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Eclipses Home | Eclipses - NASA Solar System Exploration

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An eclipse is an awe-inspiring celestial event that drastically changes the appearance of the two biggest objects we see in our sky: our Sun and Moon. On Earth, people can experience solar eclipses when Earth, the Moon, and the Sun line up.

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The alternating advance and retreat of seawater along a coastline is called a tide. High tide is when water advances to its furthest extent onto the shoreline. Low tide is when it recedes to its furthest extent. Some freshwater rivers and lakes can have tides, too. A high tide that is significantly higher than normal is called a king tide. It often accompanies a new moon and when the moon is closest to the Earth.

The moon's gravitational pull on the Earth and the Earth's rotational force are the two main factors that cause high and low tides. The side of the Earth closest to the Moon experiences the Moon's pull the strongest, and this causes the seas to rise, creating high tides. On the side facing away from the Moon, the rotational force of the Earth is stronger than the Moon's gravitational pull. The rotational force causes water to pile up as the water tries to resist that force, so high tides form on this side, too. Elsewhere on the Earth, the ocean recedes, producing low tides. The gravitational attraction of the Sun also plays a small role in the formation of tides. Tides move around the Earth as bulges in the ocean.

Most shorelines experience two high and two low tides within a twenty-four-hour period, though some areas have just one of each. A coastline's physical features, such as a wide sandy beach or a rocky cove, along with the depth of the water just offshore, affect the height of the tides.

Tides affect marine ecosystems by influencing the kinds of plants and animals that thrive in what is known as the intertidal zone-the area between high and low tide. Because the area is alternately covered and uncovered by the ocean throughout the day, plants and animals must be able to survive both underwater and out in the air and sunlight. They must also be able to withstand crashing waves.

For example, plants and animals that can anchor themselves to the rocks along a shoreline can survive the lashing from waves and the less violent movement of the changing tides. On sandy beaches, survival means being able to swim in shallow water or burrow under the sand as the waves arrive and depart overhead. Sand crabs not only burrow to survive, they actually follow the tides to maintain just the right depth in the wet sand.

Along many shorelines, tides form tide pools. These small pools of water are often left behind among the rocks at low tide. They can include a diverse population of tiny plants and animals that may serve as food for larger species.

The rise in sea levels will affect tides and their impacts, not just on marine ecosystems but on coastal areas that are home to millions of people and animals. The more that is known about how tides work and how they shape our coastlines, scientists say, the better prepared we will be for future changes in the oceans.

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The gravitational pull of the moon and the rotational force of the Earth cause tides to rise and fall across the planet. The species living in coastal areas most affected by changing tides have unique ways of surviving.

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These animations help to explain the science behind how the Moon affects the tides on Earth.

See Tides article where these animations are used.

1. The Moon and Earth exert a gravitational pull on each other. On Earth, the Moon's gravitational pull causes the oceans to bulge out on both the side closest to the Moon and the side farthest from the Moon. These bulges create high tides. The low points are where low tides occur.

2. The Moon's gravitational pull on Earth, combined with other, tangential forces, causes Earth's water to be redistributed, ultimately creating bulges of water on the side closest to the Moon and the side farthest from the Moon.

3. Rising and ebbing tides happen as Earth's landmasses rotate through the tidal bulges created by the Moon's gravitational pull. Our observer sees the tides rise when passing through the bulges, and fall when passing through the low points. Of course, in reality the Earth isn't a smooth ball, so tides are also affected by the presence of continents, the shape of the Earth, the depth of the ocean in different locations, and more. The timing and heights of the tide near you will be affected by those additional elements.

4. Twice a month, when the Earth, Sun, and Moon line up, their gravitational power combines to make exceptionally high tides, called spring tides, as well as very low tides where the water has been displaced. When the Sun is at a right angle to the Moon, moderate tides, called neap tides, result. From our view on Earth, these tides coincide with certain lunar phases since they occur when the Moon reaches specific positions in its orbit.

5. Earth's tidal bulges don't line up exactly with the Moon's position. Because the Moon is orbiting in the same direction as the Earth rotates, it takes extra time for any point on our planet to rotate and reach exactly below the Moon. This means that the high tide bulges are never directly lined up with the Moon, but a little ahead of it.

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Animations to explain the science behind how the Moon affects the tides on Earth

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High tides and low tides are caused by the moon. The moon's gravitational pull generates something called the tidal force. The tidal force causes Earth-and its water-to bulge out on the side closest to the moon and the side farthest from the moon. These bulges of water are high tides.

As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

More About Tides

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. Even though the moon only has about 1/100th the mass of Earth, since it's so close to us, it has enough gravity to move things around. 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.

One thing to note, however, is that this is just an explanation of the tidal force-not the actual tides. In real life, the Earth isn't a global ocean, covered in an even layer of water. There are seven continents, and that land gets in the way. The continents prevent the water from perfectly following the moon's pull. That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic.

That explains the first high tide each day, but what about the second high tide?

The ocean also bulges out on the side of Earth opposite the moon.

Wait, what?

If the moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the moon? It does seem a little weird. It's all because the tidal force is a differential force-meaning that it comes from differences in gravity over Earth's surface. Here's how it works:

On the side of Earth that is directly facing the moon, the moon's gravitational pull is the strongest. The water on that side is pulled strongly in the direction of the moon.

On the side of Earth farthest from the moon, the moon's gravitational pull is at its weakest. At the center of Earth is approximately the average of the moon's gravitational pull on the whole planet.

To get the tidal force-the force that causes the tides-we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.

Tidal force =
moon's gravitational pull in a specific location on Earth -
moon's average gravitational pull over the whole Earth

The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges.

These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day.

Does anything else affect tides?

The Sun causes tides just like the moon does, although they are somewhat smaller. When the earth, moon, and Sun line up-which happens at times of full moon or new moon-the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides. When lunar and solar tides act against each other, the result is unusually small tides, called neap tides. There is a new moon or a full moon about every two weeks, so that's how often we see large spring tides.

Wind and weather patterns also can affect water level. Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much less noticeable.

High-pressure weather systems can push down sea levels, leading to lower tides. Low-pressure systems-brought on by strong storms and hurricanes-can cause tides than are much higher than predicted, so watch out!

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Tides are a complicated dance between gravity and inertia.

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Eclipse - lesson. Science State Board, Class 7.

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Eclipse lesson. Theoretical materials and tasks in Science State Board, Class 7.

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How do you know if what you're reading is good?

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How do you know if what you're reading is good?

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I thought this was pretty good!