• Member Since 5th Nov, 2013
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Wellwater


Voracious reader of fanfics for canons I haven't seen, sometime proofreader, and quiet Luna fan.

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  • 410 weeks
    Why attempts at wedging heliocentrism into Equestria don't actually work

    There are some stories that have the plot point that Celestia and Luna do not actually have the control over their respective heavenly bodies that everypony believes they do — that they are merely pretending to raise and lower them, while natural causes are really responsible. Sometimes this is done in an attempt to make Equestria more realistic and believable, with the nasty character

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    6 comments · 736 views
  • 433 weeks
    Robust societal changeling integration without yellow stars

    There are a number of fics that explore the consequences of integrating friendly changelings into Equestrian society en masse. One of the obvious problems is that of unrestricted disguises and the ever-present possibility that creates that anyone might be, or have been, a changeling, meaning that anyone now has a (theoretically) plausible alibi ("It was a changeling taking my shape, I

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  • 447 weeks
    A Question of Proper Terminology

    If Applejack grew chiefly maize, would we call her a corn pone?

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  • 492 weeks
    My Bookshelf Arrangements

    I have the faint hope that this writing may either suggest organization methods to others, or explain my own system more perfectly to authors and other readers alike. Perhaps even both. Also, my OCD called and I could not but heed. If you want a quick reference to how to most efficiently use my shelves, just start at the top-left of my library and go from there. The first shelf in which a

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    0 comments · 588 views
May
15th
2016

Why attempts at wedging heliocentrism into Equestria don't actually work · 12:59am May 15th, 2016

There are some stories that have the plot point that Celestia and Luna do not actually have the control over their respective heavenly bodies that everypony believes they do — that they are merely pretending to raise and lower them, while natural causes are really responsible. Sometimes this is done in an attempt to make Equestria more realistic and believable, with the nasty character implications of such prolonged and enormous deception merely coming along for the ride. Sometimes it's the other way around, where the character implications are the point, and the realism is (presumably) a side benefit. (Bad Horse's story called Trust is an example of the latter, and the motivation for actually writing all this up. It drops into this with a wham line at the very end… and gives absolutely no explanation for any of the complicated physical problems this has, as though it were so straightforward anyone could figure it out quite simply.)

Generally, I don't think it makes a lot of sense for the Element of Honesty to be attuned to someone who would knowingly perpetrate such a massive hoax, whether for power, in a misguided attempt to reassure their subjects, or because it was always just too awkward to tell the truth. And it's hard to imagine the canon characters really going through with this anyway, even if their cutie marks are actually "avoids planetary tidal lock" and "directs artificial moon surface display changes". But those characterization issues, serious as they are, pale in comparison to the big problem: heliocentrism in Equestria, especially of the naive "works exactly like Earth" variety, is blatantly unrealistic.

You heard me. It's unrealistic. It doesn't work. It's bad science. It makes no sense. That's not the kind of theory Johanneighs Kepler* and Galihayo and Clopernicus† would have come up with. Why is that? Well, because of a little something I like to call orbital mechanics.

You see, in the case of Earth, the Moon's phases, along with its varying rise and set times, come from its ~29-day orbital period. As it moves around the planet, it gets a little further every day, and so the time Earth's rotation brings it into (or out of) view changes accordingly. Depending on how much overlap there is between the half of the Moon that faces the Sun (and is therefore sunlit) and the half that faces Earth (and is therefore visible), we see anywhere from a new moon (if it's almost directly between us and the Sun) to a full moon (if it's almost directly opposite)… or a solar or lunar eclipse respectively if the conjunction is perfect.

Immediately we are faced with problems attempting to apply this to Equestria: Nightmare Moon, Discord, and Twilight Sparkle all took control of the Sun and Moon and shifted their times of rise and set radically at their whim. Most of these stories suppose that on those occasions (and indeed whenever it was necessary to demonstrate the "control" of the bodies, perhaps even every single day for a short while), all that was really being done was an enormous illusion covering as much of the world as necessary. This can't be directly disproved, but it's a bit of a hack: the power requirements are enough to make one seriously wonder if it would not almost be easier to simply create illusions of the Sun and Moon directly all the time, if Nightmare Moon was able to maintain such an illusion for hours, days, or weeks while fighting Celestia at full strength. (How unicorns managed to miss the traces of such illusions is another question, of course. And how and why Twilight did the same thing is also a puzzle, unless perhaps she, unique among all beings, actually wasted staggering amounts of power on legitimately nudging the sun around?)

Also, the precisely synchronized moonrise/moonset so characteristic of Equestria cannot be achieved with our own moon's orbit. The timing would drift too much from day to day for that to work at all, and logically, it would be far easier to accept a tale of unicorn (or alicorn) superiority if the absurd and superfluous claim to raising the Moon at the same time as the sun was set was never made, rather than going to great lengths to patch up the Moon's actual schedule with continuous illusions or other shenanigans. So whatever causes the Moon's bizarre behavior must be either natural, or a plausible result of some artificial project or disaster — but either way, a real phenomenon, not simply part of a story invented to secure the throne.

"What if we suppose that Equestria's moon is in a sun-synchronous orbit?" someone might say. "Then from day to day the orbital progress won't change the time of rising or setting." This is true, but it has several show-stoppers. A conventional rocky body at the altitude necessary for a typical sun-synchronous orbit (less than a thousand kilometers) is well inside the Roche limit and will break apart due to tidal forces over a relatively short time — never mind the way it would set less than an hour after rising. And if a true SSO is used, there are no phases: the Moon's illumination by the Sun is at the same angle whenever it rises. But Equestria's Moon definitely has phases, as we can see even from Princess Luna's crescent-moon cutie mark‡. If the orbit is pushed out to where Earth's Moon is, to allow phases and prevent Roche breakup, the position of moonrise will drift along the horizon from North to South (or vice versa) over the course of a few weeks. Worse, over the next few weeks, it will be on the same horizon as the Sun, and will rise and set alongside it! There is no possible way for that to be missed by any observer. (Forcing the orbit to precess at the necessary yearly rate — almost 19 times the Moon's precession rate — is yet another challenge.)

At least one story posits that the sisters' job is actually to keep the planet's rotation speed up: that due to some long-past catastrophe, tidal locking slows it down and locks it into sync with the star. Unfortunately, tidal locking works best with significant asymmetry in the body's mass, as well as an awful lot of time: Earth is expected to eventually lock with the Moon, but this is still billions of years away… and there's no estimate on how long tidal locking with the Sun would take. Equestria's gravity is presumably fairly similar to Earth's, so it can't be much smaller unless it's somehow far denser. Even if it was almost entirely solid lead with a thin skin of dirt and water on top, it couldn't be much less than a quarter the radius, which is still large enough to reach hydrostatic equilibrium fairly readily and avoid most asymmetry. Unless Equestria is actually an artificial body with simulated gravity, then, it cannot suffer tidal lock on any timescale less than millions of years, never mind weeks or days.

Another idea I had was that Equestria's Moon is actually a white dwarf star, a binary companion a few AU away from its Sun, and that Equestria itself is at a Lagrange point directly between them. This explains the synchronized rise and set, but does little to explain the phases. Perhaps you could hypothesize some planet orbiting the Moon-star that's the right size and the right distance to create the phases by regular eclipse, but that would produce visible corona effects, and would have a lot of trouble reflecting enough from the Sun-star (at a distance of several AU, and then another AU or so reflecting back to Equestria) to fit the surprisingly bright shadowed area of Equestria's moon. (Long-term stabilization of an object as massive as a planet in a Lagrange point is also seriously problematic, but perhaps that's really what the sisters do with all that magic twice a day.) But this stumbles completely at explaining the craters, which we even sometimes see on the night side.

Speaking of Lagrange points, putting the Moon at the star-planet L2 point seems intriguing. Unfortunately, Lagrange points don't really work for bodies with more than negligible mass relative to the main bodies, and the distance of our own Sun-Earth L2 is roughly three times the Moon's orbital distance, meaning the Equestrian Moon would need to be a good 27 times the mass of our own, relatively, or about a third the mass of Earth. (This also does not handle phases well: the Moon would always be full.)

If you don't care about comic continuity at all, though (specifically, the bits where the Moon's surface is shown to be rather similar to our own, rocks and all), you can probably get away with a large solar sail. It supports itself in a permanent pseudo-L2 position with radiation pressure from the star — the closer it is to Equestria's surface, the more lightness it needs to stand off from the increased planetary gravity and maintain the semi-hovering orbit. The reason for and method of its construction, its very-long-term survival in space despite almost no maintenance, and the imprisonment of Luna within it are left as an exercise for the reader. (Or, preferably, the author. Hint hint. Put a few thousand words to good use already and explain something!)

I don't like this idea much, though. What kind of moronic precursor civilization puts that much effort into something that's purely decorative? Especially given that adding the crater/phase support makes it something like five or ten times as expensive for scarcely any additional benefit: it would need a mechanism to switch areas of the surface between reflecting more and less light at Equestria without changing overall radiation pressure or thrust vector in that area. That makes things much more complex and also reduces lightness, forcing it further away and thus requiring geometrically more area to cover the same visual angle. The only way this makes sense is if you have an essentially galactic-scale civilization that has such enormous resources that wasting a few planets' productivity for a few years on this nonsense doesn't hurt anyone's pocketbooks. Where all the other pointless public works produced by this civilization went is a whole story in itself. (Hint hint.)

In sum, then, there is no natural explanation from human science for all the varied phenomena of the Equestrian system — moonrise synchronized with sunset, phases, craters, a solid walkable surface. Even the artificial explanations that could be constructed rely on far-fetched motivations and enormous efforts for very little concrete benefit: rather than giving a perfectly reasonable story that happens to conclude with the observed cosmos by following the natural chain of events to its logical conclusion, these are elaborate Rube Goldberg machines to arrive at the desired end by hook or by crook. That's unrealistic.

If you're going to write fiction about how the idea most ponies have about their world's functioning is just a lie (even a "lie-to-children"), at least take the time to make your supposed truth believable.

To my mind, the simplest explanation is the best one: Despite numerous superficial appearances, Equestria's physics are fundamentally different from Earth's from top to bottom, and its Sun is not a hydrogen-fusion star, but some long-lasting magical emanator responsive enough to be moved by horn; likewise, its Moon is a magical emitter in its own right, not a passive reflector. These two bodies are necessary to sustain the inherently magical Equestrian ecosystem with their light and magic output. While certainly circular, they may or may not be spherical, and their size would be hard to pin down, although they're probably less than a light-second away. Presumably the patterns the Moon shows are variations in less-important bands of its emissions, created as a side effect of complex internal magical processes (or deliberate dramatic decoration). (Maybe the Moon's emissions are mostly invisible, and the Sun's mostly visible? That would be very fitting.)


*We don't talk about Tycho Brayhe.
†I regret none of these puns. None of them, I tell you!
‡I had someone try to tell me that her cutie mark wasn't based on anything Equestrians would ever observe. A couple of screenshots put paid to that nonsense.

Comments ( 6 )

I like this model. It makes sense, if you've read any of Meep the Changeling's books, he has it in the distant future, but the sun is a black dwarf, the moon we have has flown away a long time ago, and both sun and moon now are magical constructs. Also, humans existed, became a type 3 civilization, then self-destructed.

4151847 Interesting. I should give those a look.

4151992
Yeah! I recommend The Queen is Dead first.

Very interesting. You sir have provided much needed technical background for my own (more or less intuitive) reasoning process.

I say 'intuitive' because unlike you I do not know enough about orbital mechanics to be quite so mathematical about it. But I do know enough to discern what was physically reasonable. And a lot of stories which assume a 'heliocentric' approach just did not work out. Neither did any assumption of 'co-orbital' approach.

No - I agree. The Equestrian Sun and Moon are magical objects in nature and they can be moved independantly of each other.

More food for thought.

1) Have you noticed that Celestia sets the sun, and Luna raises her moon from the same part of the horizon?

2) The heavens are also magical - not an infinite void of space. (Refer to Season 4 episode 1. Where Celestia and Luna were both captured.) In that episode we saw the two skies (daytime and nighttime) co-adjacent to each other - like some kind of 'blind' or 'tapestry'.

3) Have you noticed that Luna's night sky is always luminous? It always has a dim purplish glow to it. Probably for the same reason the daytime sky is 'blue'.

I don't have the attention span left to read this all now (directed here from a link in a story's comments section... hah, grouchopony's there to be exact), but I'm coming back later. For now though, there's blatant, in show evidence that it's no hoax. Twilight has to do it in the season 4 finale when she has all the Alicorns' powers. She's a bit shaky... but that's all the more evidence that it's no hoax. The alternative would be... Twilight... in on a conspiracy like that? Pfffft... come on now, some writers are just silly.

This is really interesting though, bookmarking to finish it later, when I'm not just coming off a day's work and editing a story for another author here:twilightsmile:.

Those punny names though... :ajbemused:

4195145

Those punny names though... :ajbemused:

I regret nothing! :rainbowdetermined2:

I don't know why I forgot about Twilight moving the sun, though. That's a derp.

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