I understood the literal meaning of the red-underlined sentences, but frankly speaking, I don't know the processes. I mean I want to know how Earth's rotation makes a cycle of two high and two low tides in the above context and why, of course how, the Moon experiences only slight tidal forces.

And in the second sentence that I underlined in red, the phrase 'as its distance from Earth varies throughout each month' means that the Moon has slowly gotten away from the Earth, I think. Am I right? Honestly, I can not understand the process of the second sentence entirely.

Thank you.

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    $\begingroup$ Please, cite the book. $\endgroup$ Commented Nov 8, 2016 at 1:56

3 Answers 3


I think it's a somewhat poorly written page and Xakarus Alldredge's answer is very good. I thought I'd add some pictures.

The Tidal force the Earth exerts on the Moon is much stronger than the one the Moon exerts on the Earth and it's technically not a force either, it's a secondary effect from the force of gravity. But the tidal "force" on the moon is strong enough that the Moon is tidally locked and the same side of the Moon always faces the Earth. Any object in a very strong tidal force tends to become tidally locked over time.

Being tidally locked, the moon has a permanent tidal bulge and that bulge doesn't move very much but it does shift side to side as the moon wobbles slightly and it does grow larger and smaller as the Moon moves closer to and further from the earth.

Planets that rotate like the Earth have an equatorial bulge. The Moon, which rotates very slowly, only enough to keep the same size facing the Earth has a bit of an egg shape or tidal bulge, not rounder in the middle like planets with fast rotation, but stretched at 2 points, the part facing the earth and the part facing away from the earth both stretch outwards.

This drawing is exaggerated but it gives an idea of how the Earth's gravity tidally affects the moon.

enter image description here


This diagram is also exaggerated, but, as Xakarus Allderdge points out, when the Moon is closer to the Earth at perigee, the tidal force is stronger and the Moon's tidal bulge grows bigger and at apogee, it's smaller. The Moon's sidereal orbital period, going from from apogee to perigee back to apogee takes 27.3 days so the rise and fall and slight shift side to side of the lunar bulge happens slowly, much slower than the tides on the Earth which circle the Earth about daily. The Earth also has a land bulge usually called an earth tide, land tide or body tide, caused by the moon, but it's quite a bit smaller than the ocean tides and because there's nothing to measure it against, it can only be observed by very precise satellite measurement.

enter image description here

Here's a solid article on the Moon's bulge, Sometimes called a lunar body tide. It's been measured by satellite.


As stated in the quoted book excerpt, the tidal forces that arise from the Moon's gravity create a bulge in Earth's oceans on the side of the planet facing the Moon and on the side facing opposite the Moon.

Now imagine that you are on a shoreline of some arbitrary continent. As the Earth spins, the shoreline you are on is carried into the oceanic bulge that faces the Moon. This is your first high tide of the day and is the strongest, since this bulge is closest to the Moon and therefore larger. As the Earth continues to spin, you are carried out of this bulge and the water level recedes, reaching your first low tide when the Earth has rotated a further 90 degrees and your shoreline is perpendicular to the tidal bulges. When the Earth rotates another 90 degrees, your shoreline will enter the oceanic bulge opposite the Moon and you will experience your second high tide of the day. Then, just as before, the Earth's rotation will carry you out of the bulge and to a location perpendicular to the tidal bulges, and you will experience your second low tide of the day.

It is true that the Moon is slowly receding from the Earth at a rate of about 3 centimeters per year. However, that is not what the second underlined sentence was referring to. It is referring to the slight tidal stretching the Moon experiences during its orbit. The Moon's orbit is not perfectly circular; as with most orbits, it has a slight eccentricity. This causes the Moon to vary in its distance from Earth between 362,600 and 405,400 kilometers over the course of its ~28 day orbit. At its closest approach to Earth (perigee), it feels a stronger pull from Earth's gravity and its crust stretches slightly. Then once it reaches its farthest point away from Earth (apogee) the tidal forces are diminished due to the distance and its crust relaxes, which can lead to moon-quakes.

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    $\begingroup$ I might add a bit more about the Moon's" slight tidal forces" -- I think what the book really means is "small changes in..." . Because the Moon is in tidal lock, the tidal forces vary a bit as you describe but are always applied to the same regions of the Moon. By comparison the Earth, spinning madly, sees the tidal forces change from +max to -max at each location on the surface. $\endgroup$ Commented Nov 8, 2016 at 12:45

The text in the book leaves something to be desired. It's not true that the moon "experiences only slight tidal forces" as written. The tidal forces on the Moon due to the Earth are much greater than those on the Earth due to the moon because the Earth is much more massive than the moon.

However, the existing tidal lock means that those tidal forces are nearly constant in time and so less interesting than the steadily changing effect experienced by most regions of the Earth.

A better statement would be

The moon experiences only slight variations in tidal forces


The moon experiences only slight periodic distortion due to tidal forces

Both of which are comprehensively explain in the answers by Xakarus Alldredge and userLTK.


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