How do tides work?
Gravity is weird sometimes.
We all know tides as high and low. On or off. When it’s high tide you have less room to build sandcastles and when it’s low tide you can actually walk out to your favorite sandbar without fear. While all coasts on this beautiful blue globe of ours experience tides, they never experience them the same way. In fact, every single tide, every day, in every place is different, and there are a lot more reasons why than you might think. (Don’t worry, I promise this will be about space)
As you probably know, tides are caused by the Moon’s gravitational force. While too small to move the Earth as a whole, the Moon still has significant power over the world’s oceans; which, relatively, can move more freely. Imagine you have two magnets. You place one on top of a thin table and hold the other underneath it. This allows you to move the top magnet around by moving the bottom one. While extremely impressive at my middle school lunch table, this isn’t all that different from the tides. Gravity is a force of attraction. Any object with mass pulls on other objects with mass. Although it’s not nearly as strong as magnetic forces, if you’re the size of the moon, it can still be pretty powerful. This is what happens with the Moon and the oceans. The water is literally pulled towards the moon. This creates what is essentially a massive wave—with the peak corresponding to the Moons position over Earth, and the trough somewhere else. As the Earth spins and the Moon orbits, the water is pulled around the globe with it.
Your next question is likely, “where is the trough?”. Ugh, I was hoping you wouldn’t ask that. Fine, we’ll get into it. SciJinks has some cool information about tides, but I want to point out this picture: [1]
This is a diagram of the forces that the Moon exerts on Earth. In this image, the Moon is off to the right. Because it’s closer to that side, the gravitational force it exerts is greater. But to understand how these forces translate to tides, we have to look at the relative forces that they experience. Imagine that you’re in a car, driving down the road. You take a banana peel and throw it out your window. If you throw it hard enough, it will exit the window faster than your car. If you weak-arm it, it will go slower. Regardless of how hard you throw it, however, it will always be moving in the same direction. This is the same as our picture above—with respect to the Moon, the oceans are all pulled in the same direction. But, if we look at the relative speed of the banana peel to us in the car, the solid throw has a positive speed forward, and the weak throw has a negative speed backward. By that same logic, the relative force that our oceans experience is actually negative on the left side because you have to subtract the average force that the whole Earth experiences (the speed at which you’re driving the car). This results in what is called a tidal force that looks more like this: [1]
This means that there are two high tides and two low tides circling Earth at any given time—which is why we experience two high and two low tides each day.
Now there are many other factors that play into this including but not limited to the geometry of the ocean floor, depth of the ocean, and shape of the coastline. Keep in mind that the water still has to navigate obstacles like this. Even if the Moon is pulling it in a certain direction, it’s still constrained heavily to the container that it’s in. Therefore, we cannot simply time up the tides with when the Moon will be overhead, we must take into account all obstacles. Which, I would argue, is why we’ll be leaving the analysis up to the experts.
One last thing I’d like to point out is the other pretty massive thing in our solar system that we run into (figuratively) time and time again: the Sun. The Sun has enough mass to keep us locked in orbit, so naturally, it has enough to affect our tides. While it isn’t as much as the Moon, because it’s much, much farther away, it is significant. The force from the Sun, depending on its position in the sky, can be summed with the Moon’s to find the net tidal forces that the oceans experience. If the Sun and Moon are offset 90 degrees, their forces destructively interfere, meaning they partially cancel each other out. This means tides are less severe. If they’re aligned (in the case of a New and Full moon) then their forces constructively interfere, which means they combine to be even stronger. This is why you’re likely to experience higher tides and surges during times of New and Full Moons.
This has plenty of effects, from increasing damage during storm surges to stranding boats for several days as the high tides are not large enough to get the boat out. Regardless it suffices to say that the Moon’s influences are much closer to home than we tend to believe. While it’s always important to shoot for the Moon, sometimes you have to take a step back and roll with the tides.
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Cover Image: https://www.basicplanet.com/tides/
[1] https://scijinks.gov/tides/