Eclipses blow my mind

March 29, 2006 8:34 PM

Are there any reasonable scientific explanations for the fact that the width of the moon as viewed from the surface of the Earth is precisely the right width to match the apparent width of the sun, and give us total eclipses? It does seem spectacularly unlikely that it turned out that way by mere chance.
posted by stavrosthewonderchicken to science & nature (50 comments total)

Uh...

It's not really matching the apparent width of the sun. When you hold your hand up to the sun on a sunny day, and it casts a shadow on the sidewalk, you've just created a total solar eclipse for the ants walking along underneath it.

The moon is a lot bigger than your hand.
posted by autojack at 8:41 PM on March 29, 2006

no, but the moon does fit just over the sun, with that little halo but nothing more. This was actually a part of a short story I once wrote when it first occurred to me... anyway, the only thing I could find to sort of make it less weird was that the moon is in the process of moving away from the earth, so it's just that way for now, and it's not a perfect match... and the earth's shadow in a lunar eclipse is actually much bigger than the moon. but yeah, it did/does seem like an unlikely coincidence.
posted by mdn at 8:46 PM on March 29, 2006

Answer 1: it's coincidence.

Answer 2: It depends on where the moon is in its orbit at the time that a solar eclipse takes place. If it's at the outer reaches of its elliptical orbit, it doesn't have an apparent disk big enough to block the sun, and you get what's called an "annular eclipse". In that case, at "totality" there's a ring of the sun's surface all around the apparent disk of the moon. It looks like this.
posted by Steven C. Den Beste at 8:46 PM on March 29, 2006

stavros, I'm afraid it really is just a coincidence that the moon is about 250000/93000000 the diameter of the sun.

[Not sure what autojack's on about.]

[On preview - excellent point mdn, but was the moon visibly closer while there were people wondering about it? Still a pretty good coincidence if the anthropic principle doesn't apply...]
posted by nicwolff at 8:51 PM on March 29, 2006

It's an intriguing question, but look at it this way. The moon is in an elliptical orbit around Earth, meaning that the distance between Earth and its moon changes.

When the moon is further away from Earth, portions of the solar photosphere remain visible during an eclipse. This is called an annular eclipse, and they tend to happen more frequently than total eclipses, as the moon is generally too far from the Earth to allow for a total eclipse. Hence, the moon as viewed from the surface of the Earth is generally not the right width to match the apparent width of the sun.

For that reason, a skeptic might argue that you are anthropomorphizing a beautiful coincidence. Total eclipses are, after all, a scientific inevitability.
posted by scoria at 8:55 PM on March 29, 2006

Metafilter: the moon is about 250000/93000000 the diameter of the sun.
posted by rleamon at 8:55 PM on March 29, 2006

It's not really matching the apparent width of the sun. When you hold your hand up to the sun on a sunny day, and it casts a shadow on the sidewalk, you've just created a total solar eclipse for the ants walking along underneath it.

What? Was I unclear somehow? Try being a little less condescending when you seem to have no idea what you're talking about.

Perhaps I should have specified 'total eclipse'. During total eclipses, the moon, when it crosses in front of the sun (except, as SDB and scoria note, in an annular eclipse) is precisely the same angular width when viewed from the surface of the Earth. That's why during such an eclipse, we can see coronal streamers, polar plumes, and prominences.

Answer 1: it's coincidence.

Yes, that's what I've always assumed. But it's a staggeringly unlikely one, you have to admit.

a skeptic might argue that you are anthropomorphizing a beautiful coincidence.

How so? If I said something silly like 'well, maybe it's just proof that there's a god and humankind is all special and stuff', sure. I'm just asking if there is any science of which I'm unaware that can explain such a beautiful coincidence. 'It's a coincidence' sometimes sounds to me a lot like 'it's a miracle'.
posted by stavrosthewonderchicken at 9:04 PM on March 29, 2006

that can explain such a beautiful coincidence.

Sorry, that's unclear. I meant '...that can explain such a situation without using the word 'coincidence'.
posted by stavrosthewonderchicken at 9:06 PM on March 29, 2006

There's reason to believe that the actual process which formed the moon was quite unlikely (according to current theory), but once that happened, and knocked enough material loose, it was not in fact unlikely that the resultant satellite would have an apparent disk larger than the sun. In fact, for most of the history of the earth the apparent disk of the moon was quite a lot larger than the apparent disk of the sun, because the moon was a lot closer to the earth than it is now. (Due to the gravitational effects of the earth's tidal bulge, the orbital radius of the moon is increasing by about 3.8 meters per century.)
posted by Steven C. Den Beste at 9:16 PM on March 29, 2006

(Due to the gravitational effects of the earth's tidal bulge, the orbital radius of the moon is increasing by about 3.8 meters per century.)

Hmmm, I did know that at some point (but apparently forgot). The corollary questions would be, then: how much does that 38metres/millenium change the apparent angular width of the moon from the perspective at a nominal 'earth surface' with the passing of time? How close to totality is totality now, and how far back in the past and into the future would totality have been possible?
posted by stavrosthewonderchicken at 9:31 PM on March 29, 2006

stavros: keep in mind that when people here tell you the current recession velocity of the moon - thats all it is - the current recession velocity. To answer your follow-on questions would require knowing what it was in the past and future which is somewhat speculative.

The increased orbital angular momentum from the increased earth-moon distance is balancing the loss of momentum from earth's spindown due to drag/tidal forces. So it hasnt been constant.

That said, in 10,000 years of human history the moon might have moved away by a kilometer or so. Compare this to the fact that the moon's orbit is an ellipse and the difference between its closest approach and farthest is 40,000 kilometers, or about 10% of its average distance and thus a 10% width change over the course of its own orbit.

So, since the change in angular width is linear: a 1% width change every 40 million years?

Finally, there's one more thing nobody has mentioned: The apparent size of the sun varies over the course of the year too. When we are farthes from the sun, at aphelion, the sun is smallest and decreases the angular size of the sun allowing for total eclipses now. When we get aphelion (small sun) and perigee (close, big moon) we get a massive eclipse - a double whammy.

How close to totality is totality now,

It can exceed totality even today. The moon is fully covering the sun's main orb and then some. What you see around the edges is the sun's extended atmosphere or corona.
posted by vacapinta at 10:13 PM on March 29, 2006

stavrosthewonderchicken write...
[Coincidence is] what I've always assumed. But it's a staggeringly unlikely one, you have to admit.

No, not really. For every physical phenomena that appears to contain unlikely order, there are about a gazillion that don't. The human brain is a pattern recognition engine -- it would bestaggeringly unlikely for a human to look at a large set of data and not spot the any coincidences: It's what we do.

In short, one fact coinciding out of billions available is not particularly unlikely.
posted by tkolar at 10:23 PM on March 29, 2006

...sigh, I speak english goodly indeed...

bestaggeringly --> be staggeringly
"not spot the any" ---> "not spot any"

tkolar go bed now.
posted by tkolar at 10:25 PM on March 29, 2006

I meant '...that can explain such a situation with using the word 'coincidence'.

There's nothing to explain. It's just a side effect of a meaningless arrangement of objects. The only word I can think of for such an effect is "coincidence".

When you say it's spectacularly unlikely, what's your sample? Umpteen squillion gajillion stars, with many times that number of planets, with many times that number of moons, and at least one such arrangement looks like ours? Doesn't seem particularly unlikely to me - with a sample size like that, it's a certainty.
posted by obiwanwasabi at 10:40 PM on March 29, 2006

Don't misunderestimate the value of a word like bestaggeringly!

I don't suppose there's a relationship between the distance the moon is from the earth and the required mass of the moon (hence, size) for same to have a stable orbit?
posted by five fresh fish at 10:52 PM on March 29, 2006

obiwan: it's unlikely per planet, we can be pretty sure. It would be a certainty that one of them would be that way, but it wouldn't be a good bet that any particular planet would have that property.


Here's one thing to think about though:

Many people think tidal forces helped life get started on earth, which would require a pretty large moon. If the moon were in a different place or different size we might not have evolved.

If the sun were bigger or smaller we might not have liquid water, and thus life might not have started.

So it could be a requirement that for life to evolve, a sun and moon are required, and required to be at least somewhat similar in size.

That would increase the odds of intelligent life that can wonder about such coincidences coming about on planets where such coincidences exist.

That's pure speculation, though.
posted by delmoi at 10:54 PM on March 29, 2006

I don't suppose there's a relationship between the distance the moon is from the earth and the required mass of the moon (hence, size) for same to have a stable orbit?

Nope, lots of planets have stable moons with diffrent moon/sun sizes.
posted by delmoi at 10:55 PM on March 29, 2006

NASA says:

Annular vs. Total Eclipses: Lovely solar eclipses are possible because of a lucky coincidence. Although the Sun is about 400 times larger than the Moon, it is also about 400 times farther away. From our point of view, the Sun and the Moon seem to be the same size: 0.5 degrees wide -- but not always! The Moon's orbit around our planet is an ellipse, not a circle, so the width of the Moon waxes and wanes each month by ±7%. Earth's orbit around the Sun is elliptical, too. The angular diameter of the Sun varies by ±2% throughout the year. When the Moon happens to be the same size as or bigger than the Sun, total eclipses are possible. When the Moon is smaller, eclipses can be only annular or partial. On May 31, 2003, the Moon will be smaller than the Sun, and the maximum eclipse will be annular.
posted by frogan at 10:56 PM on March 29, 2006

-10 pts (reduce your fractions)

That's a ratio, not a fraction, with miles as the units of measure. While the comment lacks the appropriate units of measure, it's necessary to keep the zeros in this case for a sense of scale.

Furthermore, these two comments, yours and mine, don't help answer the question, so excuse my derail.

The devil is in the details. The corona is throwing many of you off. In a total solar eclipse, you are not seeing the photosphere, you're seeing the corona, amongst other solar phenomenon that reach beyond the photosphere.

Secondly, solar eclipses happen at least four times a year, but are not necessarily visible where you live. (It's four if you include ones that aren't really visible from the ground. The Wikipedia article says it's only two, but every intro text to astronomy I have seen says four.) Most of these eclipses are partial, hybrid, or annular. Even when there is a total solar eclipse, it is not a total eclipse everywehre the eclipse is visible from earth. In fact, it's likely you've never even seen one live yourself. In the past 1000 years, there have only been tens of total solar eclipses visible from the US.
posted by sequential at 11:03 PM on March 29, 2006

When you say it's spectacularly unlikely, what's your sample? Umpteen squillion gajillion stars, with many times that number of planets, with many times that number of moons, and at least one such arrangement looks like ours? Doesn't seem particularly unlikely to me - with a sample size like that, it's a certainty.

Well, no. I think you're sampling the wrong data there. Looking at the earth/moon system, given the umpteen squillion gajillion different possible numbers for orbital distance of the moon, size of the moon, distance of the planet from the star, and size of the star, it remains (to me) bogglingly unlikely that such size/distance ratios of the planet-moon system would exist.

I don't deny it's random chance, of course, something that people seem to be inferring from my question. Again, I was just wondering if there was any cause-and-effect action going down there of which I was unaware.
posted by stavrosthewonderchicken at 11:07 PM on March 29, 2006

I don't suppose there's a relationship between the distance the moon is from the earth and the required mass of the moon (hence, size) for same to have a stable orbit?

Nope, lots of planets have stable moons with diffrent moon/sun sizes.

More importantly, there is no such thing as "required mass." Orbits are independent of mass. An orbiting object follows the same laws as two objects dropped off a tower by Galileo.
posted by vacapinta at 11:11 PM on March 29, 2006

Also, I know nobody clicks on links, but the "double whammy" link I posted above actually shows the relative sizes of the sun and moon when the size difference is greatest in both directions - moon bigger than sun and sun bigger than moon.
posted by vacapinta at 11:16 PM on March 29, 2006

I actually was in the path of a full eclipse about 30 years ago. Problem was, it was overcast that day in Portland (grumble groan) so all I saw was the clouds getting really, really dark and then light again.

Sigh.
posted by Steven C. Den Beste at 11:29 PM on March 29, 2006

I was just wondering if there was any cause-and-effect action going down there of which I was unaware.

I'm not aware of any. I think delmoi's speculation about a certain size moon being a necessity for intelligent life to develop may be the closest you'll come to cause and effect.
posted by tkolar at 11:33 PM on March 29, 2006

Regarding the moon and its effect on helping life develop, I've got my own opinion.

There are two effects, both of which require a large moon. First, there's tidal heating of the core, which helps keep the surface plates thin and keeps plate tectonics going, which means that volcanos keep erupting.

Volcanic eruptions are the primary means by which the planet produces new atmosphere.

Second, gravitationally the moon acts like a vacuum cleaner which sucks gas away from the earth. Gas which reaches the gravitational midpoint between the earth and moon can go into orbit around the moon, get a free ride to the far side of the moon, which places it outside of the earth's magnetic field where it can be blown away by the solar wind.

The first effect runs more or less at a constant rate. The second one is more efficient when the atmosphere is thick, and less efficient when it is thin. The combination of the two keeps the atmosphere approximately at the thickness it is now. It prevented the earth from having the kind of atmosphere that Venus has (surface pressure 90 times that of earth) and likewise prevents the earth from having the kind of atmosphere that Mars has (surface pressure 2% of earth).

On Venus, there's no way for the gas to get away because there's no large moon acting like a vacuum cleaner, so the atmosphere has built up.

On Mars, the problem is that plate tectonics and vulcanism have ended. Also, the core has cooled and the magnetic field of Mars has largely collapsed, permitting the solar wind to blow away most of the atmosphere that Mars once had.

But the earth is "just right" because new production through vulcanism is just balanced with losses to the solar wind. We have enough to permit liquid water and rain, without having so much as to result in runaway greenhouse effect like on Venus.
posted by Steven C. Den Beste at 11:38 PM on March 29, 2006

Steven C. Den Beste:
Have you any proper evidence from that? The tidal forces of the moon won't be enough to significantly heat the core and lead to vulcanism and plate tectonics - that's powered by radioactive decay in the core as I understand it.

Secondly, the atmosphere is incredibly thin compared to the distance to the Moon. If your idea was right there'd still be an appreciable fraction of the Earth-Moon Roche lobe filled by atmosphere, but it's not even remotely thick in comparison.

Sorry, I think you're idea's fatally flawed in several ways.
posted by edd at 12:54 AM on March 30, 2006

stardate.org does a daily 2 minute podcast about what's happening in the heavens courtesy of the McDonald observatory. during the last couple of days they have been talking about eclipses.
posted by busboy789 at 3:42 AM on March 30, 2006

i've worried about this - it does seem too good to be true, but i can't find any connection, and no-one i've talked to know of one (not an appeal to authority, but to give you some idea of context:i have a phd in astronomy; my partner is an astronomer and give 101 level lectures on it; i work in an observatory).
posted by andrew cooke at 5:12 AM on March 30, 2006

All of this astronomy wonkery is fun ( I mean that seriously), but the answer to your question can not be any deeper than:

it's pure coincidence.
posted by teece at 8:43 AM on March 30, 2006

It remains (to me) bogglingly unlikely that such size/distance ratios of the planet-moon system would exist.

Once again, you really shouldn't be boggled by this as your potential dataset for astronomical coincidences is, well, astronomical.

Allow me to present for your pleasure some astronomical non-coincidences:

1) If you compare Pluto's orbit to that of Venus, you'll find that ... they have virtually nothing in common!

2) Asteroid 3345Xi's rotational rate is: somewhat similar but not identical to that of Mars!

3) Of the craters on the moon, none line up precisely as a giant circle facing the earth!

4) When seen from earth, certain stars appear to form the shapes of objects in the sky, but you're more likely to see them if you've been drinking heavily!

My point here is not that the moon/sun coincidence isn't interesting. It is, and frequently these apparent coincidences turn out to have causal effects. My point here is that there are literally countless astronomical phenomena, so you really shouldn't be "boggled" when coincidences occur.
posted by tkolar at 9:05 AM on March 30, 2006

I think the big coincidence here has nothing to do with the relative sizes of these two planetary bodies, but rather that we, the observers, are here on earth as opposed to Venus or Io or somewhere else entirely where these sun and any other nearby moon might not appear to be nearly as similar in size/
posted by luriete at 9:40 AM on March 30, 2006

stavros -- Just between us, this is my favorite question ever on the green. But wotta buncha dumb answers. After reading them all, I'm still where you are: Baffled and amazed by all the charming coincidences that make earth such a delightful place to live.
posted by Faze at 10:28 AM on March 30, 2006

Actually, some pretty interesting answers in this thread, but "baffled and amazed" is probably not a such bad way to stumble through life, so shine on you crazy diamond!
posted by gigawhat? at 1:18 PM on March 30, 2006

gigawhat wrote...
Actually, some pretty interesting answers in this thread, but "baffled and amazed" is probably not a such bad way to stumble through life, so shine on you crazy diamond!

I stand corrected.
posted by tkolar at 2:07 PM on March 30, 2006

But wotta buncha dumb answers. After reading them all, I'm still where you are: Baffled and amazed by all the charming coincidences

If you can see that it's just a coincidence, no matter how "charming", then you're way ahead of stavros. The answers above are accurate. It's a coincidence, and not a particularly amazing one. You just place a lot of emphasis on it because this particular coincidence involves stellar objects.

To paraphrase a quote from somebody I can't remember:

On the way to work this morning I saw a green 2002 model Peugeot 307 with registration YCE-31Q. What are the odds? I'm baffled and amazed! That car, that model, that year, that colour, those plates in that combination, right as I was looking at it! Sure, when you consider all the cars in the world, I guess it has to happen somewhere, but the odds of getting that arrangement for that particular car is spectacularly unlikely. What a charming coincidence! What a delightful place to live!
posted by obiwanwasabi at 2:21 PM on March 30, 2006

If you can see that it's just a coincidence, no matter how "charming", then you're way ahead of stavros.

WTF? Nice throwaway insult, there. Read the goddamned thread. I've acknowledged several times that I UNDERSTAND that it's got to be random chance in lieu of science, and I have no problem with that. I think it's mind-exalting. You choose to live unawed by the miracle of math, if by no others. Jesus jellohead fuck, you people can be annoying.
posted by stavrosthewonderchicken at 3:37 PM on March 30, 2006

(OK, that was precoffee, and a little harsh. My wtf?! stands, though.)
posted by stavrosthewonderchicken at 4:06 PM on March 30, 2006

No insult intended, and you're perfectly entitled to give as good as you (thought you) got, coffee or no.

You did say, though, they you wanted somebody to explain it to you without using the word "coincidence". You also said that you thought using such a word was akin to writing it up as a miracle, though you also said you acknowledge there was no intelligent agent that orchestrated the situation. My comment was simply prompted by the fact that you seem to be extremely reluctant to simply accept the phenomenon as just a coincidence, and not an especially good one at that.

You seem determined to believe that there's some elegant mechanism behind the whole thing, rather than it just being a chance alignment of fractions of distance. You seem to want this to be the case because you think it's awe-inspiring, and that writing it off as a simply coincidence somehow diminishes it. I don't understand why you think it's awesome, but the fact that, say, two people share the same birth date, or you just flipped three heads in a row isn't.

Your question seems to be "look, up in the sky - it's not a coincidence, it's not a miracle, so what is it?". Another way of asking that question is to ask "what it is, if not a natural effect and not a supernatural design?" I'm pretty sure such a question is meaningless, because it has to be one or the other, and you've ruled out the other.

It's not a good thing to see wonder and grandeur where none exist - we may as well read chicken entrails or throw runes. The eclipse thing is no more "mind-exalting" than the fact that, of all the computers in the world, I'm writing this on a Dell Latitude D510, or that of all the watches in the world, mine is a Citizen.

It's just how things are, an expected consequence of a large number of variables. It's no more a "miracle of math" than rolling a 6 on a standard die. Is it noticeable? Sure - to human beings, anyway, for some eclipses, at least. Does it mean anything, or require an explanation beyond pointing out the fractions? Not at all.

Trying to read some grand meaning into it, or claim that it's some kind of intrinsic mathematical miracle smacks of superstition, however much you rule out a deity. It's precisely the same argument those Bible code nuts use - what are the odds that this book gives these awesome messages, they ask? The answer is that it's entirely normal and expected, and not in the slightest bit exalting. E=mc^2, e^(i*(pi)) = -1, Maxwell's equations - they're unexpected, unusual, useful, elegant, tying together calculus, geometry, physics and so on in a way that helps us to make sense of and shape the universe. That's exalting - a true "miracle of math". A disc of area x fits over a disc that's y times larger but also y times further away? My reaction is "well, the y's cancel, so what did you expect to happen?"
posted by obiwanwasabi at 5:47 PM on March 30, 2006

OK, I apologise. You are perfectly entitled to derive the wonder in your life from whereever you can get it. It was wrong of me to compare you in any way to entrail readers, rune throwers or Bible coders. The moon and the sun, the two brightest objects in our sky, seem to have an eerie symmetry, and that's thought-provoking and cool. The math shouldn't diminish that at all.
posted by obiwanwasabi at 6:02 PM on March 30, 2006

obiwanwasabi wrote...
It's not a good thing to see wonder and grandeur where none exist

I think you've gone a little bit overboard, obiwan. Eclipses are pretty damn cool, and the fact that the moon (roughly) covers the sun is nifty too.

And as far as being grand, it's (literally) hard to conceive of something larger and more dramatic than a hunk of rock 1/4 the size of the earth locked in an near eternal embrace with our planet and periodically blotting out our major source of light.

In short, understanding the mechanics of an eclipse doesn't detract one whit from the grandeur and wonder of it for me.

If it does for you, more's the pity.
posted by tkolar at 6:07 PM on March 30, 2006

You seem determined to believe that there's some elegant mechanism behind the whole thing, rather than it just being a chance alignment of fractions of distance.

Right, let's try again. I appreciate the time you took to try and edumacate me, but I'm going to repeat that I don't need it. As I've said, repeatedly, I'm totally happy for it to be random chance. I don't know why you haven't seemed to read any of my comments to that effect upthread, and instead decided to attribute to me some dogged determination to see volition or divine order or something inthis -- things in which I firmly do not believe. FWIW, my bachelor's degree was in mathematics. I've had formal training in psychology, and understand the pattern-seeking mechanisms of the human brain. I understand that stuff.

Perhaps it's just that our capacities for wonder differ. One of the reasons I went into mathematics as a field of study in the first place was my absolute gobsmacked awe at things like the manifestations in nature of the fibonacci sequence, or the human perceptual inclination towards the golden mean, or the magnificent imagery I could create on the 127x47 resolution b&w monitor of my TRS Model III when I was 14 years old, simply by plotting algebraic equations, or later, when I saw that solving problems like working with the closest packing of spheres in 255-dimensional space has real-world applications.

These things, and a whole lot else, fill me with a childlike wonder at the beauty and complexity of the universe in which we live. If they don't do the same for you, that's fine, if sad, from my perspective. Whether that beauty and complexity is explicable without invoking magic daddies in the sky or not is immaterial to me, though. It's my direct experience of wonder that makes my life richer. No matter the explanation (or lack thereof), I still freakin' love it.

So implications that I'm a some kind of dullard because I experience awe in the face of the beauty of our world are insulting. I hope you can see that.

I don't understand why you think it's awesome, but the fact that, say, two people share the same birth date, or you just flipped three heads in a row isn't.

Two people in a room sharing the same birthdate wouldn't make me go 'whoa! cool!', nor would three heads in a row.

Everyone in a room of 200 people sharing the same birthday, or 200 heads in a row, though, those would.

Even though I understand statistics, and I would be happy to accept that there's nothing supernatural going on, and that trying to explain it in terms of gravity and friction and inertia would fail. I understand that purely in terms of statistical chance, the 200th head is as likely as the first. It's still going to make to go 'holy shit!'

But again, I was just asking a question about physics, really, and if there were any new ideas about it of which I was so far unaware. It mystifies me, given how many times I've said that, that you insist on imputing a weakness of mind to me under which I do not labour. That's fine, I guess. Whatever.

It's not a good thing to see wonder and grandeur where none exist

Wonder and grandeur exist everywhere. When I'm in full rhetoric (usually beer-fueled) mode, I've been known to declaim that it is the sole important function of being human to perceive them.

Sorry for the length. On preview, what tkolar said.
posted by stavrosthewonderchicken at 6:26 PM on March 30, 2006

Oh crap. I didn't see obiwan's apology, there. Ah well, that was fun to write, anyway.
posted by stavrosthewonderchicken at 6:27 PM on March 30, 2006

Well I mentioned the anthropic principle, which is the possible counter to "it's just a coincidence". If, as SDB theorizes, there are reasons that having a certain size of satellite at a certain distance makes a planet more hospitable to life, and we can all agree that there are reasons that being a certain distance from its star (or a certain ratio between your star's size and your distance from it) makes a planet more hospitable to life, then we'd expect to find most sentient beings in the universe wondering why the heck their moon looks like it's the same size as their sun.

Of course, unless the reasons for the two are related, it would still be a coincidence - it just wouldn't be a unique and special situation here on Earth that should surprise us.
posted by nicwolff at 7:10 PM on March 30, 2006

then we'd expect to find most sentient beings in the universe wondering why the heck their moon looks like it's the same size as their sun.

That would be so cool. I wonder if there's ever been any science fiction written (I kind of lost track of most of the genre 20 years back) where humans and other species found common ground in the fact that their eclipses are spectacular for that very kind of reason.
posted by stavrosthewonderchicken at 8:02 PM on March 30, 2006

any cause-and-effect action going down

Given that the phenomenon is peculiar to our perspective point, that would indeed imply some cause that could move or resize the moon for our benefit. Personally, I find that more unlikely than the coincidence. The fact that we're here at all is incredibly unlikely, stavros.

This conversation butts up against the watchmaker principle. You can either accept some really far out stuff has happened (especially difficult now that we know the universe is so very finite) or you can believe in God (so remarkably easy for so many to do that it seems, like language, to be a behavior we're almost pre-wired for).

You gotta make of it what you will. Either we're blessed by some god or it's just a really freakin' cool coincidence. I personally don't see much difference.
posted by scarabic at 12:19 PM on April 1, 2006

scarabic wrote...
Given that the phenomenon is peculiar to our perspective point, [cause-and-effect] would indeed imply some cause that could move or resize the moon for our benefit.

Umm... No?

The fact that the moon rotates exactly in sync with the earth is a pretty wild coincidence -- until you look closely enough to find out that it's not really a coincidence. And it wasn't a watchmaker that did it.

That there might be some similar link for the size of the moon versus the size of the sun is not an unreasonable proposition. I can think of several explanations (none of which pan out) that involve the relative masses and orbits involved.

In short, despite the fact that I think it's a coincidence, and not an unlikely one at that, it also wouldn't surprise me in the least if tommorow someone came up with a theory that showed the relative sizes were an inevitable outcome of the formation of the solar system. No gods need apply.
posted by tkolar at 2:53 PM on April 1, 2006

Not that it matters much now, but my original point was that the moon could be way bigger or way smaller than it is, and would still produce solar eclipses... just for a larger or smaller area of the earth at a given time. That's why it seemed to me that the moon's size relative to the sun wasn't relavent. No?
posted by autojack at 7:43 PM on April 3, 2006

Not that it matters much, but I asked about total eclipses.
posted by stavrosthewonderchicken at 8:59 PM on April 3, 2006

Just stumbled across this question and got interested. Has no-one watch 2001 recently? Why, aliens did it! What better way to speed up the evolution of a brain slowly learning how to find patterns in chaos. Alternatively, ancient humans did it.

The two planets shine on the earth in turns, one during the day and the other at night. There is not another example of such phenomena in our solar system. The prestigious scientist, Isaac Asimov, once said that, according to all the data available, the moon in principle should not exist in that position. He also said, 'The moon is big enough to result in a solar eclipse, yet small enough to generate corona. Our astronomy just can't explain the coincidence among the coincidences.' Is it really a coincidence? Not really, according to some scientists. William R. Sheldon, a scientist, said, 'In order to orbit around the earth, a spaceship has to maintain a velocity of 10,800 miles per hour at a height of 100 miles. Similarly, in order for the moon to
keep itself in its orbit to balance the earth's gravitational force, it also needs accurate velocity, self-weight, and altitude.' The question is: if the current set of conditions is impossible to achieve by nature, why are they this way?

Conclusion:

'In the past, the situation was different than that of today's science. The people of the different time periods were of course developing in the ways of those times. People in one of those times recognized the inconvenience that the dark of night caused people, so they constructed a moon up there to bring light to Earth during the night....In fact, the moon was made by the prehistoric human beings. It is hollow inside. The prehistoric mankind was well developed. '

posted by MetaMonkey at 6:54 AM on April 21, 2006

I haven't done the math, but the same distance/size ratio doesn't seem to be common among moons in this solar system. Which seems to rule out any simple physical law causing it.

So the anthropic principle is about the only thing we would have left.
posted by RobotHero at 3:28 PM on September 24, 2006