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Why does the tide rise on the "back" side of the Earth, even when it is facing the Moon?

caused by the fact that the gravitational force is inversely proportional to the square of the distance

By Mao Jiao LiPublished 2 years ago 5 min read
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High tide

Earth is the only planet in our solar system that has a surface ocean, so vast that about 71% of the planet's surface is covered by oceans. Living on Earth, we have become accustomed to the regular high and low tides of the oceans, which we call "tides" when they occur during the day and "tides" when they occur at night.

Why do tides occur in the Earth's oceans? One well-known reason is the gravitational effect of the Moon. Although the Moon's mass is not very large in the solar system, its distance from the Earth is "close" compared to other planets in the solar system, so the Moon's gravity can have a significant effect on the Earth's surface oceans.

Perhaps we can simply assume that since gravity is inversely proportional to the square of the distance, the side of the Earth "facing" the Moon is subject to a stronger lunar gravity, and the closer to the Moon, the stronger the lunar gravity.

In this case, this side of the ocean will be slightly elevated, so that the formation of high tide, but because the Earth is rotating, this elevated area and the moon's distance will become more and more distant, and the moon's gravitational force is weakened, the ocean's elevation also gradually disappeared, so the formation of the ebb tide, with the continuation of this process, the Earth's oceans also appeared a regular tidal phenomenon.

According to this understanding, the Earth's "back" side of the Moon should not appear at high tide, but people have long observed that the Earth's oceans usually appear twice in a day, the interval between the occurrence of its time is about half a day, and we all know that each time the Earth completes a rotation, almost a day.

This means that the Earth's "back" to the side of the moon will also rise tide, why is this? Let's reveal this seemingly strange tidal phenomenon

Usually, we imagine the Earth-Moon system to be in motion, with the Moon orbiting the Earth under the Earth's gravitational pull, while the Earth remains stationary, but this is not the case.

According to the law of gravity, gravitational forces are reciprocal, and when the Earth "attracts" the Moon, the Moon actually exerts the same amount of gravitational force on the Earth, so if no other factors interfere, the Earth and the Moon will inevitably collide.

The reason why the Earth and the Moon are as stable as they are now is actually that they are both rotating together around the center of mass of the Earth-Moon system, and the gravitational force between them acts as a centripetal force.

In this process, both the Earth and the Moon tend to move away from the center of rotation due to their inertia, so that the Earth and the Moon can always keep a certain distance and not be "attracted" by the centripetal force to collide together. "centrifugal force".

It is important to note that the essence of "centrifugal force" is a manifestation of the inertia of the object, it is not a real force, but a virtual force introduced for the sake of discussion.

"The magnitude of "centrifugal force" can be calculated by the formula "F = mω^2r", where m, ω, and r represent the mass of the object, the angular velocity of rotation, and the distance between the object and the center of rotation, respectively. centrifugal force" is proportional to the distance between the object and the center of rotation, that is, the farther away from the center of rotation, the greater the "centrifugal force", and vice versa.

Since the mass of the Earth is much larger than that of the Moon, the center of mass of the Earth-Moon system is located within the radius of the Earth, about 4700 km from the center of the Earth on the line between the center of mass of the Earth and the Moon, compared to the diameter of the Earth itself, which is about 12,742 km, which means that the side of the Earth "back" to the Moon This means that the "centrifugal force" on the "back" side of the Earth is greater than on the "facing" side of the Moon.

On the other hand, the "backward" side of the Earth is farther away from the Moon, and the Moon's gravitational force is weakened, which means that the "centrifugal force" dominates. Because the "centrifugal force" is opposite to the Moon's gravity, the ocean on this side will rise in the direction away from the Moon, thus forming a high tide.

To summarize, during the motion of the Earth-Moon system, the Earth's oceans are affected by both the Moon's gravity and the "centrifugal force", and on the side of the Earth "facing" the Moon, it is the Moon's gravity that triggers the rising of the oceans, while the reason why the Earth The reason why the "back" side of the Earth is also tidal is because of the "centrifugal force".

It is worth mentioning that as the "boss" of the solar system, the Sun also triggers the tides in the Earth's oceans, but the Sun is so far away from the Earth (compared to the Moon) that the magnitude of the tides is smaller than that of the Moon.

When the Moon is in the same line with the Earth and the Sun, the tides triggered by the Sun and the Moon will have a superposition effect, thus making the tides of the Earth's oceans larger, and the tides at this time are called "big tides", while when the Moon is in the vertical direction of the line between the Earth and the Sun, the tides triggered by the Sun will significantly weaken the Moon's tides. When the Moon is orbiting in the vertical direction of the line between the Earth and the Sun, the tides triggered by the Sun will significantly weaken the tidal magnitude of the Moon's gravity, and the tides at this time are called "small tides".

astronomy
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About the Creator

Mao Jiao Li

When you think, act like a wise man; but when you speak, act like a common man.

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