What You Didn’t Know You Wanted To Know: The Surprisingly Elegant Physics behind Tides
We all know that the Moon causes the tides; in fact, it’s the focal point of most folklore and mythology about the moon and the ocean, like the water-benders from Avatar: The Last Airbender. However, what exactly is the relationship between the Moon and water, and does it have other effects in the universe?
What Are Tides, Anyways?
First, to dissect the Moon’s role in this phenomenon, we need to figure out what tides are. The water isn’t actually moving, but the Earth is spinning under it. The Moon’s gravity causes the water on the side closest to it to bulge out of Earth, and Earth keeps spinning under it. The bulge never moves, you just go in and out of it throughout the day.
However, we go through 2 high and 2 low tides a day, how is that possible if thee is only 1 bulge? It’s a really counterintuitive concept, but because the bulge is outside of what is normally considered Earth and it has mass, the center of mass of the Earth has moved. So, we observe 2 bulges, even though only one actually exists.
What About The Sun? It Has Gravity Too!
Obviously, the Sun has mass (the biggest in the solar system), and it has gravity (the strongest in the… you get the idea), so it must have some contribution to this, right? Absolutely. When the Sun, the Earth, and the Moon are lined up (it’s called a syzygy, it happens every full moon and new moon), the Sun and Moon team up to cause super-bulges, and we call those spring tides. However, when they are perpendicular to each other, they cancel each other out, and we get smaller bulges, called neap tides.
So, How Is This Relevant Anywhere Other Than Fishing?
The tides actually make the days get longer. Earth’s rotation around itself is faster than the Moon’s rotation around Earth (a day is shorter than a month, duh), so the bulging water is moving ahead of the Moon, even though the moon is resisting it, causing Earth’s rotation around itself to get slower. Furthermore, the bulge also has gravity, and it pulls the moon forward. The Moon responds to that by going up to a higher orbit; the moon is getting farther and farther away from us.
But, we’re forgetting something, something else involved in this that also has gravity and mass… the Earth! It also pulls on the moon, and there’s an imbalance of that gravity between the near side and the far side, so the moon also must have tides! Thus, the phenomenon we just described about Earth slowing down also happened to the Moon’s rotation, and it’s much more significant since the Earth is much more massive than the Moon, and that’s exactly what happened. The Moon’s rotation around itself slowed down so much that it rotates around itself at the same rate that it rotates around the Earth, which is why we always see the same side of the Moon. This is called tidal locking.
If Tides Happen Anywhere Massive Objects Are, Where Else do They Exist?
Everywhere! And a great example of tides is in black holes. The have the strongest gravitational pull of anything in the universe, so their tides must be epic. Imagine if you fell into a black hole feet first. Your legs are experiencing a stronger gravitational pull than your head, so just like the water on Earth, you bulge up. But because you’re not fluid and you don’t move that easily, you actually get stretched, you get longer and longer until you resemble a spaghetti strand, and then you get ripped into pieces and get sucked into the abyss. And because apparently astronomers have a twisted sense of humor, this is called spaghettification.
From water-bending, to werewolves and witches, the Moon has a huge effect on our lives, and tides are a glaring example on that. However, we learned that the incredibly beautiful and elegant forces behind that extend far beyond our imagination, all the way to deep space.
Sources
Neil deGrasse Tyson Explains the Tides. (2018, April 10). [Video]. Startalk. https://www.youtube.com/watch?v=dBwNadry-TU
CrashCourse. (2015, March 5). Tides: Crash Course Astronomy #8 [Video]. CrashCourse. https://www.youtube.com/watch?v=KlWpFLfLFBI
What Causes Tides? (n.d.). NOAA SciJinks — All About Weather. Retrieved February 25, 2021, from https://scijinks.gov/tides