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So, I'm developing a world for my fantasy setting, but I thought it'd be neat if this particular world had a nebula visible in its night sky. (So, it might not be a nebula like the kind we have in the real universe.) Anyway, I thought maybe there might be a point during the year where the moon would be completely black but it would still be visible because the nebula would be located behind it. Problem is, I can't figure out how to make that happen, except perhaps during a lunar eclipse. But the moon isn't black during a lunar eclipse, it's red. So, how would I be able to get this black moon with the nebula as a backdrop to happen?

Update: So, I've received a lot of great feedback, but I should probably clarify something about the nebula for people so we're all on the same page. It isn't a nebula in a literal sense. This is a fantasy setting, after all. The nebula was created when one god sucker-punched another god during The Divine War, a period where a whole bunch of gods were fighting with each other. Most of their fighting was on another plane of reality, but sometimes the fallout carried over into other universes.

The nebula, which the Orcs refer to as "The Sky Wound," isn't composed of the same materials as nebulae are in the real universe. It's a massive cloud of divine energies that bled over from the plane where the gods were fighting. The god who got sucker-punched lost pieces of himself, which were scattered across the mulitverse. One piece might be at the center of the nebula, but I'm not sure of that. (It seems a bit obvious.) Anyway, the nebula is generating light, not reflecting it. As the god has been waking up from being KO'ed for thousands of years, the nebula has shifted from crimson to green. Currently, because he's fully awakened, it's completely green. As he locates the pieces of his being that were broken off of him, the nebula will shrink and ultimately vanish.

Regarding the moon, it's probably around the same size as ours. I don't want it to be so close that it will be causing all kinds of tidal and other issues for the Orcs, so it can't be in a position where the red wavelengths filtered through the planet's atmosphere will be unable to reach it during a lunar eclipse. That leaves us with finding a way to negate the red wavelengths from being reflected back to the planet during an eclipse.

The moon having an atmosphere is the most appealing explanation for me, since it presents some interesting opportunities. As I said in one of my comments, portals between planets are a common thing in this setting. I've been considering the idea of some worlds having portals that connect to their moons for a while now. It'd be fun if the Orcs could go to their moon to gather certain resources. Mind you, this doesn't mean the atmosphere would be breathable. In fact, I'd prefer that it not be. All it should do is give them enough protection from the sun and the vacuum of space that they don't have to worry about those issues. Beyond those two things, however, the atmosphere should be of no benefit. The Orcs would need to come up with some kind of magical life support system to protect themselves from it enough that they could mine for resources.

So, how do you guys think we could pull something like that off? Just to recap, the moon will have an atmosphere that does not reflect red light and will prevent the Orcs from decompressing, freezing, or burning to death when they use a portal to reach the moon. There also needs to be something keeping them safe from the lethal radiation coming from the sun. Magic can be a solution to these problems, of course, but I'm looking for non-magical explanations since I don't want to be using magic as a crutch. In other words, I'd prefer that the only solution magic offers to setting up mining operations on the moon is in the form of the Orcs having a life support system providing them with breathable air.

This is getting fun.

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    $\begingroup$ See also What are the effects of a planet staying long-term inside of a nebula? (Full disclosure: The accepted answer is my own.) $\endgroup$ – a CVn Jul 28 '18 at 8:38
  • $\begingroup$ If you changed the material of the moon from your standard rock to one that absorbs light very effectively (possibly a fantasy material), then the moon wouldn't really show up in general, unless there was light behind it. The moon would practically matte black, and wouldn't provide light at night, making the earth darker, but the night sky would be brighter in comparison, assuming light pollution (lots of light, like from cities or such) isn't present. $\endgroup$ – Iter Jul 28 '18 at 19:39
  • $\begingroup$ A fantasy material that absorbs red light specifically seems to be the best route to go at the moment, since I do want the moon to provide light for the inhabitants of the planet. And it's not like lunar eclipses only happen when the nebula is behind it. Most of the time, the moon is full when it passes between the planet and the nebula. A lunar eclipse happening at the same time is a rare event, and thus very significant to the locals (who are Orcs, if you're wondering.) I wanted the moon to contrast with the nebula as much as possible, though, so black is the best color for it to take. $\endgroup$ – Patrick-Leigh Jul 28 '18 at 19:50
  • $\begingroup$ How important is it to see the moon at other times? What if we change the composition of the moon's soil to seriously reduce its albedo we can get black. Basically, serious light absorption. That would make it black - all the time. $\endgroup$ – JBH Jul 28 '18 at 22:27
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    $\begingroup$ The moon doesn't need to be as bright as ours is, but I do want the black moon to be a rare and significant occurrence for the Orcs, so it needs to be visible in the night sky, regardless of whether the nebula is visible or not. That doesn't mean the moon has to be as luminous as ours is, of course. I just want it to turn black during a lunar eclipse, regardless of what the planet's atmosphere is like at the time. And it doesn't have to be a white moon, either, just so we're clear. It just needs to absorb red light instead of reflecting it. $\endgroup$ – Patrick-Leigh Jul 28 '18 at 22:39
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Let's see the moon cycle :

moon phases

New moon is sort of dark. You could have an convenient cycle where there is one single day each year where the new moon passes in front of the nebula.
Problem is, new moon is only visible at day. So I don't think it fits your goal.

Back to the Lunar eclipse. Most pictures are red (because of Rayleigh scattering), but it's not always the case. A dark moon during a lunar eclipse does happen :

Lunar eclipse brightness (Danjon scale)

So you can have a dark moon over your nebula, at night. But... (there is another but)

Rayleigh scattering depends mostly on the earth's atmosphere, not astral bodies positions. Long story short, your earth would need a lot of volcanic activity to ashen the sky and block the light refraction. Seems unlikely to happen once a year, and would certainly have severe consequences on your ecosystem.

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  • $\begingroup$ Speaking of the eclipse brightness, I believe the one just the other day, the pictures I saw were very close to the 1. Dark Eclipse seen here. I wasn't able to find an official brightness value though and other pictures ranged from 2 to 4 on this chart... $\endgroup$ – Draco18s Jul 29 '18 at 4:43
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  1. Red wavelengths are scattered least by the atmosphere. Make your atmosphere or that of your moon absorbent of red rays.

Longer wavelengths (reds) scatter less than shorter (blues). That is why when there is a straight path from source to eye thru much scattering atmosphere (as at sunset) the reds get thru best, with shorter wavelengths scattered proportionately to their shortness.

So too with refraction through the atmosphere into the umbra, or shadow of the planet. The less a wavelength is scattered, the better it traverses the atmosphere (bending along the way) to hit a far target, in this case the moon.

The converse of scattering is absorption. Long wavelengths are absorbed the best by absorbent stuff in the way, with shorter wavelengths bouncing off to continue more or less on their way. Water absorbs long wavelengths well, and this accounts for why blood looks green at depth. The intervening water has absorbed red rays. https://www.pinterest.com/pin/356910339188690671/ green blood at depth

If your planet has an atmosphere absorbent of long wavelengths it would soak up the red light on its way to the shaded moon. Absorbent molecules include water, nitrous oxide and methane. This would also work for your moon if it had an atmosphere. Methane in the atmosphere of Uranus absorbing red light is why that planet looks blue - that is what is not absorbed. If Uranus were lit only with red light it would look black.

2. Make the totally shaded area bigger.

Here is a schematic from wikipedia. I have added a suggested dark moon - small and close to its planet. https://en.wikipedia.org/wiki/Lunar_eclipse

eclipse schematic with dark moon added

Suppose the moon were much closer to the planet, so close that it was within that central dark triangle. The red rays could not bend enough to illumnate it and it would be dark. It would help if the moon were comparatively small and the planet large. I thought maybe the little Martian moon Phobos was close enough to Mars for it to be completely dark during a martian "lunar" eclipse. Also I thought maybe the thin Martian atmosphere would refract less light than is the case on Earth. Not quite - there is still a little bit of Mars light lighting Phobos. But the principles are sound.

phobos eclipse

https://mars.nasa.gov/mer/gallery/press/spirit/20051115a/Phobos_Eclipse_639A-A643R1.jpg

So to make your moon black during the eclipse:

1: Put red-absorbing gases (water, methane, N2O) in the atmosphere of your planet or moon.

2: Make your planet big and your moon close and small. Distant star would work too to maximize planet shadow size.


I was wondering about whether enough starlight might have reflected off Mars to light Phobos. Then I realized I forgot one thing for this scenario: the nebula. It is going to be bright - probably a lot brighter than a moonless sky on earth. That starlight / nebula light will reflect from the planet and back onto the dark moon. The solution I can think of is to make the planet as light absorbent as possible - which probably means forests.

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  • $\begingroup$ The moon having an atmosphere with red absorbing gasses actually seems like a neat thing to do. In many of the science fiction and fantasy works I've encountered, the moons of planets are nearly always devoid of any atmosphere, depicted as barren balls of rock. I like the idea of giving the moon an atmosphere, but that does raise the question of what sort of atmosphere would be the most interesting. Would it block out the view of the moon's surface entirely? How would it appear from the planet below? What size does the moon have to be to retain such an atmosphere? I'm open to suggestions $\endgroup$ – Patrick-Leigh Jul 29 '18 at 0:24
  • $\begingroup$ Give it an atmosphere that works for your story, then figure out how that came to be. Titan is a moon with a righteous atmosphere - but Titan is big. If you want a little moon with a big atmosphere you will need a dense moon made of metal. Or maybe a pure hydrocarbon moon that is slowly evaporating? $\endgroup$ – Willk Jul 29 '18 at 0:52
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    $\begingroup$ Oh, the Orc planet is definitely going to have forests. Lots them. Lumber is a pretty big export of theirs, in fact. I like the idea that the moon has a high metal content. Since portals between different planets are common in my setting, I realize that this could apply planets and their moons as well. That doesn't mean the Orc moon has a breathable atmosphere, but with magic, they could be able to survive in it long enough to collect some of the ore from their moon and bring it back to the planet. This moon just started to get a lot more interesting... $\endgroup$ – Patrick-Leigh Jul 29 '18 at 1:17
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I am answering your question as if it was for a science fiction story instead of a fantasy story. Realism in many parts of the setting certainly doesn't harm a fantasy story.

One problem with a black moon is that the Moon of Earth is made of very dark material, slightly darker than coal, and yet shines silvery white when seen from Earth because of the intense sunlight falling on it and reflected to Earth from it, and the contrast with the light less black of the night sky around it.

Another problem with a black moon is getting the background nebula to be bright enough to be brighter than the moon to make the moon look black by contrast.

If your television or monitor screen is turned off, you can see that it's surface is somewhat lighter than pure black. But when the screen is on black parts of a picture appear to be pure black, because the contrast with lighter areas of the screen makes them seen darker to your eye and brain.

Nebulas are considered to be very bright and opaque because commonly seen photographs of them are made with large telescopes and with hours-long exposures, thus capturing many thousands or millions of times as much light from them as you see if you look at them through small telescopes. Actually nebulas tend to look much dimmer and more transparent to the eye than in most photos.

So your solar system should be very close to a nebula or maybe inside it to make the nebula look brighter.

And maybe there is another star in your solar system that emits a lot more invisible ultraviolet and even X-rays than the star your planet orbits. The atmosphere on the planet would shield the surface of it from those rays. And those invisible rays from that star would excite the gases in the nebula and make them emit lower energy light in the visible spectrum, thus making the nebula seem very bright at such a relatively close distance.

Possibility One.

A planetary nebula is a shell of glowing ionized gas emitted from a red giant star. Planetary nebulae are usually about one light year, or about 63,241 Astronomical Units (AU) in diameter. So a planetary nebula would cover a large portion of the sky if your planet's solar system and the star that emitted the planetary nebula happened to pass unusually close, within a few light years. According to calculations three stars have or will pass within one light year in a six million year period centered on today, and four more have or will pass within two light years.

https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs1

The planetary nebula would probably look very dim and transparent compared to most photographs you see of planetary nebulae. But it should look brighter than interplanetary or interstellar space, noticeably brighter than blackness. So when a black moon passed in front of the planetary nebula the contrast should make the black moon look blacker.

Possibility Two.

Maybe the bright "nebula" is actually an outer moon of your planet and the "moon" of your planet is actually an inner moon of your planet. Your planet could be about the size of Earth and have a moon about the size and distance of the Moon, which would thus look about the same as Earth's Moon. And it could have another, smaller moon that orbits much closer to the planet and - if the orbits of the two planets are in the same plane - it might sometimes pass in front of the outer moon.

Possibility Three.

Because the star of your planet will have many times the diameter of your planet, the shadow of your planet in space will be a cone. A very long, narrow cone, but one that gets narrower and narrower with increasing distance from the planet. Actually it is the umbra, the darkest part of the shadow where all the light is blocked, which is a long tapering cone. Earth's umbra is about 1,400,000 kilometers or 870,00 miles long, and ends in a point.

The penumbra of an astronomical body is a cone which gets wider and wider the father it is from the body and extends forever into space, a zone where some of the light from the source is blocked by the nearer body and some is not.

https://en.wikipedia.org/wiki/Umbra,_penumbra_and_antumbra2

Because Earth's atmosphere bends light a little, some red light reaches the Moon when it is in Earth's umbra and makes the Moon look reddish. But if the Moon was much closer to Earth, the light would not be bent enough to reach the Moon and the Moon would appear very dark when in Earth's umbra. So you want the moon of your planet to be much closer to the planet than the Moon is to Earth so it is not illuminated by light bent by the planet's atmosphere.

Possibility Four.

The moon has an orbital period of 27 days, 7 hours, 43 minutes, and 11.5 seconds, or 27.321661 days. And at an average distance of 384,399 kilometers the Earth's shadow is so narrow that during lunar eclipses the moon usually spends about a few hours in the shadow of the Earth, about 1 percent of less of its orbital period.

If your planet is similar to Earth, the orbital speeds at various heights should be similar to those of Earth. Satellites as low as 99 miles above sea level would orbit with a period of 88 minutes. Similarly satellites 35,786 kilometers or 22,236 miles above Earth's equator are in geostationary orbits taking a sidereal day (23 hours, 56 minutes, 4 seconds) to orbit the Earth. Satellites higher up take longer than a day to orbit the Earth.

Artificial satellites in low Earth orbit spend almost half of their orbital periods hidden in the shadow of the Earth and thus invisible. People who watch artificial satellites know that some of them can be seen with the naked eye at night or even sometimes during the day, and keep track of when they are in sunlight and when they are in the dark shadow of the Earth.

So you should want your large, medium, or small moon to orbit very close to your planet so it spends as large a percentage of its orbit as possible invisibly hidden in the umbra part of the shadow of your planet.

Possibility Five:

Unlike artificial satellites, natural satellites or moons are held together mostly by their gravity, and so will be broken up by tidal forces if they are below the Roche limit of their primary.

The Roche limit of Earth is 9,492 kilometers for an object with the density of the Moon and 17,887 kilometers for an object with the density of an average comet. These are well below geostationary orbit so your planet could possibly have a natural satellite orbiting below geostationary orbit.

And as a general rule you probably want your planet's moon to orbit as close to the planet as possible so as to spend as large a proportion of its orbit as possible invisible in the umbra part of the shadow of the planet. Especially if the bright nebula doesn't surround the solar system but occupies only a part of the planet's sky.

But there is a problem. Tidal interactions with the planet cause moons beyond geosynchronous orbit to move farther and farther away from the planet, while making moons below geosynchronous orbit move closer and closer to the planet and eventually reach the Roche limit and break up. Phobos, the inner moon of Mars, is moving inward and predicted to reach the Roche limit and be destroyed in about 30 to 50 million years.

If your Earth like planet is old enough to have interesting features like a breathable oxygen-nitrogen atmosphere, or multi celled life, or intelligent natives, which should be several billion years old, any moons below geosynchronous orbit should have already spiraled in and been destroyed, while moons just outside geosynchronous orbit should have spiraled out for tens of thousands or hundreds of thousands of kilometers and be much farther out than you want.

Possibility Six.

So maybe any visiting Earth people may note that the close moon of the planet is just there, but unexplained, like the origin of the close moons of Mars is unexplained.

Possibility Seven.

Or maybe the close moon of your planet was a wandering object that was captured by the planet very recently on a planetary time scale, maybe just a hundred million years or something ago, and was captured at a distance close to the geosynchronous orbit and hasn't had enough time to spiral very far in or out.

Possibility Eight.

Another possibility would be to have instead of a nebula, another nearby planet that reflects the light from their sun.

The "planet" in your story could be an Earth sized moon of a gas giant planet which has the role of the "nebula" in the story. The gas giant planet would probably have many times the apparent diameter of Earth's Moon. The "moon" in your story could be another moon of the gas giant planet that orbits closer in. Sometimes when that inner moon was between the habitable moon and the gas giant it would enter the shadow of the habitable moon and appear black against the illuminated background of the gas giant planet. The shadow of the habitable moon could reach to the inner moon but not to the surface of the gas giant.

Or maybe the shadow of the habitable moon could reach to the gas giant planet and darken a small part of the gas giant planet. But if the gas giant planet has intense auroras in its atmosphere the shadowed part of the planet could glow dimly with the delecate colors of the auroras. The inner moon would have no auroras and would be pitch black when it entered the shadow of the outer moon and covered part of the planet.

This possibility has the feature that the gas giant planet could appear very large in the sky of the habitable moon orbiting it, perhaps the closest of any other possibility to that of Possibility One, a very close planetary nebula.

Possibility Nine.

Or the "moon" of habitable moon could actually orbit the habitable moon, thus being a moon of a moon.

It is possible for a moon to have a moon, but the orbit would be unstable and a moon of a moon would soon escape from the moon into interplanetary space. Soon by astronomical and geological standards of course.

Possibility Ten.

Another possibility would be to have the planet a planet orbiting the star with its moon and the "nebula" another planet in an outer orbit.

In our solar system planets never get closer to other planets than tens of millions or hundreds of millions of kilometers. But in a few other solar systems planets sometimes get much closer than that.

The closest known planetary orbits to each other are in the Kepler-70 system. The orbit of Kepler-70c is about 240,000 kilometers farther out than the orbit of Kepler-70b. So the two planets are only about 240,000 kilometers apart when closest, and Kepler-70c can look five times the apparent size of the Moon in the sky of Kepler-70b. Any moon of Kepler-70b would have to be very close to it.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes3

There is some evidence for a planet orbiting between Kepler-70b and Kepler-70c, but it is unconfirmed.

https://en.wikipedia.org/wiki/Kepler-704

Potentially habitable exoplanets that are very close together are the four potentially habitable planets of TRAPPIST-1.

The system is very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Except for TRAPPIST-1b, they orbit farther than the Galilean satellites do around Jupiter,[42] but closer than most of the other moons of Jupiter. The distance between the orbits of TRAPPIST-1b and TRAPPIST-1c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth.[41]

https://en.wikipedia.org/wiki/TRAPPIST-15

https://www.space.com/35790-seven-earth-size-planets-trappist-1-discovery.html6

According to my rough calculations, when TRAPPIST-1e is closet to TRAPPIST-1d it would appear to be about 37.8 arc minutes wide, and when TRAPPIST-1g is closest to TRAPPIST-1f it would appear to be about 40.2 arc minutes wide. The Moon as seen from Earth is 29.3 to 34.1 arc minutes wide.

astronomers consider it highly unlikely for the TRAPPIST-1 planets to have moons in stable orbits.

Assuming that you would want the moon of your planet to orbit at a distance of 10,000 to 50,000 kilometers from the surface of the planet, and that your planet has exactly the same mass and density as the Earth, your planet would have about 58.9 to 1,477.6 times the gravitational attraction on its moon as Earth has on the Moon.

And if you assume that your planet should have at least 100 times as much gravitational attraction on its moon as the outer planet does, the outer planet should have less than 0.589 to 14.776 times the gravitational attraction on the moon as Earth has on Earth's moon.

Possibility Eleven.

If the outer planet had the mass of Saturn, the least dense planet in our solar system, it would have 95.159 times the mass of Earth. Thus it would have to be at least 9,515,900 to 47,579,500 kilometers away at the closest approach to your planet. At those distances Saturn with a diameter of 116,464 kilometers would have an apparent diameter of 8.4 to 42.04 arc minutes.

Possibility Twelve:

Or you could have a habitable planet with a smaller planet orbiting about 1,000,000 kilometers farther out and a Saturn sized planet orbiting about 6,289,232 kilometers farther than the first planet. When the Saturn sized planet was closest to the habitable planet it would appear to have an apparent diameter of 400 arc minutes or 6.66 degrees, about 11.73 to 13.65 times the apparent diameter of the Moon from Earth. And sometimes the intermediate planet would pass in front of the Saturn sized planet if all the planets orbited in the same plane.

I hope that my 12 possibilities will give you and the others who have answered your question things to think about and discuss.

By the way, if you think it may be a big problem to arrange for a black moon to pass in front of a nebula that is brighter than black, there are been a few science fiction stories and programs that feature a white sky with black stars. The thought of trying to explain and justify that gives me shudders.

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    $\begingroup$ Wow. That's.... a lot of ideas. Perhaps I should have mentioned that the nebula is a magical phenomena that resulted from one god sucker punching another, so it's composition isn't the same as a traditional nebula, even if it looks like one. It's a mass of chaotic magical... stuff that's in a state of flux until the god who got sucker punched finds the missing pieces of himself, at which point the nebula will probably disappear. Still, your suggestions have given me some ideas for other worlds in my story setting, so thank you for all the hard work you did. $\endgroup$ – Patrick-Leigh Jul 29 '18 at 3:17
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    $\begingroup$ A great answer! I'm all in favour of science-fictional reasoning in fantasy. There is an article claiming more & better science appears in fantasies compared to science-fiction works. About 'white sky with black stars", there is a paper in Nature, in the mid-1960s that postulated something like that: a so-called Faustian universe. The idea gave me the shudders too. Plus one. $\endgroup$ – a4android Jul 29 '18 at 6:09
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The Moon looks red during an eclipse because it reflect the light refracted through the Earth atmosphere, which is mostly red.

Now, if the Moon was made of substances which absorb red light, there would be no reflection during an eclipse, giving the black color.

Think of sapphire, aquamarine or lazurite. They absorb red light, so when exposed to it they would look dark.

Or, alternatively, you can have an atmosphere of the planet which strongly absorb red light, diminishing the amount of it scattered on the satellite during the eclipse and also transmitted to the ground.

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  • $\begingroup$ The albedo of the moon is already slightly lower than coal, do it is already absorbing as much light as any natural material can. Only a small number of artificial materials are darker. $\endgroup$ – pojo-guy Jul 28 '18 at 10:20
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Let's put a gas giant in a near orbit between the sun and your habitable planet

I'm not going to worry about the math, but the perfect alignment of a double lunar eclipse, the eclipse from homeworld and the eclipse from the gas giant, would all but blacken the moon. Change the moon's chemical makeup so that the longer wavelengths are absorbed, and boom — dark moon with a lovely nebula behind it.

Best of all, you can ignore the celestial mechanics of just how effective this would factually be. The average reader doesn't have that much astrophysics under their belt, and those that do will enjoy the story despite the (*cough*) minor factual indiscretion.

And if you really want to strech the idea, the sun casting rays around the gas giant could taint the color of the nebula to make the moment even more spectacular. That's stretching the math to the point that angels weep — but it would be cool.

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