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xkcd #3135: Sea Level (imgs.xkcd.com)
submitted 3 weeks ago by [email protected] to c/[email protected]

xkcd #3135: Sea Level

Title text:

They're up there with coral islands, lightning, and caterpillars turning into butterflies.

Transcript:

Transcript will show once it’s been added to explainxkcd.com

Source: https://xkcd.com/3135/

explainxkcd for #3135

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[-] [email protected] 105 points 3 weeks ago

There are places in the solar system where the tide rips new mountains up every go around.

[-] [email protected] 56 points 3 weeks ago
[-] Evil_Shrubbery 18 points 3 weeks ago* (last edited 3 weeks ago)

... some metal band (Dethklok?) recording their song/filming their vid/having the concert on such a moon - the guitar solo intensifies as the band is lifted upwards by a soaring mountain, epicly, with lava flows.

[-] [email protected] 17 points 3 weeks ago

I think it's funny that Dethklok is both a parody metal band and one of the best metal bands around. I don't even really like metal and I love Dethklok.

[-] [email protected] 7 points 3 weeks ago

I would love to see them live. Brendon Smalls seems awesome.

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[-] [email protected] 8 points 3 weeks ago

that's nothing compared to Disaster Area, who need to evacuate a whole planet due to their sound system

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[-] [email protected] 28 points 3 weeks ago
[-] Evil_Shrubbery 21 points 3 weeks ago* (last edited 3 weeks ago)

True!

Going further - "tides" actually rip planets & stars fully apart (binary star systems, around black holes, yo mamma casually strolling through a galaxy).

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[-] [email protected] 11 points 3 weeks ago

Where is that? Is that on one of the moons of Jupiter?

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[-] [email protected] 65 points 3 weeks ago

I remember never believing my parents when they explained it to me as a kid. Clouds being caused by cigarette smoke was reasonable but the moon pulling out the ocean seemed too outrageous.

[-] [email protected] 31 points 3 weeks ago

As a child, my friend was told by her mother that wind occurs because people group up and blow really hard.

[-] [email protected] 5 points 3 weeks ago

aw fuck. i missed a chance to say "people suck" when asked that. does anyone have a way to get me back to say 2012?

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[-] [email protected] 51 points 3 weeks ago

Been trying to learn about the tides around here so I can tell what I'm seeing on the water. Imagine my joy when I found a Casio, which I collect, with tide and moon phase indicators!

And that's when I learned the Gulf Coast is strange, has diurnal tides (twice a day) the watch can't predict. Took me an hour and a half to figure out it would never function. The moon phase works!

[-] [email protected] 55 points 3 weeks ago* (last edited 3 weeks ago)

Huh. TIL that there are three common types of tidal cycles and which one you get depends on geography, location, ocean currents. https://beltoforion.de/en/tides/tidal_cycles.php

And yeah, dinural is apparently the most rare of the three. Wild.

[-] [email protected] 8 points 3 weeks ago

Thanks. I didn't know either that there are places, where the sea level does not rise and fall twice a day.

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[-] [email protected] 14 points 3 weeks ago

the Gulf Coast is strange, has diurnal tides (twice a day)

Diurnal tides are once a day (semidiurnal is twice a day). By the Gulf Coast, I guess you must mean the Gulf of Mexico. I'm living on the other side of the world in the other diurnal region, so I assume our tides are synchronised!

[-] [email protected] 7 points 3 weeks ago

Gulf of America, you extreme left Antifa socialist!!1!one

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[-] [email protected] 10 points 3 weeks ago

Tidal prediction requires a harmonic analysis of observed tides, and its location specific. Not sure how a watch is supposed to do that other than holding a database of tidal coefficients.

This video contains a lot of interesting history of tidal analysis and prediction:

https://youtu.be/IgF3OX8nT0w

[-] [email protected] 17 points 3 weeks ago

There are adjustments you can make on the watch. Requires tables and whatnot. That's why it took me so long to figure out it wouldn't work!

[-] [email protected] 7 points 3 weeks ago

Tides go in, tides go out, you can’t explain that.

[-] [email protected] 6 points 3 weeks ago

Casio can't!

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[-] [email protected] 32 points 3 weeks ago

"Tide comes in, time goes out - you can't explain that!"

[-] [email protected] 9 points 3 weeks ago

That's some next level transformation, tide becomes time.

[-] [email protected] 22 points 3 weeks ago
[-] [email protected] 19 points 3 weeks ago

They'll just say the moon pulls the water around as it circles above the flat disc or something idk

[-] [email protected] 12 points 3 weeks ago* (last edited 3 weeks ago)

But the moon alone doesn't orbit at the same rate as the tides. Alternatively they are fucking morons and shouldn't be respected so who cares.

[-] [email protected] 6 points 3 weeks ago

"The earth tilts when the moon hits far end of earth during moonset and that's why there is low tide. A high tide happens during moonrise when the moon drags it up slightly."

[-] [email protected] 8 points 3 weeks ago

Thats just the tilting from where the elephants shrug

[-] [email protected] 17 points 3 weeks ago

Everyone was taught the tides look like two giant water bulges going around the earth in line with the moon.

That representation is oversimplified and false.

This is how the tides look like at a global level. It’s messed up.

https://www.youtube.com/watch?v=5zi7N06JXD4

[-] [email protected] 5 points 2 weeks ago

Ok, that clarifies things so much better for me. Thank you.

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[-] [email protected] 15 points 3 weeks ago

Imagine being out exploring new islands, not realizing its low tide. You setup camp for the night on an island that's relatively flat and close to current sea-level. Then while you're sleeping the tide comes in and washes your whole camp out to sea...

[-] [email protected] 20 points 3 weeks ago

Come to the beach here in Bordeaux (well, on the coast) and see tourists set up their stuff at low ebb but forgetting they have to watch out for the flood.

[-] [email protected] 12 points 3 weeks ago

There's an island near me that has a pedestrian causeway at low tide. There are huge signs warning to check tide times or get cut off, but still people don't get it.

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[-] [email protected] 10 points 3 weeks ago

if you are exploring islands you probably have a solid idea of how tides work.

[-] [email protected] 13 points 3 weeks ago

You probably should, but that doesn't mean you do. It's not like anyone makes you take a quiz to go wander around outside.

Plenty of people get themselves into trouble all the time exploring places/things they know nothing about.

[-] [email protected] 12 points 3 weeks ago

It's not like anyone makes you take a quiz to go wander around outside.

I knew it! I am so not answering any more of his questions, next time I go out.

[-] [email protected] 8 points 3 weeks ago

WHAT! is your favourite colour?

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[-] [email protected] 6 points 3 weeks ago

i lived near the beach for a while and, uh, i have seen tourists do the stupid. i posit they count.

[-] [email protected] 14 points 3 weeks ago

And when Mercury is in retrograde, you can make an excuse for anything being kind of shitty or off.

[-] [email protected] 12 points 3 weeks ago

There better not be mercury in my Gatorade!

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[-] [email protected] 13 points 3 weeks ago

Nah, that's just Poseidon having a bad mood today. Just have to sacrifice your first child to make it stop for 10 years.

[-] [email protected] 11 points 3 weeks ago

"Their moon is tidally locked" is an absolutely metal thing to say about a planet.

[-] Evil_Shrubbery 10 points 3 weeks ago

Well, tides on moons (eg around gassy supergiants :)) are fairly usual.

So perhaps it's not as much as the tides being "sci-fi" (sci??) but the relative size of Earth's Moon (it's basically a binary planet situation where they orbit around each other).

So, the sheer size of our moon as seen from the planet's surface is the rarity.
(Then again, on a moon around a supergiant the same experience could be had from one tiny beings pov.)

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[-] [email protected] 8 points 2 weeks ago

Tides in the Bay of Fundy, Canada are 16 metres (50 feet).

[-] [email protected] 5 points 2 weeks ago

See, that's the part that confuses the hell outta me. How can water be higher in one spot than others just due to the Moon's gravity? Yeah it's the geography of the area, got it. But still, how?

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[-] [email protected] 8 points 3 weeks ago

I wonder if anyone has ever done the math on how much (in L or kg) water is moved by the moon each day. It's got to be something absurd.

[-] [email protected] 7 points 3 weeks ago

I have to assume it's about one moon's worth, divided by the distance squared.

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[-] [email protected] 7 points 3 weeks ago* (last edited 3 weeks ago)

Just wanted to mention I see your pfp on every post.

Thank you for keeping lemmy alive and making like 4% of the total posts. Seeing you post brightens up my day.

No homo.

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[-] [email protected] 7 points 3 weeks ago

I think lightning would be crazy to anyone who never experienced a planet with it. Like, "WTF, sometimes your sky does what?"

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this post was submitted on 30 Aug 2025
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