https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&user=81.138.95.57&feedformat=atomexplain xkcd - User contributions [en]2024-03-28T17:01:03ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=1276:_Angular_Size&diff=506081276: Angular Size2013-10-14T10:38:51Z<p>81.138.95.57: /* Explanation */ micrometer -> micron</p>
<hr />
<div>{{comic<br />
| number = 1276<br />
| date = October 11, 2013<br />
| title = Angular Size<br />
| image = angular size.png<br />
| titletext = If the celestial sphere were mapped to the Earth's surface, astronomy would get a LOT easier; you'd just need a magnifying glass.<br />
}}<br />
<br />
==Explanation==<br />
{{incomplete}}<br />
This comic is a comparison of the {{w|angular diameter|angular diameters}} (or apparent diameter) of various celestial objects at the surface of the earth relative to a vertex at the center of the Earth as diagrammed in the opening panel. The objects' scales are compared to actual objects on earth for comparison. Each size given is for the object at its closest approach to earth.<br />
<br />
London's {{w|M25 motorway}} is around 60 km (35 miles) across, a {{w|soccer field}} is about 100 meters long, a {{w|Table tennis table|ping pong table}} is 274 centimeters long, a {{w|laptop}} is about 35 centimeters across, the {{w|tilde}} symbol on a keyboard is about 5 millimeters long, and a cell of ''{{w|Escherichia coli|E. coli}}'' is about 2 microns long.<br />
<br />
A simple {{w|Intercept theorem|formula}} can be used to find the size on earth of a celestial object when the size of and distance to the object is known. This is done by taking the radius of the earth, multiplying by the diameter of the object, and dividing by the distance to the object from the center of the earth.<br />
<br />
The space objects referenced in the panels are:<br />
<br />
* The {{w|Sun}} and the {{w|Moon}}, and also the open cluster {{w|Messier 25}}, have approximately the same size (around 0.5 degrees of arc) when seen from the Earth.<br />
<br />
* {{w|Mercury (planet)|Mercury}}, {{w|Venus}}, {{w|Mars}}, {{w|Jupiter}}, {{w|Saturn}}, {{w|Uranus}}, and {{w|Neptune}} are the other planets of the {{w|Solar System}}.<br />
<br />
* {{w|Io (moon)|Io}}, {{w|Europa (moon)|Europa}}, {{w|Ganymede (moon)|Ganymede}}, and {{w|Callisto (moon)|Callisto}} are the main moons of Jupiter; {{w|Titan (moon)|Titan}} is the largest moon of Saturn; and {{w|Triton (moon)|Triton}} is the largest moon of Neptune. {{w|Ceres (dwarf planet)|Ceres}} and {{w|Pluto}} are {{w|dwarf planet}}s.<br />
<br />
* {{w|Phobos (moon)|Phobos}} and {{w|Deimos (moon)|Deimos}} are the moons of Mars. {{w|Eris (dwarf planet)|Eris}} is another dwarf planet. {{w|R Doradus}} and {{w|Betelgeuse}} are giant stars, respectively around 180 and 640 light-years away. R Doradus is the star with the largest apparent diameter (other than the sun, of course).<br />
<br />
* {{w|4942 Munroe}} is an asteroid [http://blog.xkcd.com/2013/09/30/asteroid-4942-munroe/ named] after xkcd author [[Randall]] Munroe. {{w|Proxima Centauri}}, {{W|Alpha Centauri|Alpha Centauri AB}}, {{w|Barnard's star}} and {{w|Sirius}} are nearby stars (all within 10 light-years from the Sun).<br />
<br />
* {{w|HD 189733 b}}, {{w|Gliese 581 b}}, {{w|Gliese 667 Cc}}, {{w|82 G. Eridani#Planetary_system|HD 20794 c}}, {{w|Tau Ceti#Planets|Tau Ceti c}}, and {{w|KOI-1686.01}} are {{w|extrasolar planet}}s; the parenthetical names are references to the comic [[1253: Exoplanet Names]]. However, some of the planets' parenthetical names do not match the table in the previous comic. For example, HD 20794 c is called "Legoland" rather than "Moonchild" in [[1253: Exoplanet Names]]. The {{w|black hole}} at the center of our Galaxy is {{w|Sagittarius A*}}, a massive object containing a mass more than 4 million times of our Sun.<br />
<br />
* {{w|Voyager 1}} and {{w|Voyager 2|2}} are space probes launched in 1977, and currently around 125 and 100 {{w|astronomical unit}}s away, respectively.<br />
<br />
The title text is about astronomy would be much easier if the celestial sphere were mapped to the earth, should come as no surprise; essentially, it's equivalent to asking "what if the earth were a giant telescope." That is, although there's no actual optical setup which would enable such a mapping, an earth so mapped would be functioning as a giant telescope.<br />
<br />
Approximate values for the mappings to the Earth sphere (based on mean Earth radius at 6,371.0 km):<br />
{| class="wikitable sortable"<br />
|-<br />
! Panel<br />
! Object<br />
! Distance<br />
! Diameter<br />
! Size at Earth sphere<br />
|-<br />
| rowspan="4"| Panel 1<br />
|| Sun<br />
|| 149,600,000 km<br />
|| 1,392,684 km<br />
|| 59.3 km<br />
|-<br />
|| Moon <small>Semi-major axis</small><br />
|| 384,399 km<br />
|| 3,476.28 km<br />
|| 57.6 km<br />
|-<br />
|| Moon <small>Perigee</small><br />
|| 363,295 km<br />
|| 3,476.28 km<br />
|| 60.9 km<br />
|-<br />
|| Messier 25<br />
|| 2,100 light-years <br/> 19.867 × 10^15 km<br />
|| 19 light-years <br/> 179.753 × 10^12 km<br />
|| 57.6 km<br />
|-<br />
| rowspan="7"| Panel 2<br />
|| Mercury <small>closest approach</small><br />
|| 92 million km<br />
|| 4,879.4 km<br />
|| 0.34 km<br />
|-<br />
|| Venus <small>closest approach</small><br />
|| 41 million km<br />
|| 12.103.6 km<br />
|| 1.9 km<br />
|-<br />
|| Mars <small>closest approach</small><br />
|| 56 million km<br />
|| 6,792.4 km<br />
|| 0.77 km<br />
|-<br />
|| Jupiter<br />
|| 778,547,200 km<br />
|| 139,822 km<br />
|| 1.14 km<br />
|-<br />
|| Saturn<br />
|| 1,433,449,370 km<br />
|| 120,536 km<br />
|| 0.54 km<br />
|-<br />
|| Uranus<br />
|| 2,876,679,082 km<br />
|| 51,118 km<br />
|| 0.11 km<br />
|-<br />
|| Neptune<br />
|| 4,503,443,661 km<br />
|| 49,528 km<br />
|| 0.07 km<br />
|-<br />
| rowspan="8"| Panel 3<br />
|| Io<br />
|| 778,547,200 km<br />
|| 3,643 km<br />
|| 29.8 m<br />
|-<br />
|| Europa<br />
|| 778,547,200 km<br />
|| 3,122 km<br />
|| 25.5 m<br />
|-<br />
|| Ganymede<br />
|| 778,547,200 km<br />
|| 5,262 km<br />
|| 43.1 m<br />
|-<br />
|| Callisto<br />
|| 778,547,200 km<br />
|| 4,821 km<br />
|| 39.5 m<br />
|-<br />
|| Titan<br />
|| 1,433,449,370 km<br />
|| 5,150 km<br />
|| 22.9<br />
|-<br />
|| Triton<br />
|| 4,503,443,661 km<br />
|| 2,705.2 km<br />
|| 3.8 m<br />
|-<br />
|| Ceres <small>Perihelion</small><br />
|| 380,995,855 km<br />
|| 974.6 km<br />
|| 16.3 m<br />
|-<br />
|| Pluto <small>Perihelion</small><br />
|| 4,437,000,000 km<br />
|| 2,306 km<br />
|| 3.3 m<br />
|-<br />
| rowspan="6"| Panel 4<br />
|| Phobos<br />
|| 56 million km<br />
|| 26.8 × 22.4 × 18.4 km<br />
|| 3.05 m<br />
|-<br />
|| Deimos<br />
|| 56 million km<br />
|| 15 × 12.2 × 10.4 km<br />
|| 1.71 m<br />
|-<br />
|| Eris <small>Perihelion</small><br />
|| 5.723 × 10^9 km<br />
|| 2326 km<br />
|| 2.59 m<br />
|-<br />
|| Eris <small>Aphelion</small><br />
|| 14.602 × 10^9 km<br />
|| 2326 km<br />
|| 1.01 m<br />
|-<br />
|| Betelguse<br />
|| 643 ± 146 light-years <br/>max. 7.464 × 10^15 km<br />
|| 950–1,200 solar radii <br/>max. 1.671 × 10^9 km<br />
|| 1.43 m<br />
|-<br />
|| R Doradus<br />
|| 178 ± 10 light-years <br/>max. 1.778 × 10^15 km<br />
|| 370 ± 50 solar radii <br/>max. 0.515 × 10^9 km<br />
|| 1.85 m<br />
|-<br />
| rowspan="6"| Panel 5<br />
|| 4942 Munroe<br />
|| 2.2 AU (1.2 AU closest to earth, 179.4 × 10^6 km)<br />
|| 9-10 km<br />
|| 35.5 cm<br />
|-<br />
|| Proxima Centauri<br />
|| 4.243 light-years <br/> 40.142 × 10^12 km<br />
|| 0.141 solar radii <br/> 0.196 × 10^6 km<br />
|| 3.11 cm<br />
|-<br />
|| Barnard's Star<br />
|| 5.980 light-years <br/> 56.574 × 10^12 km<br />
|| 0.196 solar radii <br/> 0.272 × 10^6 km<br />
|| 3.06 cm<br />
|-<br />
|| Alpha Centauri B<br />
|| 4.366 light-years <br/> 41.305 × 10^12 km<br />
|| 0.865 solar radii <br/> 1.204 × 10^6 km<br />
|| 18.6 cm<br />
|-<br />
|| Sirius<br />
|| 8.6 light-years <br/> 81.362 × 10^12 km<br />
|| 1.711 solar radii <br/> 2.382 × 10^6 km<br />
|| 18.7 cm<br />
|-<br />
|| Alpha Centauri A<br />
|| 4.366 light-years <br/> 41.305 × 10^12 km<br />
|| 1.227 solar radii <br/> 1.708 × 10^6 km<br />
|| 26.3 cm<br />
|-<br />
| rowspan="9"| Panel 6<br />
|| HD 189733 b<br />
|| 63 light-years <br/> 596.024 × 10^12 km<br />
|| 1.138 Jupiter radii <br/> 159,117 km<br />
|| 1.7 mm<br />
|-<br />
|| Gliese 581 g<br />
|| 20.3 light-years <br/> 192.052 × 10^12 km<br />
|| 1.3 to 2.0 Earth radii <br/> max. 25.484 km<br />
|| 0.85 µm<br />
|-<br />
|| Gliese 581 (Parent star)<br />
|| 20.3 light-years <br/> 192.052 × 10^12 km<br />
|| 0.29 solar radii <br/> 403,878 km<br />
|| 1.34 cm<br />
|-<br />
|| Black Hole at the center of our Galaxy<br />
|| 25,900 light-years <br/> 245.032 × 10^15 km<br />
|| Mass 4.31 × 10^6:<br/>12.684 × 10^6 km<br />
|| 0.33 mm<br />
|-<br />
|| Gliese 667 Cc<br />
|| 22.7 light-years <br/> 214.757 × 10^12 km<br />
|| 2.0 Earth radii <br/> 25.484 km<br />
|| 0.76 µm<br />
|-<br />
|| Gliese 667 (Parent star)<br />
|| 22.7 light-years <br/> 214.757 × 10^12 km<br />
|| 0.42 solar radii <br/> 584,927 km<br />
|| 1.74 cm<br />
|-<br />
|| HD 20794 c (Parent star)<br />
|| 20 light-years <br/> 189.214 × 10^12 km<br />
|| 0.92 solar radii <br/> 1,281,269 km<br />
|| 4.31 cm<br />
|-<br />
|| Tau Ceti C (Parent star)<br />
|| 11.905 light-years <br/> 112.629 × 10^12 km<br />
|| 0.793 solar radii <br/> 1,104,398 km<br />
|| 6.25 cm<br />
|-<br />
|| Koi-1686.01 (Parent star)<br />
|| 1033.8 light-years <br/> 9.780 × 10^15 km<br />
|| 0.52 solar radii <br/> 724,195 km<br />
|| 0.47 mm<br />
|-<br />
| Panel 7<br />
|| Voyager probes<br />
|| (Voyager 1 at 126.10 AU) 1.886×10^10 km<br />
|| 20 meters (with antennas)<br />
|| 6.76 µm<br />
|}<br />
<br />
==Transcript==<br />
{{incomplete transcript}}<br />
<br />
{{comic discussion}}<br />
[[Category:Math]]<br />
[[Category:Astronomy]]</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=Talk:1275:_int(pi)&diff=50260Talk:1275: int(pi)2013-10-09T10:43:35Z<p>81.138.95.57: </p>
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<div>Well, I get the int(Pi) thing, but what's with avoiding 3's? [[Special:Contributions/95.35.58.168|95.35.58.168]] 05:10, 9 October 2013 (UTC)<br />
<br />
What is "''floor pie''"? --[[User:JakubNarebski|JakubNarebski]] ([[User talk:JakubNarebski|talk]]) 05:31, 9 October 2013 (UTC)<br />
<br />
I thought this was a reference to [http://www.strangehorizons.com/2000/20001120/secret_number.shtml Bleem] and reminds me of comic [[899]]. <span style="color:green;">Saibot84</span> 06:17, 9 October 2013 (UTC)<br />
<br />
Prudent mathematicians just refer to it as "The Scottish Number". [[User:Dr Pepper|Dr Pepper]] ([[User talk:Dr Pepper|talk]]) 06:58, 9 October 2013 (UTC) Dr Pepper<br />
<br />
I can give you one '''rational''' reason for spelling out things like INT(PI) in programming. Back in the ancient times, there was a piece of electronics dubbed then a ''personal computer'' with an NSA code name of ZXSPECTRUM. It had a built-in interpreter of the ancient language codenamed BASIC. Memory was very precious in those times, every single byte counted. The creators of the interpreter did a (somewhat) clever thing - all keywords of this particular dialect of the BASIC language were stored in memory as single-byte codes, and were only spelled out by text display routines. On the other hand, CPU cycles were precious, too, so they did another (not so) clever thing by storing number constants (like the cursed number mentioned above) twofold - both in an ASCII decimal form for display purposes and in a 6-byte internal binary form for computing purposes. Therefore each number occupied the space of six bytes plus the number of digits (or other characters like sign, decimal point, etc.) BASIC hackers exploited this (mis)features to save a few bytes on some commonly-used constants by saying INT PI (parentheses were not needed), NOT PI (to get 0) or SGN PI (to get 1), thus using only 2 bytes of memory instead of 7 if the numbers were used directly. Another trick to use with larger numbers was VAL "12345", which saved 3 bytes for each number spelled this way (number of digits plus three bytes for the VAL keyword and two quote marks instead of number of digits plus six bytes of internal representation). [[Special:Contributions/89.174.214.74|89.174.214.74]] 08:43, 9 October 2013 (UTC)<br />
: Actually the internal binary form of the number was 5 bytes, but there was a special prefix byte used for two purposes, a) when listing the program the text display routines would simply skip the six bytes b) when a digit character was encountered at run time, the prefix byte was located instead of parsing the number again. It was even possible to patch the source code to replace all the digits with a single decimal point because the syntax wasn't checked at runtime. Also the trick was originally used with the ZX81 as it was slower and had less memory. I don't think the sign was stored with the number though, as that would have caused confusion with the unary minus operator. (All of the space-saving tricks mentioned above would slow the program down, of course. Even PI had to be calculated as internally the ZX81/Spectrum only knew the value of π/2.) --[[Special:Contributions/81.138.95.57|81.138.95.57]] 10:43, 9 October 2013 (UTC)<br />
<br />
I suspect in many languages 4/INT(pi) is 1 (as it does integer division) [[Special:Contributions/193.34.186.165|193.34.186.165]] 08:51, 9 October 2013 (UTC)</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=487:_Numerical_Sex_Positions&diff=41810487: Numerical Sex Positions2013-06-24T10:24:48Z<p>81.138.95.57: /* Transcript */ √</p>
<hr />
<div>{{comic<br />
| number = 487<br />
| date = October 10, 2008<br />
| title = Numerical Sex Positions<br />
| image = numerical sex positions.png<br />
| titletext = We didn't even get to the continued fractions!<br />
}}<br />
<br />
==Explanation==<br />
{{incomplete}}<br />
{{w|Sex positions}} are positions that two people can have {{w|sexual intercourse}} in. Many of them are named, although only one generally accepted position is named after a number: the {{w|69 (sex position)|69}}.<br />
<br />
In this comic, [[Cueball]] and [[Megan]] try to approximate the shapes of numbers that they are given as sex positions. They start with the classic 69, then represent the number 99 as standing sex and the number 71 as sex over a table. They are then given ever more difficult numbers to attempt before finally giving up and leaving at ln(2π). This interpretation of the comic suggests breaking the {{w|Fourth_wall|Fourth wall}}.<br />
<br />
==Transcript==<br />
:XKCD presents a guide to numerical sex positions:<br />
:69<br />
:[Traditional sixty-nine position, mutual oral sex.]<br />
:99<br />
:[A standing doggy-style position.]<br />
:71<br />
:[Megan is bent over a table.]<br />
:34<br />
:Cueball: Uh.<br />
:[Cueball and Megan look confusedly at each other.]<br />
:√8<br />
:Guys?<br />
:[Cueball and Megan are staring blankly at each other.]<br />
:ln(2π)<br />
:Aww, c'mon...<br />
:[Megan begins walking away.]<br />
<br />
{{comic discussion}}<br />
[[Category:Sex]]<br />
[[Category:Comics featuring Megan]]<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Math]]</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=1223:_Dwarf_Fortress&diff=401981223: Dwarf Fortress2013-06-10T11:08:12Z<p>81.138.95.57: /* Explanation */ Grammar fixes</p>
<hr />
<div>{{comic<br />
| number = 1223<br />
| date = June 10, 2013<br />
| title = Dwarf Fortress<br />
| image = dwarf_fortress.png<br />
| titletext = I may be the kind of person who wastes a year implementing a Turing-complete computer in Dwarf Fortress, but that makes you the kind of person who wastes ten more getting that computer to run Minecraft.<br />
}}<br />
<br />
==Explanation==<br />
{{incomplete}}<br />
This comic is a reaction to the recent reveal of a U.S. electronic surveillance program called {{w|PRISM (surveillance program)|PRISM}}, run by the NSA. (You can read a [http://www.guardian.co.uk/world/2013/jun/06/us-tech-giants-nsa-data Guardian article] about it.) PRISM, leaked by a former NSA official, incited some controversy since it collects private data (e-mails, chats, file transfers, etc.). <!-- please expand/correct this ~Alpha --><br />
<br />
''{{w|Dwarf Fortress}}'' is a freeware strategy game in which the player builds a civilization by giving orders to a group of dwarves. Cueball compares the omniscience of a ''Dwarf Fortress'' player to government surveillance.<br />
<br />
"Big Brother" means "a tyrannical government body that constantly monitors all its citizens." The term comes from the classic dystopian novel ''{{w|Nineteen Eighty-Four}}'' by George Orwell.<br />
<br />
''{{w|Turing Machine}}'' is a hypothetical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a computer. '''Turing-complete machine''' is any machine which can be proven to be able to simulate a Turing machine and therefore any computer algorithm - doing the proving this way is much easier than proving it can run anything directly. (Note: A PC is not a Turing-complete machine because its memory is limited. Any machine built in Dwarf Fortress would still run inside that PC and therefore be even more limited. But you CAN build a proof-of-concept machine by building a limited one and showing how to extend its memory.)<br />
<br />
==Transcript==<br />
:[Cueball at a computer.]<br />
:Cueball: If the corporate surveillance state monitors and controls every aspect of my life...<br />
:Big Brother: We do.<br />
:Cueball: And I play Dwarf Fortress all day...<br />
:Big Brother: You do.<br />
:Cueball: Then you're effectively Dwarf Fortress players watching your dwarves play Dwarf Fortress.<br />
:Big Brother: ...Oh god.<br />
:Big Brother realizes he's trapped in the most tedious possible hell.<br />
<br />
{{comic discussion}}<br />
[[Category:Computers]]<br />
[[Category:Comics featuring Cueball]]<br />
[[Category:Video games]]<br />
[[Category:Politics]]</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=Talk:1193:_Externalities&diff=32047Talk:1193: Externalities2013-04-01T10:53:33Z<p>81.138.95.57: </p>
<hr />
<div>[[Special:Contributions/199.48.226.89|199.48.226.89]] 10:18, 1 April 2013 (UTC) I put in "caltech.edu" and hashed a lot of words, and "Twilight Sparkle is best pony." was the best result I managed, only off by 496 bits.<br />
<br />
All I see is a blank white 780x969 image. Nothing appears when I hover over stuff. [[Special:Contributions/109.65.100.208|109.65.100.208]] 09:04, 1 April 2013 (UTC)<br />
:Sometimes it does that. It takes a while to generate, and it doesn't always render correctly. Try updating your browser or refreshing. '''[[User:Davidy22|<u>{{Color|#707|David}}<font color=#070 size=3>y</font></u><font color=#508 size=4>²²</font>]]'''[[User talk:Davidy22|<tt>[talk]</tt>]] 09:06, 1 April 2013 (UTC)<br />
The person who provided the shopped image either isn't using a modern browser or is using IE. The font is supposed to be "xkcd-Regular", which I assume is a font that gets downloaded from XKCD's server. Loading the same page in IE 9 gave me that Times New Roman-esque font instead (Chrome, Firefox, and Opera use the special font, although it's rendered a little fuzzy in Firefox). [[Special:Contributions/129.21.119.153|129.21.119.153]] 09:25, 1 April 2013 (UTC)<br />
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The dog part now shows "FREEPRIME@AMAZON.COM" underneath the sliders for me. --[[User:Gefrierbrand|Gefrierbrand]] ([[User talk:Gefrierbrand|talk]]) 09:50, 1 April 2013 (UTC)<br />
:I think it matches the company in the first panel? (Currently CAREERS@XLINX INC for me.) --[[Special:Contributions/81.138.95.57|81.138.95.57]] 10:53, 1 April 2013 (UTC)</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=Talk:1165:_Amazon&diff=26178Talk:1165: Amazon2013-01-25T10:32:44Z<p>81.138.95.57: /* Skeletonize a cow */</p>
<hr />
<div>I believe it is not about {{w|Amazon Forest}}, but about {{w|Amazon River}}. {{unsigned|194.85.224.35}}<br />
<br />
== Skeletonize a cow ==<br />
<br />
Isn't there an urban myth that pirahnas can skeletonize a cow in under 2 minutes? [[Special:Contributions/81.138.95.57|81.138.95.57]] 10:32, 25 January 2013 (UTC)</div>81.138.95.57https://www.explainxkcd.com/wiki/index.php?title=Talk:1165:_Amazon&diff=26177Talk:1165: Amazon2013-01-25T10:32:21Z<p>81.138.95.57: /* Skeletonize a cow */ new section</p>
<hr />
<div>I believe it is not about {{w|Amazon Forest}}, but about {{w|Amazon River}}. {{unsigned|194.85.224.35}}<br />
<br />
== Skeletonize a cow ==<br />
<br />
Isn't there an urban myth that pirahnas can skeletonize a cow in under 2 minutes?</div>81.138.95.57