Back To CourseBasics of Astronomy
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Have you ever helped Mom or Dad water the garden when you were a little kid? Maybe you still do now. You probably had to lift a bucket of water in order to do so. Lifting a big bucket of water certainly takes strength. But can you imagine the forces necessary to lift an entire ocean many feet?
Those forces are not going to be produced by human power by any means. You'd need something even more powerful. The force that changes the levels of the oceans is known as a tidal force, which is technically the difference in the strength of gravity between two points on a body. The deformations of a body stemming from tidal forces are called tides.
If you've ever lived near the ocean, you've certainly experienced ocean tides stemming from tidal forces. But tidal forces are responsible for much more than just the changing levels of the oceans! They can cause mountains to rise and fall, distort galaxies, and pull a person apart as they fall into a black hole.
But for this lesson, we'll just focus on more familiar, Earth-based tides and how the sun, moon, and gravity interact to cause them.
Gravity is a mutual force. Earth attracts the moon, and the moon will attract the Earth. This is something I'm sure you already knew. Similarly, it is Earth's gravity that you have to thank for your falling down after you trip. But did you know that the moon attracts your body as well? Why then, don't you 'fall' down towards the moon when you trip? Well, since the moon is much less massive than Earth and it's so far away, it attracts you with only a tiny force, one that you don't even notice.
However, because the oceans are so much larger than your body, they are definitely affected by the moon. There's so much more to be affected to begin with compared to tiny little you. Anyways, here's how these oceans are affected by the moon's gravitational pull in more detail.
Let's look at the picture above. On the left is Earth and on the right in the moon. I know you know how magnets work, so let's pretend that the moon is like a magnet and the Earth is a little metal ball. What would happen? The ball would be attracted to the magnet. If the ball is closer to the magnet, it will be more strongly attracted to the magnet than it would be if it was farther away.
Keeping that very simple metaphor in mind, you can then understand that the side of the Earth that is nearest the moon is the side where the moon's gravity pulls a bit more strongly. Meaning, it pulls the oceans on the near side of Earth more strongly than on Earth's center because Earth's center is farther away from the moon. Thus, the oceans flow into a bulge of water on the side of the Earth that's facing the moon, just as shown below.
At the same time, you know that the Earth's center is closer to the moon than the far side of Earth. Just like before, this means the moon pulls Earth away from the oceans, and they flow into a bulge on the far side of Earth. These bulges are known as tidal bulges.
Since there are two bulges on opposite sides of Earth, that means normally tides will rise and fall twice in 24 hours. But this is just a general rule of thumb because factors such as latitude, winds, and the shape of the shore can alter this.
While all this is happening, the moon pulls the solid parts of Earth into an elongated shape, not just the oceans. But you know that rock is more rigid than water and thus the oceans will deform much more pronouncedly than the solid mountains and planes of our planet. As a result, the maximum tidal distortion of the solid parts of Earth is only about 20 cm.
Compare this to the oceans. For example, in deeper parts of the ocean, far away from land, tidal bulges are about 3 feet in height. Closer to shore, in the Bay of Fundy, tides can be over 40 feet.
But these tides are not all about the moon. The sun also causes tides. You'd think that because the sun is more massive than the moon, it would have a bigger effect on the Earth than the moon. However, the sun is much farther away from Earth than the moon, and like a magnet far away from a metal ball, the solar tidal acceleration is approximately half that of the lunar tidal acceleration.
What that complex-sounding sentence boils down to is that the tides on Earth that are caused by the sun are less than half as high as the ones that are caused by our moon. However, twice a month something happens. That something is the moon, sun, and Earth aligning at full moon and then at new moon. Thus, the tidal bulges produced individually by the sun and moon have an additive effect.
This causes spring tides, low tides that are very low and high tides that are very high at full and new moon. Spring, in spring tides, doesn't refer to the season but rather to the fact that water wells up rapidly during such tides.
In opposition to spring tides are the neap tides, unusually weak ocean tides at first- or third-quarter moon. These tides are of low amplitude because the sun and moon pull at right angles to each other. Therefore, their tides partially cancel one another out.
Finally, as a last note, as the animation at 05:20 in the video shows, things aren't so simple with the way tidal bulges actually occur. The interactions between the Earth's rotation, friction with ocean beds, and the moon actually causes the tidal bulges to be slightly ahead compared to our prior images. This is covered in more detail in another lesson on tidal friction.
This lesson mainly focused on ocean tides and how the Earth, moon, and sun interact to cause them.
Tides are the deformations of a body stemming from tidal forces. A tidal force is the difference in the strength of gravity between two points on a body.
The moon's gravitational influence on Earth is not uniform. It pulls more strongly on the near side of the Earth than it does on the center than it does on the far side. This means the oceans on the near side of the moon are pulled away from Earth into a bulge, and Earth's center is pulled away from the oceans on the far side of Earth. This is what causes tides.
The sun also causes tides, and its effects on Earth are most notable during spring tide and neap tide. Spring tides are low tides that are very low and high tides that are very high at full and new moon. Neap tides are unusually weak ocean tides at first- or third-quarter moon.
By the time you have finished this lesson you should be able to:
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons