If you spend some time around the ocean you may notice something kind of strange happens. The water level goes up and down every day! We call these rhythmic changes the tide, and even though humans have been living with it for literally thousands of years, it is still commonly misunderstood. In this video we're going to explore the science behind the tide, why ocean levels fluctuate more in some places than others, and how the moon plays a role, just not in a way you might think! If you enjoy these videos and want us to make more, check out our gear at waterlust.com. We make environmentally responsible apparel for ocean lovers that helps fund Marine Science research and education. Your purchase helps keep our small business afloat and allows us to make more videos. Thank you so much for the support Ask somebody about how the tide works, and in nearly all videos on the internet, you'll get an answer that goes something like this. The moon orbits the earth and because it has a mass it exerts a gravitational force that pulls water towards it which creates a bulge in the ocean we call high tide. But wait! It also creates a bulge of water on the opposite side of the moon... and that happens because ummmm... well don't worry about that, it's complicated. Yay tides! This incomplete explanation is so common, it's even printed in some textbooks. The idea that the ocean bulges out on the side of the earth closest to the moon just feels so intuitive, even though the logic behind it completely contradicts why there's also a bulge on the opposite side. But for whatever reason this explanation is satisfactory for many educators and has led to entire generations of ocean lovers not really understanding how the tides work. So what the heck is really going on? To understand how the tides really work we need to first learn some astronomy... The universe is full of mysteries, but we do know that when an object has a mass it exerts a force we call gravity. All massive objects like planets, stars, and black holes do this. Sir Isaac Newton famously observed gravity in action by watching an apple fall from a tree... "huh" and formed a mathematical relationship that describes the strength of gravity between two objects. It's called Newton's law of universal gravitation, and it says that gravity will produce an attractive force between two objects proportional to the mass of the first object multiplied by the mass of the second object, divided by the distance between them squared. This means that bigger objects create stronger gravitational forces than smaller ones, but more importantly for this discussion, as objects get closer together, the gravitational force between them gets stronger! And as it turns out it's that distance dependency in gravity that creates the tides. Here's how! Imagine you have two planets, Planet A and Planet B, some distance apart at rest in space. If you were to magically let them go what would happen? They would accelerate towards each other, right? If that answer felt intuitive, you've just naturally applied the second key principle to understanding the tides. It's called Newton's Second Law, or that an object will accelerate proportional to its mass when a force is applied to it. The law is commonly known through its equation F=MA, but we can rearrange it to solve for acceleration, or that the acceleration an object experiences is equal to the force applied to it divided by its mass. If we consider our two make-believe planets accelerating towards one another, we can define the acceleration of Planet A caused by Planet B by substituting the universal law of gravitation into Newton's second law. When we do this you'll notice that the mass of Planet A cancels out, meaning how fast it accelerates towards Planet B has nothing to do with its own mass! This was one of Newton's most important observations and something you can confirm at home. If you drop two objects of different masses they both hit the ground at exactly the same time, meaning they accelerate at the same rate from Earth's gravity. But what does this all have to do with the tides? Let's TIDE it all together! in our original example we had two planets accelerating towards one another, but what happens if instead of Planet A, we have four little planets that are initially spaced in a circle. One on the left, one on the top, one on the right, and one on the bottom. If we magically let everything go, what happens? We know that all the little planets will accelerate towards Planet B, but they don't all accelerate in the same way. Because of the distance dependence of gravity, the planets that are closest to Planet B will accelerate faster than the planets that are farther away, and the planets that are above and below will accelerate at angles towards Planet B. If we let this play out in time, it means that the left and right planet will get farther apart as they accelerate towards Planet B and the top and bottom planet will move closer together. Look familiar? Instead of four little planets, let's imagine a circle with tons of little particles. For the programmers out there, you can illustrate this with a simple Matlab model. What happens when they accelerate towards another planet? Over time, the circle is deformed into an ellipse. In astronomy this is called the tidal force, a phenomenon of gravitational fields that deforms the shape of planets, tears celestial bodies apart, is responsible for the formation of ring systems, and you guessed it ...creates the tides in the ocean! It's important to point out that in our example, for simplicity, we had two planets accelerating towards one another from rest. But if those planets had an initial velocity, instead of crashing into one another, they could orbit around one another. This is what happens in real life, two massive objects constantly accelerating towards one another as they orbit around and around. The forces that apply to two planets centripetally accelerating towards one another are the same as those that are linearly accelerating towards one another, it's just easier to visualize the linear case. Knowing how gravitational fields stretch objects that are accelerating through them, let's explore how that specifically affects the tides on Earth. Our planet is basically a solid rock covered with a thin layer of gas and water over most of its surface. But unlike the solids that are packed tightly together by Earth's gravity, the fluids in our atmosphere and oceans can more easily move. When our planet accelerates towards another massive celestial body like the moon, the water on the near side accelerates faster than the solid rock in the middle, and accumulates to form a slight bulge. At the same time, the water on the far side accelerates slower than the rock, which forms a second bulge. In a way, the bulge on the far side can be thought of as water that is getting left behind as the rock accelerates away from it. As the rock part of the Earth rotates, the watery high and low points stay in the same position relative to the object they are accelerating towards. And cool side note, the bulging effect doesn't just affect fluids, over time the solid rock itself can become deformed in the same way. The moon, for example, has slowly been deformed into the shape of an ellipse because of Earth's tidal force on it. And even crazier, if an orbiting planet isn't big enough to hold itself together, the tidal force from the object it orbits can literally tear it apart. This is described by what is called the Roche limit, a calculable metric in astronomy that defines the closest a celestial body can orbit another without being shredded to pieces by the tidal force! Fortunately for us, the tidal forces experienced on Earth from nearby massive objects like the Moon, the sun, Venus, Jupiter etc.. are only large enough to move fluids around, but not large enough to tear us apart. And while all nearby massive objects create tidal forces, we know from the universal law of gravitation that these forces scale with their mass and their distance from Earth squared. That means that the distance a massive object is from Earth is more influential than its mass, which is why the tides on Earth are dominated by the influence of the very nearby Moon, despite it being relatively small. If you measure the depth of the ocean where you live, you'll find it changes in time following a mixed sinusoidal signal of different amplitudes and frequencies, and each sinusoidal pattern, referred to as a tidal constituent, has its own backstory. The most commonly known title constituent is caused by the Moon and is named the lunar semidiurnal tide or M2 for short. It completes two cycles in the time it takes a location on Earth to rotate around with respect to the Moon, 24 hours and 50 minutes to account for the distance the moon moved in its orbit around the Earth. But while the M2 may be the most commonly known tidal constituent, there are dozens of other ones including those created by the sun, like the solar semidiurnal tide called the S2, that completes two cycles in 24 hours flat, the time it takes one location on Earth to rotate around with respect to the Sun. Or the solar annual tide called the SA that has a period equal to how long it takes the earth to orbit around the sun, 365.25 days. And that's just the beginning, there are many other title constituents, like the K1, P1, and N2... and while you don't need to know all of them to understand the tides, it's important to appreciate that each of these tidal signals can become aligned or misaligned throughout the year. The most commonly known alignment of different tidal forces are called Spring and Neap Tides. A spring tide happens when the Moon and Sun are on the same or opposite sides of Earth. It happens twice every month and causes the gravitational bulges from each to align. If you're measuring the water level on Earth, this happens when the peaks of the M2 and S2 tidal constituents align, creating extra high, high tides, but normal low tides. Neap tides are the opposite, occurring twice every month when the Moon and Sun are at 90 degree angles to one another, causing their respective tidal bulges to be misaligned. This creates lower than usual high tides and higher than usual low tides. And for those that have explored lakes and ponds, you may have noticed they don't rise and fall like the ocean, so what gives? Well, they do have tides, it's just that they're isolated bodies of water, so there isn't enough water moving around to produce a very noticeable change, but they do happen! They're just too small for us to notice. This is where things start to get a little complicated, because even though the underlying physics of what creates the tides is relatively straightforward, the patterns it creates in our oceans can be very complex. Different parts of the world respond to these tidal forces differently. Nowhere on Earth is this more dramatic than the Bay of Fundy, a bay in Canada that is perfectly shaped to amplify the effects of the tide. Here's how it works. If you've ever slid back and forth in a bathtub at just the right frequency, you've experienced something called resonance. Here, we're moving a paddle back and forth with the same timing it takes a water wave to hit one end of the tub and reflect back. By paddling in sync, the height of the wave grows and grows until it overflows the tub. The Bay of Fundy is like one big bathtub and the Moon is like our paddle. The timing it takes water to hit one end of the bay and reflect back just so happens to be perfectly aligned with the M2 tide, and just like in the tub, the height of water oscillating back and forth has grown to huge levels. The ocean level rises and falls here over 50 feet! But if you travel a short distance away to the Atlantic side of Nova Scotia, the tidal range is only around 6 feet, because the wide open ocean on that side prevents the resonance effect from happening. Having vastly different tidal amplitudes in locations that are geographically close together illustrates an important point, how the ocean rises and falls in the specific part of the world you live depends heavily on the shape of the coastline and the local bathymetry. In a sense, how the ocean rises and falls and the currents it produces where you live is as unique as a fingerprint. Every place on Earth is different and as sands shift, sea levels rise due to climate change, and coastlines change, so do the tides! This is why even with all the wisdom of the underlying science, there is still no substitute to local knowledge from sailors, fishers and surfers who observe the patterns in their stretch of ocean every day. So the next time you go to the beach, pay attention to how the ocean changes over time. Look up and remember how our planet is constantly accelerating towards other objects in space, and how that little distance dependence in the gravitational force has some pretty big consequences!