Hobbits and Tides - They Are Both Tricksy

Tides are tricky and can be hard to understand. Many explanations in textbooks, TV and videos often get it wrong.

Usually the explanation goes that the Moon pulls on the Earth, it pulls the bits that are closer a little bit stronger than the bits that are further away.

On the side of the Earth that is facing the Moon, the net force is that the water is slightly closer to the Moon than the underlying ground, so this lifts the water a little more strongly up and away from the ground and you get a high tide.

On the other side of the Earth, the Moon pulls the ground downwards a little bit stronger than the overlying water so the ground moves slightly down and away from the water and you also get a high tide on the opposite side of the Earth from the Moon.

These effects exist but they are extremely tiny. Something like 1/10,000 ^th of the force of Earth's gravity. So in reality the effect is just not enough to cause any observable tides.

The actual explanation goes like this.

Looking at Figure 1, the Moon pulls on every bit of matter, ground and water both. You will notice that there is a slight inward tilt to the pulling forces when you are at the edges of the Earth that are 90° away from the direction of the Moon.

This inward tilt is important so keep it in mind.

Similarly, Figure 2 shows the situation for the side of the Earth that is facing away from the Moon. You can see that again, the Moon pulls on every bit of matter, ground and water both.

You will notice that again, there is a slight inward tilt to the pulling forces when you are at the edges of the Earth that are 90° away from the direction of the Moon.

In Figures 1 and 2 I drew a lot of gravity arrows for the oceans. There will also be a similar set of gravity arrows for the rocks of the Earth. At distances slightly further away from the Moon these forces are slightly weaker and closer to the Moon they get slightly stronger. So, to avoid creating a confusing mess of arrows I only drew one large arrow for the Earth.

The understanding for the origins of the tides all comes together in Figure 3. This one can be a little tricky to understand.

Essentially the effect that is portrayed in Figure 3 is that all the large gravity arrows inside the Earth are pulling the rocks of the Earth straight towards the Moon.

However there is also a similar set of large gravity arrows inside the oceans pulling the water straight towards the Moon.

These two movements basically have the same strengths pulling in the same directions. Everything is accelerating at the same rate towards the Moon, rocks and water together. Effectively neither the water nor the rocks will see the other component moving away any faster since they are both being pulled in that direction together.

However, the forces that are left over and not nullified are those inward tilting forces.

At locations on the Earth 90° away from the direction of the Moon the net forces are downwards and you get low tides.

At locations on the Earth facing towards the Moon the net effect is to compress the water inwards toward the point directly underneath the Moon as you can see in Figure 4. This compression causes a high tide in the direction of that point.

At locations on the Earth facing away the Moon the net effect is to compress the water inwards toward the point directly antipodal to the Moon as you can see in Figure 4. This compression causes a high tide in the direction of that point.

In Figure 4 you can see that these net compressive forces are also working on the rocks of the Earth.

The ground and the rocks do in fact respond to these tidal forces but water sloshes and moves whereas rocks don't. The effect is therefore much smaller for rocks and also harder to detect.

For moons that are close to their parent planet the effect can be seen mostly in heating the underlying mantle and also as a type of friction for the rotation of the moon (the rocks will heave up and down as the moon rotates due to the tidal forces causing a heating effect).

In fact, the rotation of a lot of moons that are close to their parent planets often slow down and eventually stop altogether due to this 'friction'. The effect is a moon with one face tidally locked to the larger body with one side always facing the parent planet.

The tidal locking effect also works on a parent planets such as the Earth and this is the cause for the length of the Earth's day to be constantly getting longer. The length of the day used to be much shorter than the current 24 hours and in the far future the length of the day will eventually be far longer than 24 hours.

Closing Words

The Sun also causes tides in the oceans of the Earth. The Sun is much more massive than the Moon but it is also a lot farther away. The effect is that tides from the Sun are one-third the strength of the tides from the Moon.

When the Sun and Moon are in alignment tides are extra tall and are called Spring Tides (the term 'spring' comes from the tides "springing forward").

When the Moon is at 90° to the direction of the Sun (i.e. a half moon) then the weaker Sun tide partially cancels out the stronger Moon tides and you get a weak total tidal effect. These are called Neap Tides.

So, don't believe any textbook or other explanation that uses the differential lifting forces to explain the tidal phenomenon. Although this differential force exists it is just not strong enough to be observable.

Tides are caused by a net compressive force due to the tilt of the gravitational force vectors pulling in towards the points underneath and antipodal to the Moon.

No hobbits were harmed in the making of this post. Thank you for reading my article.

Post Sources

What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios - YouTube Video
https://en.wikipedia.org/wiki/Tidal_force
https://oceanservice.noaa.gov/facts/springtide.html