explain xkcd - User contributions [en]

3219: Planets and Bright Stars

2026-03-15T07:26:03Z

Alexei Kopylov: /* Explanation */ cannot distinguish the Sun from planets

{{comic
| number = 3219
| date = March 13, 2026
| title = Planets and Bright Stars
| image = planets_and_bright_stars_2x.png
| imagesize = 374x265px
| noexpand = true
| titletext = An old astronomer trick for distinguishing the Sun from other stars is to take multiple photos a few minutes apart and overlay them, making the Sun stand out due to its high proper motion.
}}

==Explanation==
{{incomplete|This page was created by a GLOWING SPACE DOT. Don't remove this notice too soon.}}
This comic features a chart of a handful of cosmic objects (all planets or stars) and what they look like in the night sky. The joke is that they are all nearly identical dots, making the chart almost useless.

The comic shows an identification chart for some of the planets and bright stars visible at night from Earth. Bright shiny objects are often confused by people without astronomical experience, and the chart is supposed to make this easier by placing them adjacent to one another to easily see the differences. The joke is that the pictures look almost identical to one another, and therefore the chart isn't helpful at all.

The real way of distinguishing these objects is by their location in the sky. Stars can be found by using constellations, which are an apparent pattern of bright stars that make different regions of the sky distinguishable from one another. The planets can be distinguished by not belonging to the constellations, and further differentiated by their color, brightness, and movement relative to the stars (on the scale of weeks or months).

Selected objects do indeed look similar to one another in reality, but not identical. Some of them (in particular, the star Betelgeuse and the planet Mars) have a distinct reddish color, which can be seen in good conditions. The brightness is also different, and it can serve as a guide, but it's difficult to precisely judge brightness by eye, and the planets don't have a constant brightness over time. The differences are actually visible in the comic to a degree - e.g., the spots for Venus and Jupiter are slightly larger than the others - but they're subtle enough to not recognize at first glance.

Each 'object' also has a color, albeit ''extremely'' desaturated (very nearly white). If deliberately exaggerated, the comic's planets and suns are all notably non-white.
*Planets:
**{{w|Venus}}: the yellowy-orange hue of its cloud layers.
**{{w|Mars}}: the red of its surface (given more muted saturation, in the comic, for the joke to work?).
**{{w|Jupiter}}: the general orange hue of its combined cloud layers.
**{{w|Saturn}}: a more 'peachy' orange of its clouds (no obvious hint of its ring system).
**{{w|Mercury (planet)|Mercury}}: a yellow surface (not typically noted, in true-hue images, perhaps artistic licence from its proximity to the Sun).
*Stars
**{{w|Sirius}} (binary system, primarily a {{w|main sequence}} A-type star): light blue.
**{{w|Procyon}} ({{w|subgiant}} F-type star): more light green, or yellowy-blue/cyan.
**{{w|Antares}} (M-type star, {{w|red supergiant}}): orange.
**{{w|Altair}} (A-type, main sequence): light blue.
**{{w|Betelgeuse}} (M-type, {{w|red supergiant}}): relatively dark red (usually visible as such in real eyes-only observations).
**{{w|Vega}} (A-type, main sequence): light blue.
**{{w|Polaris}} (ternary star-system, dominant member being an F-type {{w|yellow supergiant}}): ''extremely'' unsaturated cyan.

Using even a small telescope would make it easy to distinguish the planets by their brightness, size, and surface features. Additionally, using a spectroscope would allow for a measurement of the star's spectrum, which coupled with its brightness would allow an astronomer to distinguish the mentioned stars.

The title text suggests a "trick" for recognizing the Sun among other stars, suggesting measuring its {{w|proper motion}} (a measure of the change in apparent position against the more distant 'fixed' background of stars, as an {{w|angular rate}} to specify some angle per time) by overlaying several images, a similar principle to the {{w|blink comparator}}. This does indeed differentiate it from other stars, but there are much easier methods, such as its extreme brightness and large angular size.{{cn}} A disadvantage of this method is that it distinguishes the Sun from other stars, but it cannot distinguish the Sun from planets. It is also completely unnecessary, except during a {{w|solar eclipse}}, because stars are not usually visible during the day, when the Sun is out. Additionally, "proper motion" is a term usually not used for the Sun.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}
:[Caption: Planets and bright stars identification chart:]
:[A 4x3 grid of planets and stars are displayed on a black background. White text below dots of light caption which object it is. Planets and stars are represented by almost-identical slightly fuzzy dots of white light.]
:Venus
:Mars
:Jupiter
:Saturn
:Mercury
:Sirius
:Procyon
:Antares
:Altair
:Betelgeuse
:Vega
:Polaris

{{comic discussion}}<noinclude>

[[Category:Comics with inverted brightness]]
[[Category:Comics with color]]
[[Category:Astronomy]]

Talk:3200: Chemical Formula

2026-01-28T23:45:55Z

Alexei Kopylov: Amount of americium

I'm disappointed that it wasn't scrollable. [[Special:Contributions/2001:41D0:8:5062:0:0:0:1|2001:41D0:8:5062:0:0:0:1]] 20:20, 28 January 2026 (UTC)
:+1 And funny to think that the universe contains less than a few hundred mol of Americium. --[[Special:Contributions/2001:16B8:CC03:E100:8552:6543:7CF4:9AE7|2001:16B8:CC03:E100:8552:6543:7CF4:9AE7]] 20:57, 28 January 2026 (UTC)
If anyone's interested in an accessible resource for getting more data like this, may I suggest https://ptable.com/#Properties/Abundance/Universe (which I believe derives data from IUPAC sources) [[User:Dextrous Fred|Dextrous Fred]] ([[User talk:Dextrous Fred|talk]]) 20:37, 28 January 2026 (UTC)
surprised to see so much Astatine, he himself declared, that stuff doesnt want to exist so I expected yet a few powers of ten less

This does make me curious: how would neutronium be represented in a chemical formula? Or would it be? My impression is it kind of exists 'outside' of chemistry... -Kalil [[Special:Contributions/147.81.60.76|147.81.60.76]] 21:12, 28 January 2026 (UTC)
:Neutron stars would be represented with '''n''' with various mass numbers. And there are no more than 1 mmol (6.02214076×1020) of neutron stars. [[Special:Contributions/2001:4C4E:1C09:EC00:7932:264E:A9E0:8ED0|2001:4C4E:1C09:EC00:7932:264E:A9E0:8ED0]] 21:38, 28 January 2026 (UTC)
What about adding mass numbers? For example, most of the hydrogen is 1H, with small amounts of 2H and trace amounts of 3H. [[Special:Contributions/2001:4C4E:1C09:EC00:7932:264E:A9E0:8ED0|2001:4C4E:1C09:EC00:7932:264E:A9E0:8ED0]] 21:38, 28 January 2026 (UTC)
:Oh look, it's the 3200th comic! Yay I guess! --'''''[[User:DollarStoreBa'al|DollarStoreBa'al]][[User Talk:DollarStoreBa'al|Converse]]''''' 22:46, 28 January 2026 (UTC)

An unregistered user (198.48.180.159) added a note that the chemical formula "C11H15NO2" (i.e. C11H15NO2) "has 302 registered isomers". I don't know the source for that number or where those isomers are registered. (It's the formula for MDMA, which is, as noted, "not good to eat".) Would that be the CAS registry? [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 23:20, 28 January 2026 (UTC)

10^26 atoms of americium is about 40 kg. But it looks like humans produced tons of americium: https://isis-online.org/uploads/isis-reports/documents/np_237_and_americium.pdf
. If there are other civilizations in the observable Universe, then the amount of americium in the Universe is even higher. So I guess the formula counts only naturally produced elements. But even then it seems underestimated. [[User:Alexei Kopylov|Alexei Kopylov]] ([[User talk:Alexei Kopylov|talk]]) 23:45, 28 January 2026 (UTC)

2946: 1.2 Kilofives

2024-06-15T18:58:23Z

Alexei Kopylov:

{{comic
| number = 2946
| date = June 14, 2024
| title = 1.2 Kilofives
| image = 1 2 kilofives 2x.png
| imagesize = 256x342px
| noexpand = true
| titletext = 'Oh yeah? Give me 50 milliscore reasons why I should stop.'
}}

==Explanation==
{{incomplete|Created by 83.333... millidozen BOTS - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

Abraham Lincoln's {{w|Gettysburg Address}} features the phrase "four {{wiktionary|score#Noun|score}} and seven"‍ to refer to 87 (a score refers to the number 20). That is akin to French where 87 is written ''quatre-vingt sept'', which literally translates as ''four twenties [and] seven''. Cueball (possibly representing [[Randall]]) likes the idea of unusual ways to refer to numbers so he uses a metric prefix to state the population of the town.

{{w|Metric prefix|Metric prefixes}} can be added to a unit to scale up or down its magnitude; for example, {{w|Kilo-|"kilo-"}} increases the unit's magnitude by a factor of 1,000, so a kilometer is as long as 1,000 meters. Although metric prefixes can be added to all sorts of units, they're not ordinarily added to number words to modify their magnitude{{Citation needed}}. The expression "kilofive" to mean 5,000 is therefore unusual. Such modification of magnitude can be accomplished by adding the corresponding word, for example in this case "five thousand".

Taking "kilofive" to be a unit meaning 5,000, the population of East Hills, 6,000, can therefore be expressed as 1.2 kilofives.

In the title text, Cueball has apparently annoyed White Hat with his confusing expressions of numbers, but he doubles down, now directly including the word "score". 50 milliscore, or 50 × 1&frasl;1000 × 20, would be equal to 1.

The comic might refer to the village of {{w|East Hills, New York}}. As of the 2020 census, it had a population of 7,284, or 1.214 kilosixes.

==Trivia==
In Roman numerals, {{w|Roman_numerals#Large_numbers|symbols can be added to numerals}} to denote orders of magnitude. In this system, 1,000 might be written as "CIↃ". This rough pattern of marks, as typically chisled or impressed into wax by a stylus, would later be refined and expressed in the not dissimilar shape of the "M" as most often seen these days to represent the thousands value in dates/etc. Alternately "I" (nominally '1') could be given a bar above it, as would any other such numerals involve in that expression, to indicate the value being denoted being of the higher order.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[Cueball, with his palm raised, is talking to White Hat. There is a sign on the ground in the background.]
:Cueball: It's a pretty small town—the population is just 1.2 kilofives.
:[The sign reads:]
:Welcome to
:<big>East Hills</big>
:Pop. 6,000

:[Caption below the panel:]
:I don't know why Abraham Lincoln should be the only one who gets to come up with weird ways to say normal numbers.

:[Title text:]
:'Oh yeah? Give me 50 milliscore reasons why I should stop.'

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring White Hat]]
[[Category:Comics featuring real people]]

Talk:2042: Rolle's Theorem

2018-09-06T05:49:54Z

Alexei Kopylov:

Now we wait for https://en.wikipedia.org/wiki/Munroes_theorem. [[Special:Contributions/172.69.54.165|172.69.54.165]] 15:51, 5 September 2018 (UTC)
:Can't wait to see how long it takes to remove the article. [[User:Linker|Linker]] ([[User talk:Linker|talk]]) 17:05, 5 September 2018 (UTC)

:Proposed ideas for Munroe's Law:
::- Any seemingly simple idea will be difficult to prove; the simpler it seems, the harder the proof.
::- Any proof which is discovered by a layperson will have been previously discovered by an expert (or an "expert") in the field.
:[[User:Rajakiit|Raj-a-Kiit]] ([[User talk:Rajakiit|talk]]) 17:57, 5 September 2018 (UTC)

I feel like Euclid beat Randall to the punch here, a couple millennia. [[Special:Contributions/162.158.155.146|162.158.155.146]] 16:54, 5 September 2018 (UTC)

I don't see that Thales has proven Randall's theorem. Do not to be confused with {{w|Thales's theorem}}, that's about right angles. Maybe I'm blind or just dumb, but if so it has to be explained with more traceable background. I just believe that this diagonal is so trivial that even the ancient Greeks weren't engaged on a proof. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 21:38, 5 September 2018 (UTC)
* From {{w|Thales|Wikipedia}}: Other quotes from Proclus list more of Thales' mathematical achievements: "They say that Thales was the first to demonstrate that the circle is bisected by the diameter, the cause of the bisection being the unimpeded passage of the straight line through the centre." [[User:Alexei Kopylov|Alexei Kopylov]] ([[User talk:Alexei Kopylov|talk]]) 05:39, 6 September 2018 (UTC)
* On the other hand not all historian believe Proclus. But van der Waerden does: [https://books.google.com/books?id=HK3vCAAAQBAJ&pg=PA88#v=onepage&q&f=false]. [[User:Alexei Kopylov|Alexei Kopylov]] ([[User talk:Alexei Kopylov|talk]]) 05:49, 6 September 2018 (UTC)

Talk:2042: Rolle's Theorem

2018-09-06T05:49:13Z

Alexei Kopylov:

Now we wait for https://en.wikipedia.org/wiki/Munroes_theorem. [[Special:Contributions/172.69.54.165|172.69.54.165]] 15:51, 5 September 2018 (UTC)
:Can't wait to see how long it takes to remove the article. [[User:Linker|Linker]] ([[User talk:Linker|talk]]) 17:05, 5 September 2018 (UTC)

:Proposed ideas for Munroe's Law:
::- Any seemingly simple idea will be difficult to prove; the simpler it seems, the harder the proof.
::- Any proof which is discovered by a layperson will have been previously discovered by an expert (or an "expert") in the field.
:[[User:Rajakiit|Raj-a-Kiit]] ([[User talk:Rajakiit|talk]]) 17:57, 5 September 2018 (UTC)

I feel like Euclid beat Randall to the punch here, a couple millennia. [[Special:Contributions/162.158.155.146|162.158.155.146]] 16:54, 5 September 2018 (UTC)

I don't see that Thales has proven Randall's theorem. Do not to be confused with {{w|Thales's theorem}}, that's about right angles. Maybe I'm blind or just dumb, but if so it has to be explained with more traceable background. I just believe that this diagonal is so trivial that even the ancient Greeks weren't engaged on a proof. --[[User:Dgbrt|Dgbrt]] ([[User talk:Dgbrt|talk]]) 21:38, 5 September 2018 (UTC)
* From {{w|Thales|Wikipedia}}: Other quotes from Proclus list more of Thales' mathematical achievements: "They say that Thales was the first to demonstrate that the circle is bisected by the diameter, the cause of the bisection being the unimpeded passage of the straight line through the centre." [[User:Alexei Kopylov|Alexei Kopylov]] ([[User talk:Alexei Kopylov|talk]]) 05:39, 6 September 2018 (UTC)
* On the other hand not all historian believe Proclus. But wan der Waden does: [https://books.google.com/books?id=HK3vCAAAQBAJ&pg=PA88#v=onepage&q&f=false]. [[User:Alexei Kopylov|Alexei Kopylov]] ([[User talk:Alexei Kopylov|talk]]) 05:49, 6 September 2018 (UTC)

Talk:2042: Rolle's Theorem

2018-09-06T05:39:58Z

Alexei Kopylov:

2042: Rolle's Theorem

2018-09-05T16:15:35Z

Alexei Kopylov: about Thales

{{comic
| number = 2042
| date = September 5, 2018
| title = Rolle's Theorem
| image = rolles_theorem.png
| titletext = I mean, if it's that easy to get a theorem named for you ... "a straight line that passes through the center of a coplanar circle always divides the circle into two equal halves." Can I have that one? Wait, can I auction off the naming rights? It can be the Red Bull Theorem or the Quicken Loans Theorem, depending who wants it more.
}}

==Explanation==
{{incomplete|Go a little bit more into the explanation. Do NOT delete this tag too soon.}}

This comic references various theorems in calculus such as the {{w|Extreme value theorem}}, the {{w|Mean value theorem}}, and {{w|Rolle's theorem}}, which are intuitively obvious but harder to prove than they seem.

It is interesting to note that the theorem mentioned in the title text is already taken: even if this theorem is trivial, {{w|Proclus}} says that the first man who proved it was {{w|Thales of Miletus|Thales}}.

==Transcript==
:'''Rolle's Theorem'''
:''From Wikipedia, the Free Encyclopedia''

: Rolle's Theorem states that any real, differentiable function that has the same value at two different points must have at least one "stationary point" between them where the slope is zero.
: [Example graph of a downward-pointing curve - points "a" and "b" are x-intercepts, point "c" has a vertical line to the apex, and f'(c)=0 is drawn with a horizontal line.]

: Every now and then, I feel like the math equivalent of the clueless art museum visitor squinting at a pinting and saying "c'mon, my kid could make that."

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Math]]
[[Category:Wikipedia]]