explain xkcd - User contributions [en]

3135: Sea Level

2025-08-30T18:46:56Z

2600:387:4:803:0:0:0:A0: /* Explanation */ aerobiome - affects clouds, might affect lightning

{{comic
| number = 3135
| date = August 29, 2025
| title = Sea Level
| image = sea_level_2x.png
| imagesize = 321x453px
| noexpand = true
| titletext = They're up there with coral islands, lightning, and caterpillars turning into butterflies.
}}

==Explanation==
{{incomplete|This page was created by A FRIENDLY TIDAL WAVE. Don't remove this notice too soon.}}
In this comic [[Cueball]] is wondering aloud what happened to an island, possibly a {{w|sandbar}} that was visible earlier, but is not anymore. [[Megan]] explains to him that the {{w|tide}} has gone up, and thus the island is underwater. She then tells him about what causes tides, namely the {{w|Moon}}, which is large enough and orbits close enough to Earth to gravitationally affect the waters of our oceans, causing them to go up and down daily. Similarly to [[2809: Moon]], Cueball is amazed at the oddity of our planet having a natural satellite large enough to affect our oceans, and Randall seems to be as well, as the caption says that if the Moon didn't exist, it would sound extremely outlandish and sci-fi-like.

{{w|Tide|Tides}} on Earth would indeed seem strange and unusual to a visitor from almost anywhere else in the universe, along with other features such as the large expanses of liquid water and the oxygen-rich atmosphere that was created by biological activity (photosynthesis). Not to mention the biological activity. As stated in the comic, tides on Earth are largely the result of a single large moon (''the'' Moon) orbiting the Earth at a relatively short distance. This configuration is {{w|List_of_natural_satellites|unique within our solar system}}: Mercury and Venus have no moons, Mars (smaller than Earth) has two, and each of the remaining planets has several. If the Moon were absent from Earth orbit, tidal amplitudes would be much less, because the Sun's gravity has much less influence on Earth's liquid water masses than the Moon does. Also, if more than one moon were present, or the moon(s) present had much less mass, or the moon(s) present had more distant orbits, tides would have less amplitude, and, in the case of multiple moons, would have much more complex (and possibly difficult to predict) periodicity.

The overall effect of the Moon-dominated tides is that in marginally more than every 6.2 hours (at least for an ocean-dominated equatorial island, such as the comic appears to depict) something that was originally fully revealed by a retreating tide could now be covered be the next rising one. This effect is even more pronounced every 14.75 days, as the Moon and Sun act together to create even higher high tides and lower low tides. Anyone used to the behaviour of the sea would probably be unsurprised by this, but someone normally resident more inland can quite easily be caught out, potentially arriving somewhere by the coast in the morning, to witness the sight of particularly low water levels, then by noon/early-afternoon finding that the seascape has changed and high-water has been reached.

The title text mentions other elements of life on Earth, saying they also seem as outlandish as tides, including {{w|coral islands}}, islands that build up from the skeletons or secretions of small animals, {{w|lightning}}, when electrical discharge creates bright light in the atmosphere, and the {{w|metamorphosis}} of {{w|butterflies}}, when caterpillars create a chrysalis and come out as butterflies.

Lightning is an element of life on Earth since it is thought to have produced chemicals which were important to life's origins. Ozone, produced by lightning, reduces the ultra-violet light reaching earths surface. This protection may have made it easier for life to transition out of the water. Microscopic life in the atmosphere (the aerobiome) affects cloud formation and weather, for instance, some bacteria promote the formation of ice crystals which contributes to rain. As such, the aerobiome may also affect lightning. Lightning is probably a common planetary phenomenon; within our solar system, it [https://science.nasa.gov/science-research/planetary-science/lightning-across-the-solar-system/ occurs on Jupiter, Saturn], and probably Uranus and Neptune. On Mars, lightning probably is generated by electric discharge in dust clouds. The evidence about possible lightning on Venus is mixed.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

:[Single frame, with Megan and Cueball standing at a coastline, with water and waves visible.]

: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.

{{comic discussion}}<noinclude>
[[Category: Comics featuring Cueball]]
[[Category: Comics featuring Megan]]
[[Category: Astronomy]]

Talk:3135: Sea Level

2025-08-30T18:04:58Z

2600:387:4:803:0:0:0:A0: The Life on Earth part is important

Holy crud empty page! F1RST P0ST! [[User:RadiantRainwing|RadiantRainwing]] ([[User talk:RadiantRainwing|talk]]) 01:38, 30 August 2025 (UTC)
:2038: Last of the original Star Wars cast dies. <span class="nowrap">—megan [[user talk:megan|talk]] [[special:contribs/megan|contribs]]</span> 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. [[User:RadiantRainwing|RadiantRainwing]] ([[User talk:RadiantRainwing|talk]]) 02:50, 30 August 2025 (UTC)
:::Alright, sorry, just realized what you did is a reference to [[493: Actuarial]]. Sorry about that! [[User:RadiantRainwing|RadiantRainwing]] ([[User talk:RadiantRainwing|talk]]) 02:53, 30 August 2025 (UTC)

This one is related to the [https://xkcd.com/2809/ Moon] comic. [[User:Pgn674|Pgn674]] ([[User talk:Pgn674|talk]]) 01:39, 30 August 2025 (UTC)
:Clearly. [[User:RadiantRainwing|RadiantRainwing]] ([[User talk:RadiantRainwing|talk]]) 01:49, 30 August 2025 (UTC)
::it's almost a repeat. is he running out of ideas? [[user:lett‪herebedarklight|raeb]] 09:54, 30 August 2025 (UTC)
:::Hope not. [[User:RadiantRainwing|RadiantRainwing]] ([[User talk:RadiantRainwing|talk]]) 16:56, 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. --[[User:PRR|PRR]] ([[User talk:PRR|talk]]) 02:41, 30 August 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. [[Special:Contributions/2600:387:4:803:0:0:0:A0|2600:387:4:803:0:0:0:A0]] 18:04, 30 August 2025 (UTC)

3135: Sea Level

2025-08-30T17:54:03Z

2600:387:4:803:0:0:0:A0: /* Explanation */ these are elements of life on earth

{{comic
| number = 3135
| date = August 29, 2025
| title = Sea Level
| image = sea_level_2x.png
| imagesize = 321x453px
| noexpand = true
| titletext = They're up there with coral islands, lightning, and caterpillars turning into butterflies.
}}

==Explanation==
{{incomplete|This page was created by A FRIENDLY TIDAL WAVE. Don't remove this notice too soon.}}
In this comic [[Cueball]] is wondering aloud what happened to an island, possibly a {{w|sandbar}} that was visible earlier, but is not anymore. [[Megan]] explains to him that the {{w|tide}} has gone up, and thus the island is underwater. She then tells him about what causes tides, namely the {{w|Moon}}, which is large enough and orbits close enough to Earth to gravitationally affect the waters of our oceans, causing them to go up and down daily. Similarly to [[2809: Moon]], Cueball is amazed at the oddity of our planet having a natural satellite large enough to affect our oceans, and Randall seems to be as well, as the caption says that if the Moon didn't exist, it would sound extremely outlandish and sci-fi-like.

{{w|Tide|Tides}} on Earth would indeed seem strange and unusual to a visitor from almost anywhere else in the universe, along with other features such as the large expanses of liquid water and the oxygen-rich atmosphere that was created by biological activity (photosynthesis). Not to mention the biological activity. As stated in the comic, tides on Earth are largely the result of a single large moon (''the'' Moon) orbiting the Earth at a relatively short distance. This configuration is {{w|List_of_natural_satellites|unique within our solar system}}: Mercury and Venus have no moons, Mars (smaller than Earth) has two, and each of the remaining planets has several. If the Moon were absent from Earth orbit, tidal amplitudes would be much less, because the Sun's gravity has much less influence on Earth's liquid water masses than the Moon does. Also, if more than one moon were present, or the moon(s) present had much less mass, or the moon(s) present had more distant orbits, tides would have less amplitude, and, in the case of multiple moons, would have much more complex (and possibly difficult to predict) periodicity.

The overall effect of the Moon-dominated tides is that in marginally more than every 6.2 hours (at least for an ocean-dominated equatorial island, such as the comic appears to depict) something that was originally fully revealed by a retreating tide could now be covered be the next rising one. This effect is even more pronounced every 14.75 days, as the Moon and Sun act together to create even higher high tides and lower low tides. Anyone used to the behaviour of the sea would probably be unsurprised by this, but someone normally resident more inland can quite easily be caught out, potentially arriving somewhere by the coast in the morning, to witness the sight of particularly low water levels, then by noon/early-afternoon finding that the seascape has changed and high-water has been reached.

The title text mentions other elements of life on Earth, saying they also seem as outlandish as tides, including {{w|coral islands}}, islands that build up on top of coral reefs out of coral detritus and other natural materials, {{w|lightning}}, when electrical discharge creates bright light in the atmosphere, and the {{w|metamorphosis}} of {{w|butterflies}}, when caterpillars create a chrysalis and come out as butterflies.

Unlike coral islands and metamorphosis, which, so far as humans know, are unique to Earth. Lightning is probably a common planetary phenomenon; within our solar system, it [https://science.nasa.gov/science-research/planetary-science/lightning-across-the-solar-system/ occurs on Jupiter, Saturn], and probably Uranus and Neptune. On Mars, lightning probably is generated by electric discharge in dust clouds. The evidence about possible lightning on Venus is mixed.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

:[Single frame, with Megan and Cueball standing at a coastline, with water and waves visible.]

: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.

{{comic discussion}}<noinclude>
[[Category: Comics featuring Cueball]]
[[Category: Comics featuring Megan]]
[[Category: Astronomy]]