1738: 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.
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
|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.|
|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. 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.|
- [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 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.
- 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 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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