I Just Learned The Tide Doesn't Actually Go In And Out, And I Need To Lie Down

There are some things I take for granted without really thinking about it. For instance, I sort of accept that Wi-Fi works despite knowing nothing about it; I trust in planes to take off, though I’m ignorant as to why. 

But having recently found out that raindrops are not, in fact, drop-shaped, I shouldn’t have been as stunned as I was to learn that the tide does not, in fact, go in and out ― at least not in the way we think. 

To be clear, the distance between the beach and the edge of the water does lengthen and shorten throughout a 24-hour period.

But as CBS News writes, “While it seems like the tides move in and out over the course of the day, the tides do not actually move in and out! That’s only what appears to happen from your perspective.” 

What’s going on, then? 

Well, partly, the Earth is moving while the sea stays more still. 

Neil deGrasse Tyson explained on StarTalk that “the tide doesn’t actually come in and out ― there is a bulge of water, two of them, on opposite sides of the Earth, caused by [the gravitational pull of] the sun and the moon.”

The moon, being closer, has a stronger pull, but the other side’s bulge stays in place due to inertia. 

In fact, National Geographic explains that “The bulge may be understood as the moon’s tidal force pulling the planet (not the ocean) toward it.”

“Earth turns inside that [tidal] bulge,” deGrasse added. “When we way the water rises and falls, tidally, what’s happening is we are rotating into the bulge and then out of the bulge.” 

And because there are bulges on both sides of the planet, as we rotate throughout the day, we have two stages of tide changes. 

That’s why “Most tides are semidiurnal, which means they take place twice a day.“

What?

I know! Honestly, finding out that fact gave me some vertigo. 

Of course, tides and water heights change due to factors like landmasses, wind, and weather patterns.

“Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much less noticeable,” the National Oceanic and Atmospheric Administration of America writes.

So it’s not the same everywhere ― but yes, every time you notice the water “going in and out,” it’s the planet moving, not the sea.