravitational forces exerted on the earth by the moon, and to a lesser extent, the sun. When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide.
When the lowest point, or the trough, reaches a coast, the coast experiences a low tide. Imagine the ocean is shaped like a football pointing at the moon. The point facing the moon is formed because the gravitational pull of the moon is strongest on whichever side of the Earth faces it. This animation shows the tidal force in a view of Earth from the North Pole.
As regions of Earth pass through the bulges, they can experiences a high tide. Tides are really all about gravity, and when we're talking about the daily tides, it's the moon's gravity that's causing them. As Earth rotates, the moon's gravity pulls on different parts of our planet.
The moon's gravity even pulls on the land, but not enough for anyone to tell unless they use special, really precise instruments. When the moon's gravity pulls on the water in the oceans, however, someone's bound to notice. Water has a much easier time moving around, and the water wants to bulge in the direction of the moon.
This is called the tidal force. Because of the tidal force, the water on the side of the moon always wants to bulge out toward the moon.
This bulge is what we call a high tide. As your part of the Earth rotates into this bulge of water, you might experience a high tide. An illustration of the tidal force, viewed from Earth's North Pole. Water bulges toward the moon because of gravitational pull. Note: The moon is not actually this close to Earth. One thing to note, however, is that this is just an explanation of the tidal force—not the actual tides. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary.
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