Moon Shapes
Moon Shapes
[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.

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 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!).
[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.
