explain xkcd - User contributions [en]

2954: Bracket Symbols

2024-07-04T16:34:46Z

TheBittern: Revise explanation for the Floor brackets: the comic references a mathematician rather than someone learning maths, so I feel this explanation is more appropriate.

{{comic
| number = 2954
| date = July 3, 2024
| title = Bracket Symbols
| image = bracket_symbols_2x.png
| imagesize = 592x569px
| noexpand = true
| titletext = ’"‘”’" means "I edited this text on both my phone and my laptop before sending it"
}}

==Explanation==
{{incomplete|Created by a ([{《"complicated function"》}]) - Please~~ change this comment when editing this page. Do *NOT* delete this tag too soon.}}
Brackets, also called parentheses, are typographical symbols used to delimit a section of text. Unlike most typographical symbols, brackets usually come in pairs, and the end bracket is typically the mirror image of the start bracket.

This comic shows a variety of (mostly) real bracket symbols, along with Randall's description.

{| class="wikitable"
|+Descriptions
|-
! Symbols
! Comic text
! Real use
! Explanation of the joke
|-
|()
|Regular parentheses for setting stuff aside
|The regular curved bracket is the most commonly used in literature, and typically denotes aside remarks that are relevant to, but not part of, a sentence (for example, a clarifying explanation). It is also frequently used in mathematical expressions and programming languages as a grouping operator, to force a particular order of evaluation.
|Randall explains, accurately, that these are regular parentheses. No joke yet.
|-
|[]
|Square brackets (more secure)
|In literature, square brackets often denote meta-textual information, such as glosses, omissions, translator and editorial notes. In mathematics, they are often used for {{w|Matrix (mathematics)|matrices}} or {{w|Interval (mathematics)|closed intervals}}. Sometimes they are used as outer parentheses for easier visual matching in complicated expressions. In programming languages, square brackets are commonly used as the indexing operator, with the index being placed inside the brackets. They may also be used to denote specific data structures such as lists.
|The straight edges and sharper corners make these brackets resemble a solid box, presumably made of a hard material, which would be a more secure container than the "soft"-looking curved brackets. They also resemble staples, which are used to hold things in place securely.
|-
|{}
|This stuff is expensive so be careful with it
|Rarely used in normal text, although may be used in expanded form to 'enclose' multiple optional lines following/preceding a single element of common purpose (similar to the 'split and recombined tracks' of [[2243: Star Wars Spoiler Generator]]). In mathematics, usually used to denote {{w|Set (mathematics)|sets}}, but other usage is possible. In programming languages most often used to denote begin and end of a separate block of code, but other uses are also extant.
|All that graphical detail in the bracket, if manufactured as a physical object, would increase the production cost making it more expensive than "regular" brackets. Their relative rarity compared to "regular" or square brackets might also increase the monetary value. They may also look 'fancy', like gates with ornate ironwork.
|-
|"", ″‶, or “”
|Someone is talking
|Used to denote speech or citations in normal text. The first version is commonly used in programming languages to denote text that is not a program, such as messages displayed to the user.
|Normal quotation marks. Some languages or communities use different typographical conventions such as „German quotation marks“. See also below for British and French.
|-
|<nowiki>''</nowiki>, ′‵, or ‘’
|Someone British is talking
|British quotation marks. Some programming languages use the first version to denote non-program text. In the Pascal family of languages, for example, <nowiki>''</nowiki>s indicate character-class data, with ""s being string-class data.
|Some British media use these to note when people are talking, though in modern usage the double quotes above are more common, while single quotes are more often used as '{{w|scare quotes}}'.
|-
|‹› or <>
|An Animorph is talking
|{{w|Bracket#Angle_brackets|Angle brackets}}. Aside from telepathic speech in prose, it's often used in comics to indicate that a character is speaking a foreign language that has been translated for the reader's benefit – at least notionally.
|Books like the series {{w|Animorphs}} or science fiction novels use these when a character is communicating nonverbally, for example via telepathy.
|-
|«»
|A French Animorph is talking
|French quotation marks. Used for quotes within quotes in some languages. For quoting conventions in different languages, see [https://op.europa.eu/en/web/eu-vocabularies/formex/physical-specifications/character-encoding/use-of-quotation-marks-in-the-different-languages this document].
|These symbols are literally called French quotation marks and are used in French texts as the first-level quotes. Here Randall is mixing the SF convention described above with actual French use.
|-
|<nowiki>|</nowiki>|
|I'm scared of negative numbers but these sigils will protect me
|Vertical bars in mathematics are used for the Absolute Value function.
|The absolute value of a number is its value with all negative and positive signs stripped off; in practical terms this is used to ensure a given value is positive (ex. <nowiki>|-69| = 69</nowiki>). If for whatever reason you need to "protect" your equations from negative numbers (which does come up in programming from time to time) the absolute value function has you covered — though it may not always be denoted with vertical bars. {{w|Sigil}}s are symbols used in magic, and some kinds of magic are thought to protect people from evil.
|-
|**, __, //
|I have a favorite monospaced font
|These symbols are conventionally used in text-based computer communications (such as emails, chats, Usenet News articles) to denote *bold*, _underlined_, or /italic/ font; some client programs interpret them and display actual bold text etc.
|{{w|Monospace font}} is a font (set of shapes used for letters, numbers and symbols) in which every character has the same width, unlike {{w|Typeface#Proportional_font|variable-width (proportional) font}}, in which the letter I is much narrower than W. While proportional font is more pleasant to read, monospace is easier to represent in simple mechanical or electronic devices, and has been used almost exclusively in the advent of computer technology, specifically in text-only environments such as {{w|computer terminals}}; these most often had only one factory installed font. Today, a person still using these symbols is probably using a {{w|terminal emulator}}, which allows to select one's favorite (preferably monospace) font from a wide set of fonts installed in the operating system.
|-
|~~
|I'm being sarcastic and I had a Tumblr account in 2014
|
|Strikethrough markup commonly used on sites like Tumblr to indicate that you don't really mean something you said. This usage is somewhat archaic.
|-
|[([{()}],)]
|These Python functions are not getting along
|The square brackets denote a mutable [https://docs.python.org/3/tutorial/introduction.html#lists list], the round brackets an immutable [https://docs.python.org/3/tutorial/datastructures.html#tuples-and-sequences tuple] , and the curly brackets a [https://docs.python.org/3/tutorial/datastructures.html#sets set]. It is valid to have nested them like this. [] could also be a slice (a bit of a list or tuple) and {} could be a [https://docs.python.org/3/tutorial/datastructures.html#dictionaries dictionary], but the syntax is wrong for these.
|Random parentheses - Spaghetti code (badly maintained or written) in programming languages including Python will often be badly organized creating a mess of indentations and brackets used to create functions or loops etc.
|-
|⌊⌋
|Help, I'm a mathematician trying to work with actual numbers and they're scary
|Mathematical symbols meaning "floor" (i.e. round down to the nearest lower integer).
|Mathematicians stereotypically prefer to work with abstract symbols and concepts rather than numbers or indeed anything that might pertain to the real world. When presented with an actual number, it is possible that a mathematician may wish for it to be rounded to the nearest integer so that they can treat it as part of [https://en.wikipedia.org/wiki/Number_theory number theory] rather than anything to do with reality.
|-
|∫ <span style="display:inline-block; transform:scaleX(-1);">∫</span>
|Why are you trying to read my violin?
|∫ looks like the {{w|Integral symbol}} which itself is derived from a {{w|Long s}}. In mathematics it is usually paired with the differential of the variable of integration (e.g., dx). A reverse integral symbol is not used in Western mathematics typesetting; it occasionally appears in mathematical texts written in Arabic, along with other symbols likewise adapted to Arabic's right-to-left writing direction. The symbol also looks like a lowercase {{w|Esh (letter)|esh}} (ʃ), used in phonetic transcription.

There is no unicode symbol for the reversed version - it is displayed here as a reversed ∫. The esh symbol has a reversed counterpart in Unicode, but it's quite a bit shorter (ʅ).
|Violins are known for their characteristic {{w|F-hole}}s.
|-
|<nowiki>|⟩</nowiki>
|Don't stop here–this is quantum country
|This {{w|Bra–ket notation|notation is used in quantum mechanics}} to notate a vector. This is called a ket, and the mirrored sign <nowiki>⟨|</nowiki> is called a bra. Combining them as bra-ket gives the inner product <nowiki>⟨|⟩</nowiki>.
| This is paraphrasing "Fear and Loathing in Las Vegas" where Johnny Depp's character Raoul Duke says: "We can't stop here, this is bat country!" while wasted on drugs, though not as wasted as later in the movie.
|}


The title text includes different kinds of quotes, including the ASCII " and ', and Unicode “” (which have both an opening and closing version).
Phones and laptops often have different input systems, and one of them may use a different kind of quote, thus mixing inconsistent quotes together, something most people may not notice or understand.

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

Bracket Symbols

and what they mean

( ) Regular parentheses for setting stuff aside

[ ] Square brackets (more secure)

{ } This stuff is expensive so be careful with it

" " Someone is talking

' ' Someone British is talking

‹ › An Animorph is talking

« » A French Animorph is talking

| | I'm scared of negative numbers but these sigils will protect me

<nowiki>*</nowiki> * _ _ / / I have a favorite monospaced font

~ ~ I'm being sarcastic and I had a Tumblr account in 2014

[ ( [ { ( ) } ] , ) ] These Python functions are '''''not''''' getting along

⌊ ⌋ Help, I'm a mathematician trying to work with actual numbers and they're scary

ʃ ʅ Why are you trying to read my violin?

| ⟩ Don't stop here--this is quantum country

{{comic discussion}}

[[Category:Language]]
[[Category:Math]]
[[Category:Programming]]
[[Category:Animorphs]]

2908: Moon Armor Index

2024-03-19T11:09:51Z

TheBittern: Add in accurate formula for calculation, including error of Pluto's armour

{{comic
| number = 2908
| date = March 18, 2024
| title = Moon Armor Index
| image = moon_armor_index_2x.png
| imagesize = 740x390px
| noexpand = true
| titletext = Astronomers are a little unsure of the applicability of this index, but NASA's Planetary Protection Officer is all in favor.
}}

==Explanation==
{{incomplete|Created by a MOON ARMOR SAMURAI BOT - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, [[Randall]] hypothesizes a situation in which each planet's moons were converted into protective armor to coat their respective planets. For example, the {{w|Moon}} would coat {{w|Earth}} in a 43 kilometer layer if it were molded into protective armor for the planet. It is unknown how the moons would be converted into armor, but with current technology, this would be highly impractical, if not impossible.{{citation needed}} It is also unclear how planets such as Jupiter, Saturn, Uranus, and Neptune, which are gas giants and thus have no solid surface, would sustain such a layer of armor, nor is it immediately evident what threat might necessitate such drastic measures. Mars' moons, being small compared to Mars, would make a thinner layer over Mars. Jupiter, being very big compared to its moons, would also have a small layer of "moon" on it. This is similar for Saturn, Uranus and Neptune. Pluto, having a moon (Charon) of a comparable size to its planet, would have a layer thicker than Earth's.

The comic uses the ≈ sign to show that the formula is only an approximation: this is because it does not take account of the armour having a larger surface area as it gets thicker. This approximation would be perfect for a shield of thickness zero, but for the thickest shield (Pluto) the error is around 4% (52.5 km by approximation, but 50.4 km by calculation). If P, M and A are the Planet radius, Moon radius and Armour thickness respectively, we can use V(P+A) = V(P) + V(M) to get A^3 + (3P)A^2 + (3P^2)A - C^3 = 0 and then solve the cubic to make an accurate calculation. For multiple moons, this calculation can be used repeatedly to armour each moon with the previous moon, before using the final moon-conglomerate to armour the planet.

The title text states that astronomers are confused about the idea of protective armor made from moons, but NASA's [https://what-if.xkcd.com/117/ Planetary Protection Officer], who is responsible for keeping other celestial bodies away from Earth's contamination, is apparently strongly in favor of this idea.

{| class="wikitable"
|-
! Planet/<br>dwarf planet !! Area (km²) || Moons || Volume (km³) || Moon shield thickness 
|-
| {{w|Earth}} || 5.1007*10^8 || {{w|Moon|1}} || 2.196*10^10 || 43 km (27 mi)
|-
| {{w|Mars}} || 1.4437*10^8 || {{w|Moons of Mars|2}} || {{w|Phobos (moon)
|(5695±32)}}+{{w|Deimos (moon)|(1033±19)}} || 5 cm (2 in)
|-
| {{w|Jupiter}} || 6.1469*10^10 || {{w|Moons of Jupiter|95}} || 1.7646*10^11 || 2.87 km (1.78 mi)
|-
| {{w|Saturn}} || 4.27*10^10 || {{w|Moons of Saturn|146}} || 7.651*10^10 || 1.79 km (1.11 mi)
|-
| {{w|Uranus}} || 8.1156*10^9 || {{w|Moons of Uranus|28}} || ||
|-
| {{w|Neptune}} || 7.6187*10^9 || {{w|Moons of Neptune|16}} || ||
|-
| {{w|Pluto}} || 1.7744*10^7 || {{w|Moons of Pluto|5}} || {{w|Charon (moon)|(9.322×10^8)}}+{{w|Moons of Pluto|(approx 87100+38800+900+200)}} || 52.5 km (32.6 mi) (by XKCD)
50.4 km (31.3 mi) (by full calculation)
|-
| {{w|120347 Salacia|Salacia}} || || {{w|Actaea (moon)|1}} || ||
|-
| {{w|Haumea}} || 8.14*10^6 || {{w|Moons of Haumea|2}} || {{w|Hiʻiaka (moon)
|(17.2*10^6)}}+{{w|Namaka (moon)|(2.57*10^6)}} || 2.43 km (1.51 mi)
|-
| {{w|50000 Quaoar|Quaoar}} || 3.78*10^6 || {{w|Weywot|1}} || 4.19*10^6 || 1.11 km (0.69 mi)
|-
| {{w|225088 Gonggong|Gonggong}} || || {{w|Xiangliu (moon)|1}} || ||
|-
| {{w|Eris (dwarf planet)|Eris}} || (1.70±0.02)*10^7 || {{w|Dysnomia (moon)|1}} || ||
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
[Text above diagram:]
:Moon armor index:
:How thick the shells around various worlds would be if their moon(s) were converted into protective armor
:≈Total moon volume/Planet surface area

:[Vertical bars showing "moon armor" thicknesses for the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Salacia, Haumea, Quaoar, Gonggong and Eris. Earth's bar has a label named "43 km thick" and is compared to the height of Mt Everest. The zoom in to the Mars' bar has a thickness label of 2" and is compared to a rover wheel over it. Pluto's bar has a label named "(Mostly Charon)".]

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Bar charts]]

2908: Moon Armor Index

2024-03-19T10:47:59Z

TheBittern: Added in Pluto, with spherical approximations for small moons based on their long axes (they are rounding errors in any event)

{{comic
| number = 2908
| date = March 18, 2024
| title = Moon Armor Index
| image = moon_armor_index_2x.png
| imagesize = 740x390px
| noexpand = true
| titletext = Astronomers are a little unsure of the applicability of this index, but NASA's Planetary Protection Officer is all in favor.
}}

==Explanation==
{{incomplete|Created by a MOON ARMOR SAMURAI BOT - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, [[Randall]] hypothesizes a situation in which each planet's moons were converted into protective armor to coat their respective planets. For example, the {{w|Moon}} would coat {{w|Earth}} in a 43 kilometer layer if it were molded into protective armor for the planet. It is unknown how the moons would be converted into armor, but with current technology, this would be highly impractical, if not impossible.{{citation needed}} It is also unclear how planets such as Jupiter, Saturn, Uranus, and Neptune, which are gas giants and thus have no solid surface, would sustain such a layer of armor, nor is it immediately evident what threat might necessitate such drastic measures. Mars' moons, being small compared to Mars, would make a thinner layer over Mars. Jupiter, being very big compared to its moons, would also have a small layer of "moon" on it. This is similar for Saturn, Uranus and Neptune. Pluto, having a moon (Charon) of a comparable size to its planet, would have a layer thicker than Earth's.

The title text states that astronomers are confused about the idea of protective armor made from moons, but NASA's [https://what-if.xkcd.com/117/ Planetary Protection Officer], who is responsible for keeping other celestial bodies away from Earth's contamination, is apparently strongly in favor of this idea.

{| class="wikitable"
|-
! Planet/<br>dwarf planet !! Area (km²) || Moons || Volume (km³) || Moon shield thickness 
|-
| {{w|Earth}} || 5.1007*10^8 || {{w|Moon|1}} || 2.196*10^10 || 43 km (27 mi)
|-
| {{w|Mars}} || 1.4437*10^8 || {{w|Moons of Mars|2}} || {{w|Phobos (moon)
|(5695±32)}}+{{w|Deimos (moon)|(1033±19)}} || 5 cm (2 in)
|-
| {{w|Jupiter}} || 6.1469*10^10 || {{w|Moons of Jupiter|95}} || 1.7646*10^11 || 2.87 km (1.78 mi)
|-
| {{w|Saturn}} || 4.27*10^10 || {{w|Moons of Saturn|146}} || 7.651*10^10 || 1.79 km (1.11 mi)
|-
| {{w|Uranus}} || 8.1156*10^9 || {{w|Moons of Uranus|28}} || ||
|-
| {{w|Neptune}} || 7.6187*10^9 || {{w|Moons of Neptune|16}} || ||
|-
| {{w|Pluto}} || 1.7744*10^7 || {{w|Moons of Pluto|5}} || {{w|Charon (moon)|(9.322×10^8)}}+{{w|Moons of Pluto|(approx 87100+38800+900+200)}} || 52.5 km (32.6 mi)
|-
| {{w|120347 Salacia|Salacia}} || || {{w|Actaea (moon)|1}} || ||
|-
| {{w|Haumea}} || 8.14*10^6 || {{w|Moons of Haumea|2}} || {{w|Hiʻiaka (moon)
|(17.2*10^6)}}+{{w|Namaka (moon)|(2.57*10^6)}} || 2.43 km (1.51 mi)
|-
| {{w|50000 Quaoar|Quaoar}} || 3.78*10^6 || {{w|Weywot|1}} || 4.19*10^6 || 1.11 km (0.69 mi)
|-
| {{w|225088 Gonggong|Gonggong}} || || {{w|Xiangliu (moon)|1}} || ||
|-
| {{w|Eris (dwarf planet)|Eris}} || (1.70±0.02)*10^7 || {{w|Dysnomia (moon)|1}} || ||
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
[Text above diagram:]
:Moon armor index:
:How thick the shells around various worlds would be if their moon(s) were converted into protective armor
:≈Total moon volume/Planet surface area

:[Vertical bars showing "moon armor" thicknesses for the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Salacia, Haumea, Quaoar, Gonggong and Eris. Earth's bar has a label named "43 km thick" and is compared to the height of Mt Everest. The zoom in to the Mars' bar has a thickness label of 2" and is compared to a rover wheel over it. Pluto's bar has a label named "(Mostly Charon)".]

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Bar charts]]