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Revision as of 05:35, 4 March 2013

Welcome to the explain xkcd wiki!

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Go to this comic explanation

Moon Shapes
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.
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, 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 Earth's 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 circular from Earth). Mid-way between those extreme phases is a "quarter" moon, when exactly half of the surface facing the Earth is completely dark, the light half being one-quarter of the total moon's surface.

Because the moon is approximately spherical, as the moon progresses from "new" to "quarter" (or vice versa), the illuminated portion forms a crescent. As it progresses from "quarter" to "full" (or vice versa), the illuminated portion it is described as "gibbous" (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 centre 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 centre 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 humour 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.

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 the on the half of the moon that faces the horizon (there are points during the daytime when the orientation can go the other way).

It is also notable that the portion of the moon that is dark is still a large hunk of (unlit) rock, and therefore even at night when we can't distinguish between the dark part of the moon and the background blackness, it would still 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 the 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!).

Explanation of individual items in the list
Image Shape Explanation Examples
moon1.jpg Full moon Normal Reality
moon2.jpg Gibbous moon Normal Reality
moon4a.jpg Crescent moon with horns away from horizon Normal Reality
moon3.jpg Crescent moon with horns towards horizon 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 Van Gogh, Landscape with Couple Walking and Crescent Moon
moon5a.jpg Wide crescent-like moon where the horns don't connect through a diameter 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 ligther 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. Van Gogh, Starry Night
moon6.jpg Narrow crescent-like moon where the points don't connect through a diameter 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
moon7.jpg Crescent moon blocking stars Normal Reality (but see Trivia below)
moon8.jpg Crescent moon with stars between horns Many people (including artists) seem to forget that the dark portion of the moon is still a solid object that we can not 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. Edwin Blashfield, Spring Scattering Stars, the DreamWorks Animation logo

Trivia

moon7a.jpg The image of the crescent moon blocking the stars is slightly wrong, because there are still lights on the surface of the moon.

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.]
Only possible during a solar eclipse
[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.]
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.]
There's either a hole in the Moon or a nuclear war on its surface.


Is this out of date? Clicking here will fix that.

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You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them! Here's how.

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.

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