3135: Sea Level

Sea Level

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

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Explanation

The existence of the Moon, and its various impacts (including the movement of the tides, and reflected light it provides) are such stable and constant realities of life on Earth that people tend to take them for granted. This strip points out that, if someone were unaware of the Moon and its impacts, they would seem amazing and even implausible.

Cueball is wondering aloud what happened to an island, possibly a sandbar that was visible earlier, but is not anymore. Megan explains to him that the tide has come in and submerged the island. She then explains that the Moon, being quite large, relative to the Earth, and orbiting relatively near, creates gravitational effects that change local sea levels over the course of each day.

The fact that the net gravitational pull at Earth's surface change throughout the day, enough to make the local levels of the ocean visibly rise and fall, would be shocking if we weren't accustomed to it. A visitor to Earth from another planet would have a good chance of being completely blindsided by such an effect. Earth's moon has, by far, the highest mass in relation to its planet in the solar system, and is the only known satellite to have such major impacts on its planet's surface. (Pluto and Charon form a double-satellite system, and both are considered too small to be a true planet) Someone coming from a planet whose moons have little noticeable effect would likely be surprised at how casual humans are about the situation.

The title text mentions other elements of life on Earth, saying they also seem as outlandish as tides. Coral islands are bodies of land built entirely from the skeletons and secretions of dead animals. Lightning is an electrical discharge in the atmosphere (sometimes striking the Earth) which creates a brief but very intense current, heat, and ionization of the atmosphere. The metamorphosis of butterflies from caterpillars involves the latter building a chrysalis around themselves, then effectively dissolving into biological soup, which then reforms into a very different form of animal. As with tides, each of these occurrences would seem amazing, possibly unbelievable, to someone who hadn't encountered them before, but are so common in our world that we generally pay them little attention.

This strip is very similar to 2809: Moon, as both center around Cueball being shocked by the reality of Earth's Moon. It follows a long-running theme of expressing general amazement about the world in which we live, where things that feel like they should be impossible are so commonplace that we often ignore them. Examples include 332: Gyroscopes, 2115: Plutonium, 2540: TTSLTSWBD, 3087: Pascal's Law, and 2775: Siphon.

Transcript

[Single frame, with Megan and Cueball standing at a coastline with grass, with water and waves visible. They are looking at the ocean and are standing together.]

Cueball: Hey, where's that big island we were looking at this morning?

Megan: Oh, it's underwater. The ocean's depth here goes up and down by like ten feet every day.

Cueball: What?

Megan: It's because the planet has a big moon orbiting near the surface. It causes weird gravity effects.

Cueball: What???

[Caption below the frame:]

People here are used to them, but tides are one of the weirdest and most sci-fi elements of life on Earth.

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Discussion

Holy crud empty page! F1RST P0ST! RadiantRainwing (talk) 01:38, 30 August 2025 (UTC)

2038: Last of the original Star Wars cast dies. —megan talk contribs 02:18, 30 August 2025 (UTC)

What? If you're trying to get back at me because I was being useless and just "first posting", it's a reference to 269: TCMP, and I also, by the way, wrote the whole first paragraph of this explanation. RadiantRainwing (talk) 02:50, 30 August 2025 (UTC)

Alright, sorry, just realized what you did is a reference to 493: Actuarial. Sorry about that! RadiantRainwing (talk) 02:53, 30 August 2025 (UTC)

I guess you're one of today's lucky Ten Thousand...

Wait, no, not everyone has read comic 493 by the time they're adults. I'm too lazy right now to calculate how many people learn about comic 493 each day, so I'll leave it as an exercise for the reader. —megan talk contribs 01:01, 31 August 2025 (UTC)

This is where it pays to have read every XKCD comic :P Logalex8369 (talk) 16:45, 1 September 2025 (UTC)

This one is related to the Moon comic. Pgn674 (talk) 01:39, 30 August 2025 (UTC)

Clearly. RadiantRainwing (talk) 01:49, 30 August 2025 (UTC)

Definitely easy to see, even the references are the same…Randall we trust in you not to copy again…《プロキシ》(XKCD中毒者) 19:07, 1 September 2025 (UTC)

it's almost a repeat. is he running out of ideas? raeb 09:54, 30 August 2025 (UTC)

Hope not. RadiantRainwing (talk) 16:56, 30 August 2025 (UTC)

Why do people talk about running out of ideas? That is something I can't imagine. The world throws ideas and absurdities at you all the time. Running out of time to execute an idea - sure. Seems much more likely to be filtering error (have to check current idea against 3000 previous strips). 107.77.205.64 19:42, 30 August 2025 (UTC)

Larry Niven had a story about alien-made indestructible spaceship hulls, except the makers didn't account for tidal effects when grazing a star. The test pilot was nearly ripped apart, but figured a way to survive. He sued their butts off against the guarantee. He concluded that their home planet did not have a large moon, a Clue. --PRR (talk) 02:41, 30 August 2025 (UTC)

Isn't it frakking ridiculous that the Puppeteers, with all their brains, haven't observed or couldn't deduce the existence of tides? I did like the deduction about their homeworld making them nervous and their legal system of blackmail/extortion, though. --DW 2607:FB90:8FA9:E54A:5856:AACD:B913:6DD8 13:59, 2 September 2025 (UTC)

I think I finally figured out the common thread. All the items here are here because they are elements of Life on Earth. The way the explanation was written kind of buried that important part of the comic.

Overall, if you just look at them as unrelated phenomena, then Lightning seems quite common. Islands made by microskeletons, and life-forms which change their form during development seem like they would be pretty common where there is life. Large tides - thought to be uncommon, but don't have much data, and models are hard. 2600:387:4:803:0:0:0:A0 18:04, 30 August 2025 (UTC)

All planets with intelligent life we know have tides. In fact one could argue that tides play an important role in the development in life. Thus any intelligent observe is arguably familiar with tides. Thus the text is wrong in arguing that tides are surprising based on the observation that most known planets likely do not have large tides. --2A01:599:114:9E35:D827:C56:FF88:1858 19:09, 30 August 2025 (UTC)

Argument has problems - insufficient sample size, selection bias. Nothing in the comic talked about intelligent life.

The role of tides in development of life certainly makes sense to add.

Tides are strange in that they are very complex and hard to explain in detail. Fluid dynamics in a very complex, non-ridgid vessel, involve gravitational forces from multiple bodies. 107.77.205.64 20:00, 30 August 2025 (UTC)

Sample size is definitely a problem. Nothing in the comic talks about tides being strange in a cosmic sense. They are just very weird for one of the two observers from earth. --195.63.76.62 20:39, 30 August 2025 (UTC)

Realistic. Early Romans had no experience of tides until they ventured out of the Medditeranean, and were probably as spooked by them as the observer here. 80.189.2.17 22:38, 3 September 2025 (UTC)

All planets with intelligent life that we know of also have microwave ovens and television cameras and rubber ducks. Perhaps those are also essential for the long-term continued existence of intelligent life? PotatoGod (talk) 19:09, 31 August 2025 (UTC)

Well, I can confirm that microwave ovens are essential for the continued existence of this intelligent lifeform. 159.118.184.96 04:47, 1 September 2025 (UTC)

I'm shocked that Randal didn't include some sort of reference to climate change- and how tides effectively, at least in 2025 and for the foreseeable future, dwarf sea rise due to melting ice. Seebert (talk) 20:18, 30 August 2025 (UTC)

On a (twice-)daily basis, yes. But that's like saying an unseasonal/hyperseasonal cold snap belies the possibility of global warming. (If that's the point you're trying to make.)

And I'm not sure if you're saying that Randall "is the sort of person who would go on and on about climate change, but for soe reason he surprisingly didn't do so here" or "he really ought to be mentioning climate change at every opportunity, but he missed the opportunity to convey the concept"..? I'd disagree with both of those assessments of his (non-)inclusion here, though, and perhaps you're even coming from a completely different third direction that I might or might not understand. But really not the place to discuss it, as he obviously hasn't made that part of the joke/message in this comic. 92.17.62.87 23:27, 30 August 2025 (UTC)

Lightning should be common throughout the universe, as the ingredients for it (planetary atmospheres containing things like dust that can build up differential static charges through agitation) appear to be. It's still a very weird phenomenon, with many aspects not understood (how does the triboelectric effect work, can breakdown patterns be predicted, wtf is going on with sprites and ball lightning, etc) but it really isn't likely to be rare. --81.96.108.67 05:48, 31 August 2025 (UTC)

Ten feet tidal range on a remote island - isn't this too much? I thought it should be less, with stronger tides only in some gulfs where an amplification exists. https://en.wikipedia.org/wiki/Tide figure 15 shows 5 feet tidal range maximum

In the channel tides can get pretty high and some parts of it would be remote islands by European standards. Also the comic doesn't mention remoteness. --195.52.138.253 18:42, 31 August 2025 (UTC)

Significant tides can also bother or surprise people who are used to comparatively smaller tides. I should know, I am one of those people. I live by the Mediterranean Sea and any time I go to the beach on the Ocean, I am worried that I will once again lose my towel to the moving shoreline... 2001:861:51C2:B540:1593:36EE:75F9:1F83 18:35, 1 September 2025 (UTC)

^V^ 8th place! Learning English lol…love autocorrect. in South Carolina, tides are HELL on coast《プロキシ》(XKCD中毒者) 19:05, 1 September 2025 (UTC)

Rolled over to tuesday and no new comic?? -- SteveTheNoob (talk) 23:40, 1 September 2025 (please sign your comments with ~~~~)

It's still (as we write) Monday for Randall, even if it's slightly after midnight for some of us Rightpondians. And there are have definitely been more extreme 'delays' to the scheduled comics. Take a deep breath, man... If you have to go to sleep and wake up to the comic, then that's what you have to do... 92.17.62.87 23:48, 1 September 2025 (UTC)

I thought Randall would just publish using +00 timezone for the love of the game. Oh well, I’ll stay awake until he publishes -- SteveTheNoob (talk) 00:40, 2 September 2025 (please sign your comments with ~~~~)

It just varies so much. Sometimes it's "late afternoon 'tommorow'", for any given day, sometimes it has been so early, by ±0 UTC standards, that one wonders if he was effectively staying up to post it 'very late the night before'.

When he was on his book tour over this side of the Atlantic, I know at least some of them were 'sensibly' posted at times relative to his being in these time-zones (i.e. 5 or hours ahead of the non-'late' instances, usually.

Special comics (huge and/or dynamic ones) seem to be posted any time at all, at least by my possibly faulty combined recollection, which is perhaps due to last-minute tweaks (the lateness of April Fool comics is such that, whilst something is often posted, if 1/Apr is a M/W/F, it might have been an emergency-filler, or perhaps brought forward from the W/F/M following - and then days, or occasionally even a week or more, later, it happens).

It's only really possible, right now, to go into the edit history of various comics and see when they were uploaded by the BOT of the time (or, when BOTs have been temporarily or permanently broken, when a human editor decided it needed doing manually). This introduces some small polling-lag and response time between 'Randall time' and 'wiki time', but if you're eagerly awaiting the comic here, that's probably the end-timing that most interests you.

I can't recall any "so late, it was almost Randall's tomorrow" instances, recently, but there was a "so early, it was not too long after his prior midnight". And if Randall ever uses an automated method of posting, I think it's only for exceptional circumstances (not noticably for individual strips, that is - things like the Time comic probably were automated-and-queued, out of planned necessity).

And you asked before 00:00 UTC (I restored the time... and used the Unsigned template as a quickish way to overcome you apparently not using ~~~~ properly), so even if he were of that habit, you were asking too soon. If he was supposed to be posting at 24:00 UTC, anyway. - Quickly checking, though, and he still hasn't posted as of this moment. But it's still only late evening on Monday, where he is, and has maybe a couple more hours to spare before it becomes notably (and maybe fashionably) late. If you're under UK or one of the EU timezones, that's a long time to wait up. Not sure even I should be here, right now, though I do have my 'reasons' for it.

No, still no change on xkcd.com, just checked before I signed it off. Whether you'll get more or less sleep than myself, I leave up to your circumstances, possibly your other morning commitments. But I'm expecting to see the Monday comic in a few hours, when I check, even though that'll be actually the start of my own Tuesday. 82.132.239.156 02:00, 2 September 2025 (UTC)

The new comic is out now, although I don't think explainXKCD has updated yet.47.150.145.249 04:46, 2 September 2025 (UTC)

Not sure the premise of this comic is correct. The sun generates tides on Earth of about half the amplitude of the tides generated by the moon, and those vary on close to the same period. Tidal force is proportional to rM/R^3, where r is the radius of the planet experiencing the tide, M is the mass of the body creating the tide, and R is the distance between them. Something like Gliese 581c is 6.8 times the mass of Earth, and assuming comparable density, would have r = 1.9 times the Earth. The star Gliese 581 is 0.3 times the mass of our Sun, and the planet orbits at R = 0.07 that of earth. So we have 1.9 * 0.3 / 0.07^3, a star-induced tidal force on Gliese 581c of more than 1600 times that of the Sun on Earth, and more than 800 times that of the Moon on Earth. So, while large moons near a planet might be uncommon, those aren't the only sources of tides comparable to those on Earth.163.116.145.92 17:00, 2 September 2025 (UTC)

As we're still mentioning Pluto-Charon, just need to properly point out that (regardless of planethood), its moon is so good at causing tides, on Pluto's actual hard-mass, that Pluto would now not experience cyclic tides anything like Earth's twice-daily version. It's perhaps a bit anthropocentrically-expdriential, but we could say that Earth's sweet-spot of tides lies between those of moons too small+distant to create notable tides and those so large+near that they've dominated the 'planet' into a situation where tides are meaningless. (Ditto other twin-dwarf-planets out there.)

We also have an oceanic, but land-studded, surface; pretty much without precedent on any other body that we know sufficient information about. Tides on Titan (by Saturn, i.e. planet-on-moon) can't do quite so much with its methane 'lakes', Tides on Europa (by Jupiter) can at most flex the icy shell over its substantial water-ocean with no 'land' to flood/reveal. Which might well each produce interesting alternative tide-related effects, to the tides as we best known them, but very much not the same thing. (Titan, in particular, seems to have possible dominating 'seasonal' transference of liquid from hemisphere to hemisphere, but not by a tide-type mechanism. Any nominal Titanians would perhaps be used to that change of local shorelines, but still be mystified/surprised by our tides. What potential Europans might think is even harder to say, as we have even less unambiguous understanding about what their home-experience is, like if there's any 'spires' of base rock that reach significantly up the local column of liquid-water to make the icy shell movement a notable effect. And our world would already be considered strange enough to their 'eyes'.) 82.132.238.108 18:24, 3 September 2025 (UTC)
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