1738: Moon Shapes

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Moon Shapes

Title text: Whenever I see a picture of the moon where the points go more than halfway around, I assume it's being eclipsed by one of those Independence Day ships and interpret the rest of the image in light of that.

Explanation

The Earth's Moon, being the most prominent object in the night sky (most of the time), is a frequent subject of art; particularly art depicting a nighttime scene. Unfortunately, the moon often appears in works of art in ways that are very dramatic and would not be realistically possible. It may be done out of ignorance, or knowingly by taking artistic license. As someone interested in and who has worked in astronomy, this likely bothers Randall.

The Moon is well known to have "phases" describing what portion of the visible surface of the moon is illuminated by sunlight and highly visible, and what portion is dark, and generally only slightly visible when the moon appears while the sun is also up. These phases progress between "New" (when the surface facing the Earth is completely dark) and "Full" (when the surface facing the Earth is completely illuminated, appearing as a full disk as viewed from Earth). Mid-way between those extreme phases is a "Quarter" Moon, when exactly one-half of the surface facing Earth is completely dark - at this point the Moon is a quarter of the way in its cycle from the New Moon, either one quarter of the way into it ("First" Quarter) or a quarter of the way from completing it ("Last" Quarter).

Because the Moon is approximately spherical, its illuminated side appears as "crescent" in shape as it progresses from New to First Quarter phase. As it progresses from First Quarter to Full phase, observers on Earth see a Waxing "Gibbous Moon (which just means that the dark portion has formed a crescent). One can imagine this like a globe on which you draw a straight line from the north pole to the south pole down the center of the side facing you (appearing to create two semi-circles); upon rotating the globe, the line would become rounded as it moved away creating a crescent on the side the line was moved towards. Because of the geometry involved, a line connecting the two points (horns) of a Crescent Moon (or of the darkened crescent inverse of a Gibbous Moon) must be a diameter of the moon (i.e. it must pass through the center of the circle).

The deliberate misidentification of a Waxing Gibbous Moon ("waxing" means going from new to full; that is increasing in illuminated area) as a "wax gibbon" (a Southeast Asian ape made of a nonpolar solid) is a source of humor in this comic. This is probably a reference to H.P. Lovecraft, who had several of his stories take place under "a gibbous moon" for dramatic effect, or even more likely a reference to the Discworld by Terry Pratchett, often referenced in xkcd (as in 1498: Terry Pratchett). In the witch series the Gibbous Moon is mentioned several times as the most magic, rather than the more often used Crescent or Full Moon.

Further, because the light portion of the Moon is illuminated by sunlight (whether or not the Moon is in the sky at the same time as the Sun), the light side of the Moon will always be facing towards the Sun. If the Moon is in the night sky, the Sun must be somewhere "below" the horizon on the other side of the Earth. Thus, at night, the light portion of the Moon must always be on the half of the Moon that faces the horizon (there are points during the daytime when the orientation can go the other way); however, because of the moon tilt illusion it is possible for the light portion of the moon to appear to point up. The moon tilt illusion is generally not as severe and may only last a few hours after sunset.

It is worth noting that while the Moon's dark portion blends imperceptibly with the dark night sky, it is still a solid body. Therefore, it would be impossible to see more distant objects such as stars "through" the dark portion of the Moon's circumference. This is most dramatically exemplified by a solar eclipse during which the Moon passes in front of the Sun and is therefore completely dark (the Sun is lighting only the far side), but the Moon's circumference still blocks a circular portion of the Sun's light. Therefore, if we were to see any lights in the part of the sky the dark side of the Moon blocks, they would need to be from sources between us and the Moon's surface, such as a nuclear war on the moon.

This comic lists some of some common mistakes. In some cases, a depiction may be unrealistic in multiple ways - for example, the Flag of Tunisia has both unrealistic horns and a star visible between the horns, while the Charles VI tarot shows a Moon with over-long horns pointing towards the horizon.

In the title text, Randall is referring to the movie Independence Day and how one of the alien's ships (in the movie) 'eclipses' part of the Moon. He says that if the points go halfway or longer around the Moon, then he imagines it's caused by an alien ship and interprets the entire piece of art in that context (i.e. aliens are about to attack those shepherds!).

Table of the images

Explanation of individual items in the list

#	Image	Shape	Rating	Text	Explanation	Examples
1		Full moon	Normal	"Full" or "Quarter" or "Harvest" or "Wax Gibbon" or whatever	Reality. The full moon cannot really be drawn incorrectly, and will look like this whenever it is up at night.
2		Gibbous moon	Normal		Reality, as in this is how one of the moon's phases looks on a normal basis.
3		Crescent moon with horns away from horizon	Normal		Reality, as in this is how one of the moon's phases looks on a normal basis.	LOOK AT THE MOON
4		Crescent moon with horns towards horizon	Not normal	Not possible at night	This can only happen when the sun is above the horizon. Since a crescent moon means that the Sun and the Moon are relatively close in the sky, the Moon would not be visible with a naked eye, its light completely outshone by the sunlight. Randall comments that this is possible only during the daytime, marking it wrong as the background would not be black. According to this image on Wikipedia's article on lunar phase "Phases of the Moon, as seen looking southward from the northern hemisphere. The southern hemisphere will see each phase rotated through 180°". This might seem to indicate that shape #3, which is visible in for instance USA where Randall lives should be seen like #4 in the southern part of South America, also at night! However, because the light portion of the moon is illuminated by sunlight (whether or not the moon is in the sky at the same time as the sun), the light side of the moon will always be facing towards the sun. If the moon is in the night sky, the sun must be somewhere "below" the horizon on the other side of the Earth. Thus, at night, the light portion of the moon must always be on the half of the moon that faces the horizon; However, in the case of twilight after sunset and shortly after (like the example painting), due to the moon tiltillusion the bright side of the moon may appear to point up relative to an observer on the ground (although not to the extent show in the comic). But as the text from Randall points out there can be times during daytime when the orientation can go the other way. But then the background should not be black. Originally Randall wrote a different (wrong) sentence here and then corrected to the one currently explained, see trivia below.	Van Gogh, Landscape with Couple Walking and Crescent Moon
5		Wide crescent-like moon where the horns don't connect through a diameter	Not normal	Only possible during a lunar eclipse (#1 only, dubious) or a solar eclipse (bright part is the Sun)	This is only possible during a partial solar eclipse or the start of an annular eclipse (in which cases the lit portion is not the moon, but the sun), or else if the Earth is casting its partial shadow on the Moon, a penumbral lunar eclipse. Randall labels the lunar eclipse "dubious", since the shadow during penumbral eclipse would be much lighter than shown here, in fact barely visible as a slight darkening of the Moon's surface. The Earth's shadow, being very large, would also likely cast a less-rounded edge than depicted here.
6		Narrow crescent-like moon where the points don't connect through a diameter	Not normal		This situation is even harder to create than the previous one - unlike the previous example, here the diameter of the entire shadow is clear, and is too small for the Earth's shadow in a lunar eclipse. A huge Independence Day spaceship (as per the Title text) might be the right size. It does however resemble a partial annular eclipse if you imagine that the black area is the moon covering up the white sun.	Alcoholic Blues. Van Gogh, Starry Night but turned the other way.
7		Crescent moon blocking stars	Normal	Looks OK	Reality (as in image 3) with stars shown around the moon, but not any inside the sphere of the sky that would be blocked by the dark (but still present) side of the moon. (See trivia below though).
8		Crescent moon with stars between horns	Not normal	There's either a hole in the Moon or a nuclear war on its surface	Many people (including artists) seem to forget that the dark portion of the moon is still a solid object that we cannot see through.[1][2] If stars are visible, there are either one or more holes in the moon, or the light-source is actually on the moon, such as nuclear explosions. As the Star and Crescent, the image is sometimes considered a symbol of Islam, although it's relatively recent and there's no traditional basis for putting the star between the horns - as originally used on the Flag of Turkey, the star appears in a realistic position. The flags of Algeria, Tunisia and some other countries show the star in the dark part of the moon. In the first Edwin Blashfield, Spring Scattering Stars a God is standing on the moon throwing stars down, but then these stars are actually in front of the moon and are good. Nothing in the image suggest that stars can be seen through the dark part of the moon. Also, the DreamWorks Animation logo shows no stars. Although both show persons sitting on the seal, this is thus also clearly not an effort to make it look real. But in neither case stars can be seen in the moon. This is also the case for the live DreamWorks logo from movies. Here there are stars in the background, but they are not inside the moon as can be seen here.	An example can be found in the image on the last page of How mole got his car with the Mole from the carton series by Zdeněk Miler. This is not just showing the stars inside from the last shape, but also the type of moon shown in the sixth image.

Transcript

[Caption above the panel:]

Interpreting the shape of the moon in art

[The left part of the panel shows a two-column chart is shown with labels above the columns. The left side shows the moons shape as white on a black square. These types of moons could be seen in certain art pieces. The right side saying whether this is normal or not as indicated with a green check mark or a red X. Right of the second column there are explanations of why the specific type of moon is marked as it is and what it could be called or how it could be possible even with the red X. The upper three moons have one common explanation as indicated with a bracket that covers all three with the text on the middle part of the bracket. Similarly moon five and six also have a bracket and only one explanation.]

Shape Normal?

[Shape #1-3 shows a white circle (full moon), a more than half full moon (Gibbon) and a thin seal at the bottom right of the square.]

✓

✓ "Full" or "Quarter" or "Harvest" or "Wax Gibbon" or whatever

✓

[Shape #4 same as #3 but with the seal in the upper part of the square.]

X Not possible at night

[Shape #5-6 shows a full moon with a circular section taken out of the right side and a seal that goes almost all the way around the circumference of the moon with almost a full circle taken out of the top left part of the moon.]

X

X Only possible during a lunar eclipse (#1 only, dubious) or a solar eclipse (bright part is the Sun)

[Shape #7 same as #3 but with the seal a little smaller and more to the top and less to the left. Around the moon there are several starts represented with 29 small white dots. In the center of the black square there is a black circle, coinciding with the outer rim of the seal. Within this circle (the dark side of the moon) there are no stars!]

✓ Looks OK

[Shape #8 same as #7 but apart from the 29 small white dots from before there are now also 6 more dots inside the dark circle with no stars in #7.]

X There's either a hole in the Moon or a nuclear war on its surface.

Trivia

• Randall changed the text for the fourth moon shape the next day from "Only possible during a solar eclipse" to "Not possible at night".
  • The original can be found here. The news section now reads "Thank you to Phil Plait for the correction on #4!"

• The image of the crescent moon blocking the stars is slightly wrong because there are still lights on the surface of the moon.

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Discussion

It's a reflection of the nuclear war on the sun's surface. Mikemk (talk) 08:08, 26 September 2016 (UTC)

I can't find any photoshopped Moon that looks like the last image. Somebody has to make one. --162.158.92.207 13:22, 26 September 2016 (UTC)

Randall uncharacteristically missed an opportunity for pointing out additional errors that people make: It's interesting to note that you can get a decent estimate of the artist's latitude by looking at how they draw a crescent moon. In equatorial cultures, the crescent looks like a cup or a boat - and they interpret it like that. But if you look at most english language children's books, the crescent looks like a letter 'C' or a 'D' with a human face - suggesting that they were probably made in the tradition of northern Europe. When I first moved from the UK (more or less a 'C'-shaped crescent moon) to the southern USA (more like Randall's depiction of the correctly-drawn crescent with the points at a roughly 45 degree angle to the horizon) - I subconsciously felt that the moon "looked wrong" - it was only much later that I understood the reason.

Furthermore, this rotation of the moon relative to the observer also explains why "The man in the moon" is a common trope caused by the pareidolia interpretation of the cratering patterns of the moon in northern cultures. But in southern cultures, people tend to see a rabbit in those full-moon patterns - and that has become the source of many of their stories.

Now that I'm more acutely aware of this - it's interesting to note how many movies get the orientation of the moon wrong for the location that their story is supposedly set in! SteveBaker (talk) 13:41, 26 September 2016 (UTC)

In Japan, for example, the patterns are interpreted as a rabbit making mochi (a sort of dense dumpling made from rice pounded into a powder) on the moon - the Sea of Tranquillity forming the head, and the Sea of Clouds forming part of the pestle in which the rabbit is pounding the rice. 188.114.102.167 (talk) (please sign your comments with ~~~~)

Considering how many movies features the famous Mountains of Illinois, I would be more surprised when they get it right. -- Hkmaly (talk) 13:38, 27 September 2016 (UTC)

Not quite sure how to add this but Gibbon is the author of The Decline and Fall of the Roman Empire - or a type of Ape. It is not a phase of the moon. Also I think the moon depicted is Waning. 141.101.98.113 14:02, 26 September 2016 (UTC)

I think the correct expression is gibbous - "having the illuminated part greater than a semicircle and less than a circle" 141.101.98.68 (talk) (please sign your comments with ~~~~)

Yes, the one he says is correct has me thinking: "OMG, the moon is drunk and has fallen over on its ass." No self-respecting moon lies on its back like that.162.158.114.222 14:17, 26 September 2016 (UTC)

Indeed - but that's pretty much how it looks down here in sunny Texas. It's one of those things you never think about - but once the fact of it clicks in your head, you get this visceral feeling of how you're standing on a large ball rather than a flat plane! Ha! Take that flat-earthers! :-) SteveBaker (talk) 18:46, 26 September 2016 (UTC)

Living in Florida, our crescent moons are almost horizontal. 108.162.212.60

"Wax gibbon" is probably nothing more than a joke on mispronouncing "waxing gibbous". As drawn, it is the way a waning gibbous would appear in the northern hemisphere, but a waxing gibbous in the southern hemisphere. Harperska (talk) 16:18, 26 September 2016 (UTC)

Link to the DreamWorks logo image please? There seem to be multiple versions. 108.162.237.216 15:16, 26 September 2016 (UTC)

I'm surprised Randall missed the chance to include a joke about guys with fishing rods. --162.158.85.141 15:41, 26 September 2016 (UTC)

http://www.roadtovr.com/wp-content/uploads/2014/11/dreamworks-logo.jpg... Here's the link to the Dreamworks logo. You're welcome. --JayRulesXKCD (talk) 16:17, 26 September 2016 (UTC)

neither of the examples for the "stars in the moon" apply here. both randall's examples imply a spherical moon. spring and the dreamworks child are supported on a crescent moon for which only the light section actually exists. better examples, please. --141.101.98.84 12:26, 27 September 2016 (UTC)

While the dreamworks logo features a child sitting on the crescent moon as though the dark portion wasn't there, none of the versions of the logo which contain stars in the sky actually show stars 'inside' the moon's disk, so the logo probably isn't a good example of what Randall is complaining about. See https://www.youtube.com/watch?v=xPpy8mYHQps. Harperska (talk) 17:16, 27 September 2016 (UTC)

The first "wrong" image is also only possible if the bright portion is presumed to be the sun during a solar eclipse, assuming the sky is actually depicted as black. You can only have a crescent moon during a solar eclipse if the solar system suddenly acquired a second sun. Harperska (talk) 16:28, 26 September 2016 (UTC)

Here's a good counter-example: EXAMPLE. The bright dot is actually the ISS transiting the moon - but it certainly looks like an impossibility! SteveBaker (talk) 18:50, 26 September 2016 (UTC)

The article doesn't mention the "nuclear war" joke. Does it need explaining? 141.101.98.49 19:29, 26 September 2016 (UTC)

It does now. WingedCat (talk) 22:24, 27 September 2016 (UTC)

your welcome ;-) NotLock (talk) 22:30, 27 September 2016 (UTC)

I've always wanted to create a story, and have the horns of the moon connect on the other side, so you have a blackbody in front of the moon, in parody of this tendency. Also, interesting how the moon is at different rotations in different locations. I never did see the rabbit in the moon. Now I know why. 108.162.245.74 04:34, 27 September 2016 (UTC)

Some of the examples of "incorrect" moons are kinda questionable - like, how relevant is the position of the moon when there's literally a giant divine skyperson standing on it, grabbing stars and scattering of them? 172.68.35.80 23:09, 26 September 2016 (UTC)

I fixed part of the explanation by mentioning the title text. --JayRulesXKCD (talk) 14:41, 26 September 2016 (UTC)

I think that's enough detail for an explanation, so I removed the "incomplete" bar. --JayRulesXKCD (talk) 14:52, 27 September 2016 (UTC)

Should the table of explanations include the text in the comic? NotLock (talk) 22:30, 27 September 2016 (UTC)

The comic has been updated on xkcd. Randall revised the description of #4. Maybe this should be updated? 108.162.242.135 21:55, 27 September 2016 (UTC)

for trivia doesn't mass bend light so IMHO probably still possible for star light on that position 162.158.163.61 05:07, 28 September 2016 (UTC)

Nope - if a distant star's light were being gravitationally lensed by the moon, then its light would appear adjacent to the moon's disk, not on it. A star's light could only appear to originate on the moon's disk if its light were being lensed by another massive object between the earth and the moon (such as a second moon). Whoop whoop pull up (talk) 17:10, 16 October 2016 (UTC)

For the Dreamworks logo, it seems that the static versions don't sport the stars-through-the-moon problem, but I suspect the animated versions (the ones showing at the beginning of their movies) might and probably do. As it so happens, I'm watching Kung Fu Panda 2 for the first time right now, - the fishing kid is a turtle in this case, LOL! - and I note that this problem isn't present here, though there are no stars at all during the logo, so that might be the only reason why. (Of course, if a kid can sit there fishing, the rest of the moon is clearly absent, why wouldn't we see stars there, LOL!) - NiceGuy1 108.162.218.239 18:40, 28 September 2016 (UTC) I finally signed up! This comment is mine. NiceGuy1 (talk) 11:12, 9 June 2017 (UTC)

I swear to god I've seen an Upside down moon before.... both in the day and at night the cusps were pointed down or towards the horizon heres an example of one https://www.flickr.com/photos/dcysurfer/14631243979 i'm 100% sure this is not fake and that there is no reason why one would use a dslr upsidedown or rotate it in post (this is in the southern hemisphere so that may be why... i also swear that many times i've seen the cusps pointed directly to the side (northern hemisphere) heres a nasa photo http://www.nasa.gov/centers/wallops/news/InOMN.html Needforsuv (talk) 06:57, 3 October 2016 (UTC)

You haven't, at least not at night. The Flickr photo isn't evidence at all -- people often rotate pictures for best effect. 172.68.55.80 15:55, 3 October 2016 (UTC) i swear it was at night. also the nasa photo looks the same...

i'll take a photo of it one day... it could've been dusk/ last light and maybe it was just a touch rotated down without accounting for the slope the user is standing on but its not impossible -- Needforsuv (talk) (please sign your comments with ~~~~)

'If the Moon is in the night sky, the Sun must be somewhere "below" the horizon on the other side of the Earth.' Funny how people still talk as if Earth is the center of the universe. More correct would have been to say something about "Earth's rotation" and stuff. 162.158.83.144 (talk) (please sign your comments with ~~~~)

Not funny at all. There is no concept of 'down' except from the point of view of a person on the Earth.

The moon that does look like the last one is the one in the old Proctor and Gamble logo. https://goo.gl/images/4UE6n6 162.158.10.4 (talk) (please sign your comments with ~~~~)

Phil Plait reads xkcd

Has anyone else noticed the Phil Plait message in the news section (up top) ? In case it changes: "Thank you to Phil Plait for the correction on #4!" Blagae (talk) 20:16, 4 June 2017 (UTC)

UPSIDE down moon PROOF:

[3] 
I took the photos MYSELF Needforsuv (talk) 09:26, 28 March 2018 (UTC)