explain xkcd - User contributions [en]

2935: Ocean Loop

2026-05-16T01:23:14Z

2603:6011:1F0:2A40:C522:C804:9042:7B68: /* Explanation */

{{comic
| number = 2935
| date = May 20, 2024
| title = Ocean Loop
| image = ocean_loop_2x.png
| imagesize = 317x286px
| noexpand = true
| titletext = I can't believe they wouldn't even let me hold a vote among the passengers about whether to try the loop.
}}

==Explanation==
Upon hearing the term "ocean loop", many people think of horizontal {{w|ocean gyre}}s or {{w|ocean currents}}. This comic illustrates a vertical, rather than horizontal, ocean loop.

The comic shows a large construction, rising out of the sea to dwarf a nearby cruise ship. It involves a submerged water-jet sending water up out of the surface and round a rollercoaster-loop-like water-flume trough. The scale is such that it seems that the ship, once caught in the necessarily powerful stream of water, is also intended to be propelled around the inverting loop before "safely" exiting at the other side.

The design is reminiscent of the infamous 'Cannonball Loop' water slide - a [https://www.latimes.com/travel/themeparks/la-trb-action-park-looping-water-slide-20150305-story.html fully-functional] water slide complete with a loop-de-loop - which was one of many famously dangerous rides and attractions at Action Park; a theme park in New Jersey that became iconic for its blithe disregard for human safety, and the numerous accidents and deaths at the park as a result. This [https://youtu.be/P14W1pRRK9Y video], among other potentially dangerous water slides, shows 2 such loop-de-loop water slides ([https://youtu.be/P14W1pRRK9Y?t=487 1] and [https://youtu.be/P14W1pRRK9Y?t=522 2]). Apart from various other issues regarding {{What If|43|large "loop-de-loops"}}, the stream of water required to maintain this setup would be {{w|Entrainment (hydrodynamics)|acting upon the nearby water}} and so the nearby ship is probably already close enough to be drawn into the loop (with the best option left being to deliberately steer into it, rather than risk being swept uncontrollably into the structure), assuming that it isn't already caught in the tug of the water-jet's inward flow.

Even assuming a "successful" loop (the stresses and rotation inflicted by the loop are likely beyond the design limits of such a vessel), the emergence back into the ''relatively'' calm and stationary waters beyond the exiting outflow would be a severe challenge to navigation. On the positive side, due to the nature of buoyancy, if the loop structure itself is capable of withstanding the force of the water being forced round it then it ''should'' be equally capable of withstanding the passage of the ship, unlike an impromptu rail-based loop which might stand up on its own but then shake itself apart when the first carriage is sent around it.

Not only would there be problems for the engineers, ship and navigators, the "ride" wouldn't be pleasant for the ship's passengers in any way. Many of the passengers would suffer extreme injuries from the changes of velocity (up to 370km/h or 230mph based on a loop radius of 3 x ship length) and rotation (unlike {{w|rollercoasters}}, or even airplanes during simple take-off and landing, passengers aren't normally strapped down). It is possible that the initial extreme undercurrent would capsize the ship. Depending upon where in the ship you were, the centripetal forces and the ship's rotation may not match for all passengers, forcing anyone not properly secured out towards the bow or stern. As well as the passengers, this also is relevant to all unsecured items (e.g. knives and forks would go flying off tables), as well as the dangers of breakable glass, liquids and many other dangerous objects which could create hazards even (or particularly) against those who have strapped themselves down to prevent their own movement through the ship.

Because of all these safety concerns, the caption, "[[:Category: Banned from conferences|I don't know why the cruise line fired me]]", suggests that someone in the company realized this would not be a good idea (and they were {{What If|43|probably right}}), and shut down the concept. However, the title text, "I can't believe they wouldn't even let me hold a vote among the passengers about whether to try the loop", implies that the narrator (whether because they simply hadn't thought it through properly, or they are [[Black Hat|someone with a sadistic nature]]) actually managed to get as far as building this loop and having a ship ready to try it. Considering the vast budget that would have been required to realise this concept, it appears that oversight at the cruise company is not what it could be, and perhaps others should be in line for firing as well (if it has not already bankrupted the company).

When those in charge of the ship objected to sailing into this loop, the builder attempted to get around this by having the passengers vote on it. Presumably worried that opening the decision-making process to the passengers might favor the exciting risk over the well-founded reason of the staff, those in charge put a stop to that too. Cruise ships generally don't function as democracies, even outside of absurd situations such as the one depicted.

In the later comic [[3220: Rotational Gravity]], the same Cueball is fired by another cruise line for a similarly bad idea where he rotates the ship around to create artificial gravity. The title text, just as here, complains about the decision of the cruise line, and with explicit reference to the scenario in this comic.

==Transcript==

:[A cruise ship approaches an enormous loop-de-loop flume from the left. A large jet of water is being propelled into the loop-de-loop by an enormous submerged hot tub style jet. The loop-de-loop, that looks like a waterslide, and the structure housing the jet are connected to a slightly inclined seabed, higher on the right.]

:[Caption below the panel:]
:I don't know why the cruise line fired me.

==Trivia==
* '''This trivia section was created by a BOT'''
* The [[:File:ocean_loop_huge.png|standard size]] image was uploaded with a resolution/size larger than the supposed 2x version.
* This may have been an error.
* At the time of posting, the image was ''massive'', 4760 x 4295 pixels.

{{comic discussion}}

2809: Moon

2026-05-15T00:53:57Z

2603:6011:1F0:2A40:C522:C804:9042:7B68: /* Explanation */

{{comic
| number = 2809
| date = July 31, 2023
| title = Moon
| image = moon_2x.png
| imagesize = 313x402px
| noexpand = true
| titletext = I mean, it's pretty, but it doesn't really affect us beyond that. Except that half the nights aren't really dark, and once or twice a day it makes the oceans flood the coasts.
}}

==Explanation==
The {{w|Moon}} is a celestial body orbiting Earth, first formed approximately 4.5 billion years ago - about 50 million years after the initial formation of the solar system. As of the date of this comic, the Moon is still orbiting the Earth{{Citation needed}} at an average distance of approximately 384,400 kilometers, or about 238,900 miles.

This comic points out how weird it is to have such an enormous celestial body near to us. The Moon has a radius more than one quarter of Earth's, and is around one eightieth of Earth's mass, and is so close that major surface features are visible, even with the naked eye, and much more clearly visible with even a simple telescope. Celestial distances tend to be so large that only truly immense objects can be seen without magnification, and even those tend to appear only as points of light to the naked eye. The second nearest body of notable size, Venus, is approximately 46.576 million kilometers away at its closest. The fact that there's "another world" that's close enough that humanity has always been aware of it, but distant enough that it couldn't be reached until a space program was developed, is a striking feature of Earth that we take for granted, only because it's always been that way.

While it's not uncommon for planets to have orbiting moons, no other planet in the solar system has a moon that's so large, in relation to the planet. Of the other rocky planets, only Mars has moons, and the largest of those is only 14 miles across.

Pluto and Charon are closer in size than even the Earth/Moon system, but their center of mass lies outside of Pluto. As such, they would actually be considered a {{w|Double planet|double(/binary) planet pair}}, had Pluto not been redesignated as a "dwarf planet". There is the possibility a term such as "double dwarf planet" could be adopted, at some point, as "double minor planet" is sometimes already used for binary asteroid systems. The lesser bodies of the Pluto-Charon system may then even be considered as circumbinary moons.

Other than this, Earth is the only accepted planet we're currently aware of that has a satellite that's so visible from its surface.

The title text sarcastically claims that, other than being "pretty", the Moon doesn't impact us, then subverts it by mentioning substantial impacts it has on Earth. Having such a large satellite so close has impacts that we take for granted only because we're used to them, but if they hadn't always existed, they'd seem unbelievable. One is that, for half the lunar cycle, the moon reflects enough light to produce visibility at night. The other impact is tides, since the gravitational pull of the Moon is large enough to alter the surfaces of oceans, causing shorelines to shift on a daily cycle. The text mentions these dismissively, in a deliberate contrast with their huge significance. Moonlight alters the illumination cycle of the planet to a significant degree, which changes how both humans and other animals operate at night, especially before the advent of artificial lighting. Tides had major impacts on the development of life, continue to affect ecosystems, and play an essential role in our ability to interact with the oceans. If someone from a planet without such a large moon were to observe these impacts, they'd likely be shocked and amazed by them, but we barely notice them. Another impact is {{What If|26|leap seconds}}, since the gravity of the moon pulls on the earth, ever so slightly slowing the earth down

The comic [[3135: Sea Level]] is very similar to this one, but it starts with noting how strange and SciFi the tides on Earth are.

==Transcript==
:[Ponytail is pointing and looking up to the left, while Cueball behind her looks the same way.]
:Ponytail: That thing hanging in the sky is a second nearby world. It's close enough that you can see its surface as it passes overhead.
:Cueball: Wow. Isn't that ... weird?
:Ponytail: I dunno, it's just always been there.

:[Caption below the panel:]
:If it didn't exist, the moon would sound like such an outlandish sci-fi concept.

==Trivia==
*Earth's moon can be considered weird for additional reasons.
**It is by far the largest and most massive moon relative to the planet it orbits.
**Our moon is the only moon in the solar system to not have a proper name, not even a jumble of letters and numbers. In English it is simply given the proper name of "the Moon" (capitalized), being the ancient archetype for all other moons discovered since the time of Galileo, although it can also be described by other titles such as "Luna" (directly taken from from Latin mythology/astronomy).
**It is almost exactly the same apparent size as the Sun in the sky which at various times enables both total solar eclipses (for which it needs to be close/large) and annular ones (for which it must not be ''too'' close/large). Much earlier in history it was too close to do both and much later it will gradually drift too far away to do so – making it additionally a temporal coincidence that humanity gets to witness it as we are so used to seeing it.
**There are even a couple of hypotheses that think that to get protein shaped right for life, the tides were needed, and that to evolve for living on land the tides are needed.
{{comic discussion}}

[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Cueball]]
[[Category:Astronomy]]

3243: Crystal Gazing

2026-05-15T00:31:59Z

2603:6011:1F0:2A40:C522:C804:9042:7B68: /* Explanation */

{{comic
| number = 3243
| date = May 8, 2026
| title = Crystal Gazing
| image = crystal_gazing_2x.png
| imagesize = 281x397px
| noexpand = true
| titletext = 'Beyond that lies a vale of fire through which my vision cannot penetrate' is the kind of fun thing geologists, heliophysicists, and early universe cosmologists have a lot of opportunities to say.
}}

==Explanation==
A {{w|geochronology|geochronologist}} (someone who estimates the ages of rocks, fossils, etc.) dressed up as a wizard (assuming that isn't how they normally dress) announces — in archaic language befitting his costume — that he has determined the {{w|age of the Earth}} by analyzing {{w|zircon}}. Zircon is a crystal with the formula ZrSiO<sub>4</sub>. During its formation it can incorporate uranium instead of zirconium in its crystal lattice, but cannot incorporate lead. The uranium then decays (via several intermediates) into lead. Thus a sufficiently old zircon crystal will contain some lead, allowing geologists to {{w|Detrital_zircon_geochronology|calculate its age}}. This method is especially reliable, since uranium-238 decays into lead-206 with a half-life of 4.5 billion years, while uranium-235 decays into lead-207 with a half-life of 0.7 billion years, allowing geologists to determine the age even where some lead was lost from the crystal. The geochronologist says he predicted the age of the Earth by gazing into the [[2776|crystal]], similar to the traditional fortune tellers' method of making predictions by gazing into crystal balls.

A prophet of doom is someone who predicts impending disasters, particularly the end of the world (and there are lots of ridiculous ways that [https://what-if.xkcd.com/archive/ could happen]). These prophets and predictions are especially common in fantasy media. The joke in the caption is that someone who can determine when the world began is just doing this in reverse, and that's what geochronologists are doing when they calculate the age of the Earth. His last line, "the beginning was nigh", is a reversal of the phrase stereotypically used by prophets of doom: "the end is nigh". This is in keeping with the idea represented in the comic.

The title text points out, in similar flowery language (in keeping with the character being a wizard from a {{w|J. R. R. Tolkien|Tolkienesque}} or {{w|Dungeons & Dragons}}-type world, where [https://tolkiengateway.net/wiki/Vales_of_Anduin vales] are often part of the geographical language), that many areas of physical sciences have a limiting horizon to their ability to study their subject. {{w|Geologists}} are limited by the early molten history of the Earth, in that most of the crust has been recycled back into the molten mantle at some point, and the mantle acts as a physical 'vale of fire' for the core — each limiting what can be studied. Similarly, {{w|heliophysicists}} have difficulty knowing what happens within the Sun due to the physical 'vale of fire' that is the Sun's {{w|photosphere}}. Finally, {{w|cosmologists}} have difficulty studying the universe beyond the {{w|recombination (cosmology)|recombination epoch}}, beyond which the universe is shielded by a 'vale of fire' (the {{w|surface of last scattering }} from which photons of the {{w|cosmic microwave background}} escape) from electromagnetic observation — both in time (investigating the early universe) or in space (investigating beyond the event horizon of the visible universe).

[[:Category:Dungeons & Dragons|D&D]] is a recurring topic on xkcd.

==Transcript==

:[A character in wizard garb with long beard and hair talks to Cueball. To the left of the wizard is a poster with illegible text over a graph with two lines intersecting.]

:Wizard: By gazing into my crystals of zircon, I have divined the date of the hour of fire marking the limit of this world's existence.
:Wizard: 4.54 billion years ago, the beginning was nigh!

:[Caption below comic:]
:Geochronologists are just reverse prophets of doom.

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