How Tides Work

Podcast Transcript

Every single day, the oceans of the world go through a cycle with two high tides and two low tides.
For thousands of years, no one knew why the tides rose and fell, and even today, most people only have a vague idea of what drives the tides.
They dictate the lives of many people who live near the sea, and we might even be able to harness their power in the future.
Learn more about how tides work, and why it is more complicated than you probably think, on this episode of Everything Everywhere Daily.

Humans have been aware of the tides ever since they were aware of the ocean.
However, no one was really sure why the tides came in and out. Most cultures did realize that the tides had something to do with the moon. After all, it wasn’t hard to figure out that there was a high tide when the moon was visible overhead.
Explanations as to how the moon caused the tides were different in almost every culture.
The Yolngu Aboriginal people in Northern Australia believed that the moon would fill up with water and then be emptied into the sea.
In Ancient India, there was a belief that the waters expanded because of the heat from the moon.
Some Ancient Greeks did believe that the moon controlled the tides, but Aristotle was not one of them.
Muslim astronomers in the 12th century created tables of the tides, but they thought it was due to reasons of astrology and in fact, though it proved astrology.
As astronomy became a more serious science, most astronomers still believed the moon influenced the tides. Oddly enough, Galileo was one astronomer who ridiculed the lunar theory of tides. He thought it was due to the rotation of the Earth and its revolution around the sun.
The person who finally got it mostly right was, not surprisingly, Isaac Newton.
So, what exactly is happening with tides. What makes the tides go in and out?

Most of you probably know the basics of how the tides work, but it is actually much more complicated than you might realize.
Just to recap for those of you who don’t live next to the sea or have lives that require paying attention to the tides, there are two high tides and two low tides every day.
The high and low tides are separated by approximately six hours. However, it isn’t exact. Likewise, the height of the high tides will vary based on the time of day, day of the month, and time of the year.
Most people think that the gravity of the moon attracts the water which causes the tides to go up and down. There is a truth to that, but that would be a far too simplistic explanation.
To understand exactly how the tides work, let’s assume that the Earth is the same size, but has no land. It is just a ball of water, and the water is of uniform depth everywhere. I’ll refer to this as theoretical Earth.
Most people think that Moon orbits the Earth. Again, this is true, but the model of the Earth and the Moon you might have seen in school models or in diagrams in books doesn’t really capture what is going on.
Rather than saying the Moon revolves around the Earth, it is more accurate to say that the Earth and the Moon both revolve around the center of mass of the Earth-Moon system. The difference between these two statements is small but important.
The center of mass of the Earth-Moon system lies about 1,000 miles below the surface of the Earth at whatever point happens to be facing the Moon. That point is what the Earth and the Moon both revolve around. The Moon doesn’t revolve around the center of the Earth.
In this system, the Moon is gravitationally attracted to the Earth, and the Earth is gravitationally attracted to the Moon.
This pull is what causes a high tide. Water is a fluid so it can easily flow, and that flowing water will rise. Tide levels can change dramatically depending on where in the world you are, but the average height of a tide is about 2 feet or .6 meters in the open ocean.
Believe it or not, the same gravitational pull which causes the seas to rise also affects the land. Land tides are a thing, but because it is a solid, it doesn’t flow easily and most people don’t even know it’s a thing. It is possible for land tides to trigger geological events such as earthquakes and volcanoes.
So, if the Moon is gravitationally attracting the water which causes a high tide, why are there two high tides per day? There should only be one high tide if this was the only thing at play.
The second high tide takes place on the opposite end of the Earth. The reason why there are two high tides has to do with the fact I just mentioned that the center mass of the Earth-Moon system isn’t at the center of the Earth.
The other high tide is due to the centripetal force of the Earth. Because the center of mass of this system isn’t at the center of the Earth, it has an imbalance effect on how it pulls the water to the opposite side from the Moon.
I should note that this is a different effect from the centripetal force which comes from the rotation of the Earth. That force is applied equally and it is what causes the Earth to bulge slightly at the equator by about 7 kilometers.
These two high tides, however, are not equal. The high tide that faces the moon is known as the high high tide, and the one away from the moon is known as the low high tide.
The reason why the low high tide isn’t as high is for the same reason that the high high tide is high.
Centripetal force pulls the water away from the moon, but the moon still exerts a gravitational influence on that water too. It isn’t quite as much due to the far side of the Earth being slightly farther away, but it is enough slightly offset the centripetal force, thus making the low high tide a bit lower than the high high tide.
Just to complicate things, the high high tide doesn’t point directly at the moon. It points just slightly off line due to the rotation of the Earth and the movement of the Moon in the same direction.
Now, you might be wondering, if the Moon can exert a gravitational influence to affect the tides, couldn’t the sun do this as well? After all, the sun is vastly larger than the Moon.
The answer is, yes it can and it does.
Everything I just explained about the Earth and the Moon also applies to the Earth and the Sun. The only difference between the size of the gravitational effects is due to the distances involved. The sun is 27 million times more massive than the moon, and 400 times the distance.
It would seem that 27 million times larger would overwhelm 400 times the father. However, the inverse square law actually makes the sun have less of an effect on the Earth. For the sun to have the same influence as the Moon, it would need to be 64 million times larger than the moon at the same distance.
The solar tide is only about half as strong as the lunar tide. The ancients didn’t even know about solar tides because the effect is overwhelmed by the lunar tides.
The main solar tides manifest themselves is by augmenting lunar tides.
In addition to the tides changing every day, the size of the tides will change throughout the lunar month.
When the moon is in alignment with the sun or in opposition with the sun, or in other words when the moon is full or new, it is called a spring tide. A spring tide occurs twice each month and it is when the tides are at their highest because the solar and lunar tides are augmenting each other.
The opposite of a spring tide is a neap tide. Neap tides occur during a quarter moon, waxing or waning. During a neap tide, the low lunar tide is overlapping the high solar tide, and it results in the lowest tides in a given month.
This monthly cycle of spring and neap tides gets even more complicated.
This is because the orbit of the moon around the Earth, and the orbit of the Earth around the sun isn’t a perfect circle.
The moon varies in its distance by about 13%, but the gravitational attraction can be as much as 33% greater.
Likewise, the distance from the Earth to the Sun can vary by as much as 3-4%.
About once every 18 years, all of these forces can align to create what is known as a supertide. During a supertide, certain places on Earth can experience flooding
Earlier I talked about a theoretical Earth without any land. Of course, the real Earth does have land.
The differences in coastline and bodies of water can result in massive differences in the actual level of tides.
Looking down on the Earth from the North Pole, the Earth rotates counter-clockwise. So as the Earth rotates, the tide can result in a massive amount of water that goes across the ocean. It is as if it were a massive global wave that goes around the world.
If the geography of a location is just right, it can funnel that water to exaggerate the magnitude of the tides.
The place where this funneling effect is most pronounced is in the Bay of Fundy which is located between New Brunswick and Nova Scotia in Canada.
Here, the daily tides can rise and fall by as much as 16 meters or 52 feet, compared with the global average of about 3 feet or 1 meter. The place where you can notice the height of the tides is at Hopewell Rocks in New Brunswick. You can literally walk on the seafloor and then a few hours later, you can go kayaking in the exact same spot.
The fishing boats in the area literally will be grounded at low tide and often rest on their side until the tide comes back up. It is quite a sight to see if you are ever in the area.
On the same note, there are also places in the world where there is no funneling at all and the tides are abnormally small, like in the Gulf of Mexico.
All of these different factors made tides tables very difficult to compute. Not only are there cycles within cycles, but every location is going to have localized conditions that will result in their own unique tide levels.
Given that the tides are affected by the sun and the moon, you might be wondering if they are affected by the other planets. The answer is, technically, yes. However, the influence of other planets is so minute that it can’t be measured.
Tides also happen to be one of the biggest potential sources of clean energy. Plans to generate power from tides include massive underwater turbines that would turn when water goes in or out, as well as tidal barriers which would function like a dam and route water through turbines.
Currently, the world’s largest tidal power facility is located in South Korea and it can produce 254 megawatts of electricity.
So, I hope you can see that the tides are far more complicated than it seems. It isn’t just a matter of the moon’s gravity causing the tides, although that is a big part of it. There are cycles within cycles and geographical conditions which will determine tide levels all around the world.