The sun is shining, the sand feels warm beneath your toes, and you're watching the waves crash over one another. They rise to where you are standing, rushing over your feet and then fall back down. A few hours later, you come back to the same spot- and this time, the waves fall short of reaching you. You pause and start to wonder why... and then, you look up at the sky and spot the moon. There's your answer.
The moon's gravitational pull on Earth is what creates the tides. The reason the moon has such a large influence is not because it is the largest or most massive object around, it is because it is relatively close to the Earth. The moon is about 380,000 kilometers away from us. If you combine that with the size of the Earth, which is approximately 13,000 kilometers across, you get the perfect recipe for tidal forces. As Sir Issac Newton explained, the closer two objects are, the stronger the gravitational pull is between them. And the more massive the object, the more gravitational force it exerts.
When the moon tugs on the Earth it also pulls on the oceans. Every day, the ocean experiences four tidal changes: two high tides and two low tides. High tide, also called a flood tide, happens because of tidal bulges, these are like the ocean being stretched by the moon’s gravity. Think of the moon as a magnet drawing the ocean towards it. One bulge forms on the side of the Earth closest to the moon, where the pull is the strongest. The other forms on the farther side of Earth, where the gravitational force is actually weaker. However, due to the laws of inertia a tidal bulge is created there too. It may seem odd that both sides of the planet experience this phenomenon, but it’s all about balance. We measure gravity by the center of an object, so if one side of Earth has a force pulling it towards the moon, the opposite side is left behind, experiencing a weaker force, causing a bulge to appear. Then there’s low tide, which is also referred to as an ebb tide. This occurs in the areas outside of those bulges, where the moon’s gravitational pull is not as strong at that moment. As the Earth rotates, different regions move in and out of those tidal bulge spots. So, a beach that was experiencing high tide will, hours later, rotate out of that gravitational range and slip into a low tide. Meanwhile, another part of Earth moves into the gravitational path and starts to experience a high tide. It is this constant motion that keeps the tide changing all day long.
This phenomenon does not just affect the oceans on Earth. It also occurs even so slightly in the solid parts of Earth too. The land beneath us rises and falls by about 30 centimeters every day because of the moon’s pull. Think about it in terms of if you were sitting watching a movie marathon all day, you would rise up and down about 30 centimeters twice that day without even noticing. Pretty cool, right?
So, the next time you are at the beach watching the waves rise and fall up the sand, just look up. The moon is there: the man behind the curtain, pulling at our planet and making the oceans dance.