Diamonds in a Cloudy Sky (8 Aug 2009)

[billMe]

How is it that the moon perfectly eclipses the sun? If the sun were any larger or the moon were any farther away it would seem that a perfect eclipse would never happen. Is this the case? Is there an explanation or discussion regarding this or am I totally off base?

http://apod.nasa.gov/apod/ap090808.html

[rstevenson]

It's just a coincidence. Our moon happens to be just the right size to almost perfectly cover the sun. No significance can be attached to this coincidence.

Rob

[billMe]

thank you

[neufer]

It's just a coincidence.
Our moon happens to be just the right size to almost perfectly cover the sun.
No significance can be attached to this coincidence.

An interesting consequence of this coincidence is that the tidal forces
of the moon & sun are in the same ratio as their mean densities:

3.35 = mean lunar density = lunar tidal force
1.41 = mean solar density = solar tidal force

[3.35 + 1.41] = full/new moon tidal force
[3.35 - 1.41] = half moon tidal force

[Chris Peterson]

It's just a coincidence. Our moon happens to be just the right size to almost perfectly cover the sun. No significance can be attached to this coincidence.

It's also a temporary coincidence. The Moon is gradually moving farther from the Earth; millions of years ago it covered more of the corona during eclipses, and millions of years from now every total eclipse will be annular- too bright for the corona to be seen at all.

[rstevenson]

An interesting consequence of this coincidence is that the tidal forces
of the moon & sun are in the same ratio as their mean densities:

3.35 = mean lunar density = lunar tidal force
1.41 = mean solar density = solar tidal force

[3.35 + 1.41] = full/new moon tidal force
[3.35 - 1.41] = half moon tidal force

Surely that's not a "consequence of this coincidence", is it? I would expect the density of any moon would be proportional to its tidal force (a truism if I ever heard one), and that the tidal forces of any moon plus any star would be additive when they are in more or less alignment and subtractive when they're not?

Rob

[bystander]

Surely that's not a "consequence of this coincidence", is it? I would expect the density of any moon would be proportional to its tidal force (a truism if I ever heard one), and that the tidal forces of any moon plus any star would be additive when they are in more or less alignment and subtractive when they're not?

I would think that tidal force would be inversely proportional to distance and proportional to mass, so it would be coincidental that it is currently proportional to density.

[neufer]

Surely that's not a "consequence of this coincidence", is it? I would expect the density of any moon would be proportional to its tidal force (a truism if I ever heard one), and that the tidal forces of any moon plus any star would be additive when they are in more or less alignment and subtractive when they're not?

I would think that tidal force would be inversely proportional to distance and proportional to mass, so it would be coincidental that it is currently proportional to density.

Monopole potential wells are inversely proportional to distance.
Dipole potential wells are inversely proportional to distance squared.
Quadrupole potential wells are inversely proportional to distance cubed.

Diameter = distance times 1/107.5 radians
(i.e., the angular size of both the sun & moon [the coincidence!]).

Ergo: Quadrupole tidal potential wells are proportional to density = Mass/Volume.

[Czerno]

It's just a coincidence.

It's also a temporary coincidence. The Moon is gradually moving farther from the Earth; millions of years ago it covered more of the corona during eclipses, and millions of years from now every total eclipse will be annular- too bright for the corona to be seen at all.

Just a coincidence. Indeed, Chris.

Or is it a coincidence ? Millions of years ago, we weren't here to see eclipses, and millions of years from now, it is possible Man will no more exist either. The moon has been very slowly receding from the Earth, due to tidal friction effects, and "by coïncidence" Man is here at the epoch when the Moon and Sun are the same angular size.

I've long wondered whether maybe there's more than a coincidence - think anthropic principle if you want.
That we can't see a causal link or relation between the existence of Man (or a thinking animal?) and the almost exact superposability of the lunar and the solar disks does not preclude there exist some relation yet to be found.

Of course I'm pushing too far but... isn't it an attribute of Scientific queries to be weary of extraordinary "coincidences" ? What was the a priori probability that a random planet arouand a random star got a unique moon at the right distance, hey RStevenson, hey Chris ?

;=)

[apodman]

Moon and Sun are the same angular size. I've long wondered whether maybe there's more than a coincidence - think anthropic principle if you want.

That's all very nice food for daydreams, but all the anthropic principle does is take a truism (that all respondents to the "exist or not" survey say they exist) and dress it up as more than it is. If one wonderful coincidence indicates more than coincidence (!), why doesn't the lack of more wonderful coincidences indicate the opposite? Why are Venus and Mercury not the right sizes and in perfect orbits to give us more perfect eclipses? Why don't comets arrive in bunches to spell out messages from the gods?

That we can't see a causal link or relation between the existence of Man (or a thinking animal?) and the almost exact superposability of the lunar and the solar disks does not preclude there exist some relation yet to be found.

... isn't it an attribute of Scientific queries to be weary

Coincidences do direct us to look for relationships, but in this case relationships that satisfactorily describe the situation have already been found. We are wary, not weary, but rational.

What was the a priori probability that a random planet around a random star got a unique moon at the right distance ... ?

It's on the order of the reciprocal of the number of other equally impressive coincidences that could have happened to make you ask the same question. When you take the total odds of one of them happening, you get a fair likelihood. Besides, you're sitting in an a posteriori situation (where the outcome is already known) asking an a priori question. It's remarkable if you pick the lottery numbers before the drawing but not afterward.

---

Note that much of this line of inquiry is philosophy, not science, and therefore falls outside the defined scope of this forum. For example, the so-called anthropic principle is not testable and therefore not scientific. So I had better say no more lest I receive a warning citation from the authorities.

[Chris Peterson]

Just a coincidence. Indeed, Chris.

Or is it a coincidence ?

Well, this is a science forum, and the scientific answer is that the relationship is a coincidence, nothing more. You could find a million things like that in nature. If you want to read some kind of philosophy into it, fine, but this isn't the place to be asking those kind of questions.

[neufer]

It's just a coincidence. Our moon happens to be just the right size to almost perfectly cover the sun. No significance can be attached to this coincidence.

It's also a temporary coincidence. The Moon is gradually moving farther from the Earth; millions of years ago it covered more of the corona during eclipses, and millions of years from now every total eclipse will be annular- too bright for the corona to be seen at all.

Technically total & annular eclipses are mutually exclusive (except in the case of hybrid eclipses)...but we know what you mean.

The Moon's distance is growing by about 1% every 100 million years.

A quick calculation shows that :
there will be no total eclipses 650 million years from now
and that there were no annular eclipses about 1 billion years ago.

[rstevenson]

Surely that's not a "consequence of this coincidence", is it? I would expect the density of any moon would be proportional to its tidal force (a truism if I ever heard one), and that the tidal forces of any moon plus any star would be additive when they are in more or less alignment and subtractive when they're not?

I would think that tidal force would be inversely proportional to distance and proportional to mass, so it would be coincidental that it is currently proportional to density.

Monopole potential wells are inversely proportional to distance.
Dipole potential wells are inversely proportional to distance squared.
Quadrupole potential wells are inversely proportional to distance cubed.

Diameter = distance times 1/107.5 radians
(i.e., the angular size of both the sun & moon [the coincidence!]).

Ergo: Quadrupole tidal potential wells are proportional to density = Mass/Volume.

I stand corrected. (At least I would stand corrected if I understood. But that's my problem, not yours. Time for me to hit the books, or, nowadays, the web.)

Rob

[Czerno]

Neufer : Just curious, does your estimation of the drift take nonlinear [O(t²) and over] terms in account ? Is the recession speed increasing or decreasing by the way ? In any case the variation is very slow at the scale of the age of mankind, maybe even at the scale of the age, and probable lifetime, of the Planet. Makes the coincidence of sizes the more remarkable.

And no, Apodman, with all due respect, you can't seriously put "wandering stars" (Venus and Mercury) in the same bucket as the Sun and the Moon.

@Chris & al : I do not believe in the explicative virtue of the s-c "anthropic principle" BTW, I already posted sometime against that notion. Did you fail to notice the quotes and smileys in my previous post ? I don't really need a lesson in a-priori, conditional, or observed probabilities either - thank you. However I do believe that apparent coincidences need to be examined the more carefully as they seem extraordinary, and this coincidence of angular diameters, as seen from our perspective, is almost to good to be fortuitous. Alright, correlations to, e g, our existence here, are difficult to devise - but don't sweep the idea too fast. Remember the "principle" of the bread that always falls from the table on its buttered side... that looked like nonsense or a philosophical joke, at best, and is very well explained by relations between the fundamental physical constants (which govern gravity, our size hence also the height of tables and the number of half turns the bread slice makes in its fall) ?

[Czerno]

I stand corrected. (At least I would stand corrected if I understood. But that's my problem, not yours. Time for me to hit the books, or, nowadays, the web.)

Rob

Think of it thus : the tidal force is but a differential in attraction 'felt' by two neighbouring bits or particules of matter.
Since the gravitational attraction force itself is ~ 1/r² (thanks Sir Isaac), its tide-causing differential is as 1/r cubed.

It's really thus simple.

[BMAONE23]

Neufer : Just curious, does your estimation of the drift take nonlinear [O(t²) and over] terms in account ? Is the recession speed increasing or decreasing by the way ? In any case the variation is very slow at the scale of the age of mankind, maybe even at the scale of the age, and probable lifetime, of the Planet. Makes the coincidence of sizes the more remarkable.

And no, Apodman, with all due respect, you can't seriously put "wandering stars" (Venus and Mercury) in the same bucket as the Sun and the Moon.

@Chris & al : I do not believe in the explicative virtue of the s-c "anthropic principle" BTW, I already posted sometime against that notion. Did you fail to notice the quotes and smileys in my previous post ? I don't really need a lesson in a-priori, conditional, or observed probabilities either - thank you. However I do believe that apparent coincidences need to be examined the more carefully as they seem extraordinary, and this coincidence of angular diameters, as seen from our perspective, is almost to good to be fortuitous. Alright, correlations to, e g, our existence here, are difficult to devise - but don't sweep the idea too fast. Remember the "principle" of the bread that always falls from the table on its buttered side... that looked like nonsense or a philosophical joke, at best, and is very well explained by relations between the fundamental physical constants (which govern gravity, our size hence also the height of tables and the number of half turns the bread slice makes in its fall) ?

http://creationwiki.org/Moon_recession

[rstevenson]

http://creationwiki.org/Moon_recession

Rob

[neufer]

Neufer : Just curious, does your estimation of the drift take nonlinear [O(t²) and over] terms in account ? Is the recession speed increasing or decreasing by the way ? In any case the variation is very slow at the scale of the age of mankind, maybe even at the scale of the age, and probable lifetime, of the Planet. Makes the coincidence of sizes the more remarkable. ?

http://creationwiki.org/Moon_recession

No, Czerno, I didn't take all the nonlinear factors into account.

But it is nice to know that the Moon recession was never a Moon depression.

[apodman]

it is nice to know that the Moon recession was never a Moon depression

That's because there are never more than two consecutive quarters of waning.

you can't seriously put "wandering stars" (Venus and Mercury) in the same bucket as the Sun and the Moon

Sticks and stones may break Mercury's and Venus' bones, but just don't say anything bad about Pluto.

Alright, correlations to, e g, our existence here, are difficult to devise - but don't sweep the idea too fast.

Sweep away what idea? Unless you propose some mechanism for the correlation, there is no idea.

[Czerno]

quote : "Sweep away what idea? "

ISTM you're absolutely denying there /might/ be something to even wonder about. It's hard to understand why.

At the same time "NASA scientists" (as journalists would put it) believe in the existence of many inhabited planets in the vast Universe and even around nearby stars, enough that the US spends tax payers money (I'm not in the lot) to send messages to the green men 10 parsecs away and, who knows, try to tap their telephones <G>

Oh, well, to each his false science (I don't know if you're among the "believers" in ET life, please don't take it personally)

--
Czerno

[apodman]

quote : "Sweep away what idea? "

ISTM you're absolutely denying there /might/ be something to even wonder about. It's hard to understand why.

At the same time "NASA scientists" (as journalists would put it) believe in the existence of many inhabited planets in the vast Universe and even around nearby stars, enough that the US spends tax payers money (I'm not in the lot) to send messages to the green men 10 parsecs away and, who knows, try to tap their telephones <G>

Oh, well, to each his false science (I don't know if you're among the "believers" in ET life, please don't take it personally)

--
Czerno

For the record, extraterrestrial life is not a belief; it is a fact pending discovery.

For the record, I am a US citizen, pro-US, pro-NASA, and pro-science. And not just enough science to trash the parts I don't like, either.

For the record, there is far more to wonder about than any person can manage in a lifetime, which is why an idea must have some substance that can be examined to command my attention.

[neufer]

it is nice to know that the Moon recession was never a Moon depression

That's because there are never more than two consecutive quarters of waning.

Well, thank God it stopped waning!

[BMAONE23]

it is nice to know that the Moon recession was never a Moon depression

That's because there are never more than two consecutive quarters of waning.

Well, thank God it stopped waning!

Well, If it weren't for the "Waning" term then we might have the Karate Moon
Wax-on...Wax-off

[Czerno]

... a fact pending discovery

Now that's one scientific notion ;=)

For the record, I am ...
For the record, ... an idea must have some substance that can be examined to command my attention.

And not egocentric in the least.

We love APOD - we do - and as such are grateful to you inasmuch as you're contributing to make it a great daily source of enjoyment and wonder. Let's please keep focused on topic as much as possible and avoid that aggressive tone which but belittles the interest of the forum, IMVHO.

[apodman]

not egocentric in the least. ... keep focused on topic as much as possible and avoid that aggressive tone

Look in the mirror, man who promotes pseudo-science and non-science and who uses his posts to slur NASA, science, scientists, and the US. Your cracks about spending taxpayers' money and tapping phones are political and a violation of rule 5 of this forum. I only spoke of myself as a positive counterexample to the iconoclasm you attempt. My statement of a "fact pending discovery" obviously puts me out on a limb, but it is a very short and very thick limb and that's the point - a strong logical and statistical likelihood that can be verified any day by current and emerging technology (similar to the successful search for extrasolar planets) is so much more than a "belief" that characterizing it as only a "belief" is an insult.