explain xkcd - User contributions [en]

Talk:2561: Moonfall

2021-12-30T06:06:39Z

162.158.78.77:

Well, it did fund 8 out of 10 seasons of Mythbusters[[User:Seebert|Seebert]] ([[User talk:Seebert|talk]]) 19:11, 29 December 2021 (UTC)
:What is "it"? [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 21:58, 29 December 2021 (UTC)
::Explosions, probably. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 23:15, 29 December 2021 (UTC)

This synopsis makes me eager to never ever see this tripe, which the comic failed to achieve. Thank you, explainxkcd, for saving me time and money.[[Special:Contributions/162.158.107.18|162.158.107.18]] 20:03, 29 December 2021 (UTC)
:It doesn't sound much different from most other action blockbusters, like the "Terminator" franchise, or "Armageddon". And it will probably be better than the "Transformer" movies. As Cueball and Megan indicate, it's mostly about watching lots of things blow up, not about plausibility. [[User:Barmar|Barmar]] ([[User talk:Barmar|talk]]) 21:57, 29 December 2021 (UTC)
:”…only to find out that our Moon is not what we think it is.” – Wait, what, the moon isn’t cheese?? --[[Special:Contributions/162.158.183.164|162.158.183.164]] 22:32, 29 December 2021 (UTC)

Only thing less likely than Moon suddenly getting on collision course is that we will be able to prevent the collision. Wait. I see he lowered the bar even more with only THREE people somehow fixing it without help of rest of NASA ... how do they even get to space without help? -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 23:14, 29 December 2021 (UTC)
:"[O]ne astronaut from her past, Brian Harper and a conspiracy theorist K.C. Houseman" is grammatically confusing. That could be either three people (assuming it's an omitted Oxford comma) or one person (an astronaut named Brian Harper who spreads conspiracy theories under the pseudonym "K.C. Houseman"). It needs at least one more comma if "Brian Harper" is supposed to be an appositive [[Special:Contributions/162.158.78.77|162.158.78.77]] 06:06, 30 December 2021 (UTC)

2516: Hubble Tension

2021-09-17T12:17:29Z

162.158.78.77: /* Explanation */ As someone appended a whole para on the subject, later on, removing this earlier more condensed addition of mine. (Though really it should remain linked. New para could be moved/rewritten to be up here where the other detail is, maybe?)

{{comic
| number = 2516
| date = September 15, 2021
| title = Hubble Tension
| image = hubble_tension.png
| titletext = Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!
}}

==Explanation==
{{incomplete|Created by Dave - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
[[Ponytail]] is telling [[Cueball]] about the {{w|expansion of the universe}} telling him that there are three main estimates of the rate of expansion, and that they all disagree. She then tells him of the two well known (and very complicated) methods, and finally the joke is that the third method is performed by a guy named Dave (who replies from off-panel), and he claims to measure the distances with a radar gun, as if the galaxies were speeding here on Earth.

The fact that most {{w|galaxies}} are receding from us, and that the distance to the galaxy is directly proportional to the speed (as measured by {{w|red-shift}}) was discovered in the 1920s by {{w|Edwin Hubble}} and others. This constant of proportionality is known as the {{w|Hubble Constant}}.

One way of measuring the Hubble Constant is to measure the distance to (relatively) nearby galaxies. Once distance is obtained, speed can be easily obtained by measuring the red-shift and thus the Hubble Constant calculated. Measuring the distance turns out to be fiendishly difficult because a distant bright star looks the same as a dim star that is closer, and localized movements can influence the speed of recession — though less significantly, for multiple reasons, the further away are the objects that you study.

In practice, astronomers have a number of ways of measuring distance that work at different scales, and they can be built upon to measure distance to far away galaxies. This is known as the {{w|Cosmic distance ladder}}.

The first rung is {{w|parallax}}. As the Earth orbits around the Sun, nearby stars appear to move slightly relative to distant stars; a star that moves by one second of arc is said to have a distance of 1 {{w|Parsec}} — about 3¼ light years or 30 trillion (3x1013) kilometers.

The next rung is {{w|Cepheid variables}}, which periodically brighten and dim. The frequency of variation is related to the absolute brightness of the star, and thus by comparing the absolute to the relative brightness (subject to the {{w|Inverse-square law}} where not otherwise obscured) the distance can be measured.

The final rung is {{w|Type Ia supernova}}, which occur when an accreting {{w|white dwarf}} exceeds 1.4 solar masses. Because the initial mass is always identical, the absolute brightness of the explosion is as well, so the distance can be similarly calculated.

Putting these together, the best measurement of the Hubble Constant is 73 km/s/Mparsec.

This is in conflict with the other main way of measuring the Hubble Constant, analyzing makeup of the {{w|Cosmic Microwave Background}} (CMB) radiation, which yields a value of 68 km/s/Mparsec. The difference is statistically significant, and well outside the error bounds of each measurement.

Since the CMB technique relies on our understanding and assumptions about the early universe, as well as on the cosmological effects of General Relativity on large scales, if this discrepancy proved real it could be the gateway to new discoveries in cosmology and gravity, as well as possibly shed light on the origin of the universe and a '{{w|Theory Of Everything}}'. Cosmologists got quite excited about this. It might also be that there was a previously unaccounted-for error in any of the rungs of the cosmological distance ladder, and that once that is fixed, the two results will be consistent.

The third method introduced in this comic is a guy named Dave who is trying to use a {{w|radar speed gun}} (as used by the police for detecting speeding cars) to try to measure the movement of astronomical bodies. A radar system works by sending electromagnetic radiation from the gun and then measuring the returned radiation to determine how far away or how fast a moderately distant object is moving. Because of the transmission and return times required (and the inverse-square law), a radar device will only be able to get information about the very closest objects, such as the Moon (a type of {{w|Earth–Moon–Earth communication|Moon bounce}}) and other objects orbiting the Earth (or ''perhaps'' the Sun), where the influence of being in orbit utterly dominates over any possible Hubble-shift. And that still needs powerful radar systems like the former {{w|Arecibo Telescope}} to be able to get any useful information that far away, a hand-held radar gun would not be able to 'lock on' across even those distances.

Going by back-calculating grossly 'idealized' universe models, as suggested by the other two estimates, a receding velocity of 85 miles per hour ('mph'; about 137 kilometers per hour, 'kph' or 'km/h') should be seen at a distance of roughly 1700-1850 light-years, on the order of the thickness of our galactic disc. Much too far to use a radar gun on, also much too close to exclude any significant galactic stellar motions. Much the same is true if the figure is actually 85 kph (1050-1130 ly), as suggested it might be in the title text.

Aside from being practically incorrect, that value of 85 kph relates to around 53 mph, which might be the normally observed traffic speed on certain roads (especially if someone is conspicuously using a radar gun!) if by 'all directions' you effectively mean 'both directions' of traffic flow that Dave could possibly be measuring. Dave may have been referring to the kind of {{w|Ford Galaxy|Galaxy}} that he ''can'' more easily find out the velocity of.

The comic is likely making fun of the common internet phenomenon of amateur (wannabe?) scientists seeking to discredit established scientific facts by reporting the results of experiments made using everyday tools. Dave has probably heard of the fact that there is no agreement in the scientific measurements of the Hubble constant and decided to try to settle the controversy using the tools at his disposal, without remotely realizing that the margin of error required in the measurements is well outside the range of what can be used with conventional objects.

Dave might also lack an understanding of units of measure and dimensions. Ponytail describes the measurements of the rate of universal expansion, a speed that varies with distance, in km/s/Mparsec, having dimension 1/T or 1/time. Dave made his measurements in miles/hour or km/h, which have dimension L/T or length/time. These are not comparable with the official units. Dave does not appear to be aware of this (and Ponytail does not draw Cueball or Dave's attention to it).

==Transcript==
:[Cueball and Ponytail are walking to the right. Ponytail has her palm raised.]
:Ponytail: There are three main estimates of the universe's expansion rate and they all disagree.

:[They keeping walking to the right.]
:Ponytail: Measurements of star distances suggest the universe is expanding at 73 km/s/megaparsec.

:[They are still walking to the right.]
:Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec.

:[They continue walking to the right. Ponytail points towards Dave who replies from off-panel to the right.]
:Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions.
:Dave (off-panel): ''Those galaxies are really booking it!''
:Ponytail: Thanks, Dave.

{{comic discussion}}

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

2516: Hubble Tension

2021-09-16T08:31:05Z

162.158.78.77: /* Explanation */ Should I show my workings? I mean, feel free to check my results, I suppose, for silly magnitude errors or something.

{{comic
| number = 2516
| date = September 15, 2021
| title = Hubble Tension
| image = hubble_tension.png
| titletext = Oh, wait, I might've had it set to kph instead of mph. But that would make the discrepancy even wider!
}}

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

The fact that most galaxies are receeding from us, and that the distance to the galaxy is directly proportional to the speed (as measured by redshift) was discovered in the 1920s by Edwin Hubble and others. This constant of proportionality is known as the Hubble Constant.

One way of measuring the Hubble Constant is to measure the distance to (relatively) nearby galaxies. Once distance is obtained, speed can be easily obtained by measuring the red-shift and thus the Hubble Constant calculated. Measuring the distance turns out to be fiendishly difficult because a distant bright star looks the same as a dim star that is closer, and localised movements can influence the speed of recession — though less significantly, for multiple reasons, the further away are the objects are that you study.

In practice, astronomers have a number of ways of measuring distance that work at different scales, and they can be built upon to measure distance to far away galaxies. This is known as the cosmological distance ladder.

The first rung is parallax. As the Earth orbits around the Sun, nearby stars appear to move slightly relative to distant stars; a star that moves by one second of arc is said to have a distance of 1 Parsec — about 3¼ light years or 30 trillion (3x1013) kilometres.

The next rung is Cepheid variables, which periodically brighten and dim. The frequency of variation is related to the absolute brightness of the star, and thus by comparing the absolute to the relative brightness (subject to the {{w|Inverse-square law|inverse square}} where not otherwise obscured) the distance can be measured.

The final rung is Type 1a Supernovae, which occur when an accreting white dwarf exceeds 1.4 solar masses. Because the initial mass is always identical, the absolute brightness of the explosion is as well, so the distance can be similarly calculated.

Putting these together, the best measurement of the Hubble Constant is 73 km/s/Mparsec.

This is in conflict with the other main way of measuring the Hubble Constant, analysing makeup of the Cosmic Microwave Background radiation, which yields a value of 68 km/s/Mparsec. The difference is statistically significant, and well outside the error bounds of each measurement.

Since the CMB technique relies on our understanding and assumptions about the early universe, as well as on the cosmological effects of General Relativity on large scales, if this discrepancy proved real it could be the gateway to new discoveries in cosmology and gravity, as well as possibly shed light on the origin of the universe and a '{{w|Theory Of Everything}}'. Cosmologists got quite excited about this. It might also be that there was a previously unaccounted-for error in any of the rungs of the cosmological distance ladder, and that once that is fixed, the two results will be consistent.

The third method introduced in this comic is a guy named Dave who is trying to use a police radar gun to try to measure the movement of astronomical bodies. A radar system works by sending electromagnetic radiation from the gun and then measuring the returned radiation to determine how far away or how fast a moderately distant object is moving. Because of the transmission and return times required (and the inverse-square law), a radar device will only be able to get information about the very closest objects, such as the Moon and other objects orbiting the Earth (or the ''perhaps'' the Sun), where the influence of being in orbit utterly dominates over any possible Hubble-shift. And that still needs powerful radar systems like the former {{w|Arecibo Telescope}} to be able to get any useful information that far away, a hand-held radar gun would not be able to 'lock on' across even those distances.

Going by back-calculating grossly 'idealised' universe models, as suggested by the other two estimates, a receding velocity of 85MPH should be seen at a distance of roughly 16-17 thousand kilometres, about half the distance to geosynchronous orbit (where satellites do ''not'' move much at all, on average, on daily or even yearly timescales), and there seems no reason to believe the radar gun could discover anything at that distance at that speed. The same is true if the figure is actually 85KPH (for around 10,000km), as suggested in the titletext, although this velocity relates to around 50-55MPH, which might be the normal traffic speed on certain roads (perhaps when someone is conspicuously using a radar gun!) if by 'all directions' you effectively mean 'both directions' of traffic flow that Dave could possibly be measuring.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[Cueball and Ponytail are walking to the right. Ponytail has her palm raised]
:Ponytail: There are three main estimates of the universe's expansion rate and they all disagree.

:[They keeping walking to the right.]
:Ponytail: Measurements of star distances suggest the universe is expanding at 73 km/s/megaparsec.

:[They are still walking to the right.]
:Ponytail: Measurements of the cosmic microwave background suggest it's expanding at 68 km/s/megaparsec.

:[They continue walking to the right. Ponytail points to Dave, off screen]
:Ponytail: And Dave, who has a radar gun, says it's expanding at 85 mph in all directions.
:Dave (off-screen): ''Those galaxies are really booking it!''
:Ponytail: Thanks, Dave.

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Astronomy]]
[[Category:Physics]]

2513: Saturn Hexagon

2021-09-09T19:37:02Z

162.158.78.77: /* Explanation */ To clarify that windspeeds aren't km/h, etc.

{{comic
| number = 2513
| date = September 8, 2021
| title = Saturn Hexagon
| image = saturn_hexagon.png
| titletext = Sorry, in SI units that's "there's a big football in there."
}}

==Explanation==
{{incomplete|Created by CUEBALL'S POLAR HEXAGON - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
{{w|Saturn's Hexagon}} is a cloud formation on Saturn centered on its north pole. Similar to Jupiter's {{w|Great Red Spot}}, Saturn's Hexagon has proven a persistent feature observed by multiple space probes. The cause was not known until recently, when data from the 2006-2009 {{w|Cassini–Huygens}} probe could be analyzed in depth. This finding was widely publicized in popular science media (see for example [https://www.sciencealert.com/astronomers-think-they-figured-out-how-saturn-s-giant-hexagonal-storm-could-have-formed]) and is related to how currents flow deep within Saturn's atmosphere.

Randall proposes an alternate explanation: it is the top of a {{w|Ball_(association_football)|soccer ball}}. Soccer balls have been made in the shape of a {{w|truncated icosahedron}}, where faces alternate between regular hexagons and regular pentagons to achieve a more uniform roll, since 1968 when the design was introduced as the {{w|Adidas Telstar}}, a design now considered the "traditional" soccer ball.

BSBIT stands for Bachelor of Science in Business Information Technology [https://www.acronymfinder.com/Bachelor-of-Science-in-Business-Information-Technology-(BSBIT).html], a relatively new specialization where business majors learn programming techniques [https://vt.edu/academics/majors/business-information-technology.html]. It could be used in the comic to imply that a graduate of this major came up with the soccer ball model listed in the presentation, but more likely BSBIT stands for "Big Soccer Ball In There".

Soccer is the name given in the United States to {{w|association football}}, a game called simply "football" in most of the world. Since a system derived from {{w|Imperial units}} of measurement (inches, feet, miles, pounds, etc.) is used in the United States whereas the SI/metric system (centimetres, metres, kilometres, kilograms, etc.) is the system in use in most of the world, "football" is jokingly referred to in the title text as the SI name for "soccer". As much of the Web panders to a significantly US-based audience{{fact}}, many sites use only Imperial-like measurements and omit metric equivalents, which might annoy non-US users; Randall parodies this by sarcastically and non-seriously apologizing.{{fact}}.

Ironically, the UK is the birthplace of association football and place of the origin of the term "soccer" — originally to {{w|Names_for_association_football#Background|distinguish it}} from rugby football (sometimes "rugger"), before soccer became the most common form of football. "Football" now means association football for many in Britain, as with most people on Earth, though in particular regions and contexts it may instead mean {{w|Comparison_of_rugby_league_and_rugby_union#Naming|Rugby Union or League}}. Other international variations will usually be identified explicitly, as with 'American' football (gridiron, or jocularly "hand-egg"), '{{w|Australian rules football|Aussie Rules}}' football and {{w|Gaelic}} football (outwith its own dedicated celtic 'homelands').

The UK is also a partial hold-out for imperial measures. Officially many everyday measurements must now be primarily given in their metric forms, if not more specifically SI, but in the UK and the US road distances remain signed in miles, with road-speeds in miles per hour; glasses of brewed alcohol and doorstep milk deliveries are in pints (indeed, it is ''illegal'' in the UK to sell draught beer or cider except as a ⅓ pint or multiple of ½ pint); feet-and-inches and stones-plus-pounds are still commonly used for a person's height and weight. (It's worth noting that the American pint is 16 fluid ounces or 473 ml whereas the Imperial pint is 20 fl.oz. or 568 ml.) As a further sop to those who still think better in 'old money' measures (an allusion to how British currency itself was non-decimal in nature until 1971), a weather presenter may add to their metric-based summary to also give temperatures in Fahrenheit and rainfall in inches (though windspeeds will all be in mph, or the {{w|Beaufort scale}} as used in the {{w|Shipping Forecast}}).

Incidentally, the presentation of the truncated-icosahedral 'football', pressing one clear polygonal face up along the upper limit of the planetary sphere, has much in common with the (non-truncated) icosahedron that floats within a {{w|Magic 8-Ball}}, arranged to display just one random triangular face whenever its viewing window is upwards. This may be coincidence, without any obvious attempt to directly reference any of the [https://knowyourmeme.com/photos/1404098-safely-endangered popular memes] relating to this. Randall has previously parodied the magic 8-ball in [[1525: Emojic 8 Ball]].

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

[Cueball is presenting in front of a poster, which he is pointing at with a stick.]

Cueball: We're proud to announce that our team has finally determined the origin and nature of Saturn's polar hexagon.

[The poster represents Saturn and its ring-system. There is a massive football/soccer ball drawn as if inside the semi-transparent planet, taking up slightly less than half of it by volume.
One of the ball's hexagons coincides with Saturn's polar hexagon, and is labelled "Hexagon". Other labels are illegible. 
The poster's title is "There's a Big Soccer Ball In There". The rest of the poster is illegible, except for a section heading that reads "BSBIT Model".]

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category: Astronomy]]
[[Category:Soccer]]

2513: Saturn Hexagon

2021-09-09T14:28:33Z

162.158.78.77: /* Transcript */ The singularly illustrated 'ring' must be (some of) the multiple 'rings' (mash-up of mergy/gappy orbitals), but simplified. So let us treat it as a combined-singular for practical exposition.

{{comic
| number = 2513
| date = September 8, 2021
| title = Saturn Hexagon
| image = saturn_hexagon.png
| titletext = Sorry, in SI units that's "there's a big football in there."
}}

==Explanation==
{{incomplete|Created by CUEBALL'S POLAR HEXAGON - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
{{w|Saturn's Hexagon}} is a cloud formation on Saturn centered on its north pole. Similar to Jupiter's {{w|Great Red Spot}}, Saturn's Hexagon has proven a persistent feature observed by multiple space probes. The cause was not known until recently, when data from the 2006-2009 {{w|Cassini–Huygens}} probe could be analyzed in depth. This finding was widely publicized in popular science media (see for example [https://www.sciencealert.com/astronomers-think-they-figured-out-how-saturn-s-giant-hexagonal-storm-could-have-formed]) and is related to how currents flow deep within Saturn's atmosphere.

Randall proposes an alternate explanation: it is the top of a {{w|Ball_(association_football)|soccer ball}}. Soccer balls have been made in the shape of a {{w|truncated icosahedron}}, where faces alternate between regular hexagons and regular pentagons to achieve a more uniform roll, since 1968 when the design was introduced as the {{w|Adidas Telstar}}, a design now considered the "traditional" soccer ball.

BSBIT stands for Bachelor of Science in Business Information Technology [https://www.acronymfinder.com/Bachelor-of-Science-in-Business-Information-Technology-(BSBIT).html], a relatively new specialization where business majors learn programming techniques [https://vt.edu/academics/majors/business-information-technology.html]. It could be used in the comic to imply that a graduate of this major came up with the soccer ball model listed in the presentation, but more likely BSBIT stands for "Big Soccer Ball In There".

Soccer is the name given in the United States to {{w|association football}}, the form of football practiced in most of the world. Since the imperial system (inches, feet, yards, miles, etc.) is also used in the United States whereas the SI/metric system (centimetres, metres, kilometres, etc.) is the system in use in most of the world, "football" is jokingly referred to in the title text as the SI name for "soccer". As much of the Web panders to a significantly US-based audience{{fact}}, many sites may use only Imperial measurements and omit metric equivalents, which might cause annoyed international users to respond; Randall parodies this by sarcastically and non-seriously apologizing.{{fact}}. Randall writes for his US-based college-educated community from Massachusetts.

Ironically, the UK is the birthplace of association football, and the origin of the term "soccer" — originally to {{w|Names_for_association_football#Background|distinguish it}} from rugby football (sometimes "rugger"), before soccer became the most common form of football and is now considered the default code of football for many, though particular regions and contexts may instead give {{w|Comparison_of_rugby_league_and_rugby_union#Naming|Rugby Union or League}} that soubriquet. Other international variations will usually be identified explicitly, as with 'American' football (gridiron, or jocularly "hand-egg"), '{{w|Australian rules football|Aussie Rules}}' (more rarely discussed, without Hollywood levels of cultural exporting) and {{w|Gaelic football}} (outwith its own dedicated celtic 'homelands').

The UK is also a partial hold-out for imperial measures. Officially many everyday measurements must now be primarily given in their metric forms, if not more specifically SI, but in the UK and the US road distances remain signed in miles (though horse racing distances remain in furlongs, and their prizes in guineas), with road-speeds in miles per hour; glasses of brewed alcohol and doorstep milk deliveries are in pints (indeed, it is ''illegal'' in the UK to sell draught beer or cider except as a ⅓ pint or multiple of a half-pint); feet (plus inches) and stones (plus pounds) are still commonly used for a person's height and weight. As a further sop to those who still think better in 'old money' measures (an allusion to how the currency itself was non-metric in nature until 1971), a weather presenter may add to their summary (with the windspeeds in mph, except when in Beaufort scale) the equivalent Fahrenheit temperatures involved, in addition to the more official (i.e. 'new money') Celsius variety, and perhaps also give the inches version of any rainfall mentioned.

Incidentally, the presentation of the truncated-icosahedral 'football', pressing one clear polygonal face up along the upper limit of the planetary sphere, has much in common with the (non-truncated) icosahedron that floats within a {{w|Magic 8-Ball}}, arranged to display just one random triangular face whenever its viewing window is upwards. This may be coincidence, without any obvious attempt to directly reference any of the [https://knowyourmeme.com/photos/1404098-safely-endangered popular memes] relating to this.

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

[Cueball is presenting in front of a poster, which he is pointing at with a stick.]

Cueball: We're proud to announce that our team has finally determined the origin and nature of Saturn's polar hexagon.

[The poster represents Saturn and its ring-system. There is a massive semi-transparent football/soccer ball inside of the planet, taking up slightly less than half of it by volume.
One of the ball's hexagons coincides with Saturn's polar hexagon, and is labelled "Hexagon". Other labels are illegible. 
The poster's title is "There's a Big Soccer Ball In There". The rest of the poster is illegible, except for a section heading that reads "BSBIT Model".]

{{comic discussion}}