explain xkcd - User contributions [en]

3135: Sea Level

2025-09-01T23:53:27Z

Darrylnoakes: Undo revision 385735 by Darrylnoakes (talk)

{{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.

The way Megan phrased the last sentence, that the Moon orbits close to the surface of the Earth could also add to Cueball's outburst. Whereas the Moon is comparatively close to Earth in the solar system, not many Earthlings would describe the Moon as being close to Earths surface. Especially when they actually find out how far it is away from Earth. Most pictures depicting both the Earth and the Moon simultaneously cheat to make it possible to see them near each other, while still being able to se surface features. If the system is seen from the side, with the Moon and Earth furthest from each other, a picture shown on a computer screen would make both of them rather small. (If [https://youtu.be/Bz9D6xba9Og Earth is a basketball and the moon a baseball], the baseball should be placed around 8 meters from the basketball to represent the correct distance.)

{{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 (far, far smaller) moons, and each of the remaining planets have several, but are comparatively insignificant, individually. There are four moons in the solar system bigger than the Moon, and a few more almost as big, but they all orbit gaseous planets much larger than Earth, amongst many other minor moons, and thus have far less practical tidal influence upon their respective parent planets. The ''twelfth'' largest known moon is Charon, 12.2% the mass of the dwarf planet Pluto, that it it orbits. (The Moon has 1.2% the mass of Earth, by far the largest such ratio amongst the fully-fledged planetary moons, and is also about 20 times further away in comparison). Its much larger tidal effect has actually resulted in mutual {{w|tidal locking}} so that Charon now orbits only above the same part of Pluto and could not create Earth-like tide cycles. That is, even ''if'' there were plutonian seas quite like ours, another required feature that is effectively unique to the Earth-Moon scenario.

The Sun also influences Earth's tide, as might be expected. If the Moon were absent from Earth orbit, however, tidal amplitudes would be about a third of their current values. The Sun's gravity has about half the influence on Earth's liquid water masses than the Moon does, as the relative distances involved mean that the solar gravity gradient across different parts of the Earth and its oceans is far less. Also, if more than one moon were present, 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 there being multiple moons, they would have much more complex (and possibly difficult to predict) periodicity. If the multiple moons had {{w|orbital resonance}}, the periodicity would be relatively straightforward.

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==

:[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.

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

3135: Sea Level

2025-09-01T23:51:22Z

Darrylnoakes:

{{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.

The way Megan phrased the last sentence, that the Moon orbits close to the surface of the Earth could also add to Cueball's outburst. Whereas the Moon is comparatively close to Earth in the solar system, not many Earthlings would describe the Moon as being close to Earths surface. Especially when they actually find out how far it is away from Earth. Most pictures depicting both the Earth and the Moon simultaneously cheat to make it possible to see them near each other, while still being able to se surface features. If the system is seen from the side, with the Moon and Earth furthest from each other, a picture shown on a computer screen would make both of them rather small. (If [https://youtu.be/Bz9D6xba9Og Earth is a basketball and the moon a baseball], the baseball should be placed around 8 meters from the basketball to represent the correct distance.)

{{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 (far, far smaller) moons, and each of the remaining planets have several, but are comparatively insignificant, individually. There are four moons in the solar system bigger than the Moon, and a few more almost as big, but they all orbit gaseous planets much larger than Earth, amongst many other minor moons, and thus have far less practical tidal influence upon their respective parent planets. The ''twelfth'' largest known moon is Charon, 12.2% the mass of the dwarf planet Pluto, that it it orbits. (The Moon has 1.2% the mass of Earth, by far the largest such ratio amongst the fully-fledged planetary moons, and is also about 20 times further away in comparison). Its much larger tidal effect has actually resulted in mutual {{w|tidal locking}} so that Charon now orbits only above the same part of Pluto and could not create Earth-like tide cycles. That is, even ''if'' there were plutonian seas quite like ours, another required feature that is effectively unique to the Earth-Moon scenario.

The Sun also influences Earth's tide, as might be expected. If the Moon were absent from Earth orbit, however, tidal amplitudes would be about a third of their current values. The Sun's gravity has about half the influence on Earth's liquid water masses than the Moon does, as the relative distances involved mean that the solar gravity gradient across different parts of the Earth and its oceans is far less. Also, if more than one moon were present, 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 there being multiple moons, they would have much more complex (and possibly difficult to predict) periodicity. If the multiple moons had {{w|orbital resonance}}, the periodicity would be relatively straightforward.

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==

:[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.

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

3029: Sun Avoidance

2024-12-26T03:02:37Z

Darrylnoakes: /* Explanation */

{{comic
| number = 3029
| date = December 25, 2024
| title = Sun Avoidance
| image = sun_avoidance_2x.png
| imagesize = 311x403px
| noexpand = true
| titletext = C'mon, ESA Solar Orbiter team, just give the Parker probe a LITTLE nudge at aphelion. Crash it into the sun. Fulfill the dream of Icarus. It is your destiny.
}}

==Explanation==
{{incomplete|Created by a BOT FLYING CLOSE TO THE SUN. Do NOT delete this tag too soon.}}
This comic shows the end of a table of human missions, both terrestrial and space-based, ranked by how far they stayed away from the {{w|Sun}}.

The vast majority of these missions have been on Earth, the Moon, or in Earth orbit, so about 157 million km from the Sun. There have also been 8 probes sent to the outer planets; they would be at the top of the list if this were shown.

Most space probes try not to get too close to the Sun, because it's extremely hot{{cn}} and their equipment (especially the electronics) are not designed to work at such temperatures and radiation levels. If they have to venture into the inner Solar System, either because the mission is to an inner planet or other body there or to use {{w|gravity assist}} of Mercury or Venus, mission planners will design the trajectory so it remains tens of millions of kilometers away from the Sun, to minimize its impact.

This comic was posted the day after December 24, 2024, when the {{w|Parker Solar Probe}} made its closest approach to the Sun. As a result, it has set a new record for the worst failure in solar avoidance. This mission needs to be really close to the Sun so it can make close-up analysis of its corona and magnetic field. It has been engineered with special solar shields to protect it from the extreme heat and radiation.

The next closest mission that's still in operation (the {{w|Helios (spacecraft)|Helios}} missions ended in 1985) is the {{w|European Space Agency}}'s {{w|Solar Orbiter}}. The title text jokes that it should nudge Parker so it crashes into the Sun, so the Solar Orbiter will retake the lead among missions in operation. This would be difficult, since at Parker's aphelion (furthest distance from the Sun in its orbit) it's still only about 7 million km, 35 million km from Solar Orbiter's orbit (and the probes would be much further apart if they're not on the same side of the Sun at the time). It references {{w|Icarus}}, a character from Greek mythology who flew too close to the Sun using wings crafted by his father {{w|Daedalus}}, and fell into the sea because the beeswax in the wings melted.

==Transcript==
{{incomplete transcript|There should not be a wikitable there, just do it with Transcript-style description. Do NOT delete this tag too soon.}}

:[A table with an ellipsis indicating a large number of rows being omitted, followed by seven substantive rows. All of the rank numbers except "1." are cut off at the left, with the leftmost digit being a partially cut-off "4", except that on the fourth substantive row, the "4" is shown in full with a cut-off "2" to its left. Hence, these ranks actually represent numbers with at least eight digits.]
:{| class="wikitable"
|-
!Rank
!Mission
!Sun Nearest Miss
|-
|1. . . . ... . . . 4303857.
|colspan="2"|[All other expeditions in human history]
|-
|4303858.
|Mariner-10
|69.0 million km
|-
|4303859.
|Helios 1
|46.4 million km
|-
|4303860.
|BepiColombo
|45.8 million km
|-
|24303861.
|Messenger
|45.3 million km
|-
|4303862.
|Solar Orbiter
|43.8 million km
|-
|4303863.
|Helios 2
|43.3 million km
|-
|4303864.
|Parker
|6.17 million km
|-
|}

:[Caption below the table:]
:Congratulations to the Parker Solar Probe for setting a new record for "Worst Job Avoiding the Sun."
{{comic discussion}}

[[Category:Space probes]]

Talk:2634: Red Line Through HTTPS

2022-06-18T04:28:41Z

Darrylnoakes:

HTTPS was standardized in 2000 or so, so 2015 is quite a stretch for a site to not use it because the site was last updated before HTTPS was widely available.
With pretty much any browser now, a red line through HTTPS means that the site _is using HTTPS_, but it is _not trusted by the browser_ (due to e.g. the certificate being self-signed or expired).
[[User:Darrylnoakes|Darrylnoakes]] ([[User talk:Darrylnoakes|talk]]) 04:28, 18 June 2022 (UTC)