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July 19, 2010 12:10 PM Subscribe
Why does the moon (well, depending on the flavour of eclipse) exactly cover the sun during a solar eclipse?
What's the probability that this would happen for a given planet with a moon? Is it super rare? Or just kinda rare? Can anyone cook up a statistical probability?
What's the probability that this would happen for a given planet with a moon? Is it super rare? Or just kinda rare? Can anyone cook up a statistical probability?
According to Wikipedia, The Sun's distance from the Earth is about 390 times the Moon's distance, and the Sun's diameter is about 400 times the Moon's diameter. Because these ratios are approximately the same, the Sun and the Moon as seen from Earth appear to be approximately the same size: about 0.5 degree of arc in angular measure.
posted by shakespeherian at 12:15 PM on July 19, 2010
posted by shakespeherian at 12:15 PM on July 19, 2010
Well, the moon is pretty odd in and of itself. It's 1/4 the mass of the earth, and I don't think any other planet in our solar system has a satellite even close to this ratio.
Consider this: tens (hundreds) of millions of years ago the moon was much closer to the earth! I bet the solar eclipses looked different then.
posted by sbutler at 12:17 PM on July 19, 2010
Consider this: tens (hundreds) of millions of years ago the moon was much closer to the earth! I bet the solar eclipses looked different then.
posted by sbutler at 12:17 PM on July 19, 2010
It doesn't? Solar eclipses are only visible from certain portions of the planet, i.e. the locations where the moon actually falls between the earth and the sun.
If your question is "What are the odds that a given moon orbiting a given planet at a given distance which is revolving around a given star at a given distance will be large enough to cause a total eclipse for some part of said planet's surface?" I think the answer is "Pretty damn high," due to the distances involved. The sun is big, but we currently orbit the sun at a distance more than 100 times its diameter. Doing the math, you don't need to be all that big to cause a total eclipse of any appreciable size with those sorts of numbers. Even an orbital body a fraction of the moon's size could cause a decent-sized total eclipse.
Even the moons of Jupiter can cause eclipses.
I'll leave someone who 1) has more time and 2) knows more geometry than me to do the actual numbers.
posted by valkyryn at 12:25 PM on July 19, 2010 [1 favorite]
If your question is "What are the odds that a given moon orbiting a given planet at a given distance which is revolving around a given star at a given distance will be large enough to cause a total eclipse for some part of said planet's surface?" I think the answer is "Pretty damn high," due to the distances involved. The sun is big, but we currently orbit the sun at a distance more than 100 times its diameter. Doing the math, you don't need to be all that big to cause a total eclipse of any appreciable size with those sorts of numbers. Even an orbital body a fraction of the moon's size could cause a decent-sized total eclipse.
Even the moons of Jupiter can cause eclipses.
I'll leave someone who 1) has more time and 2) knows more geometry than me to do the actual numbers.
posted by valkyryn at 12:25 PM on July 19, 2010 [1 favorite]
If you're referring to the position, as opposed to the size, of the moon, then whether the moon is positioned to precisely block the sun depends on where you are on the planet. It's only seen as a total eclipse in a relatively small area. People in other places will see a partial eclipse, or just a near miss.
(pretty sure, anyhow)
posted by jon1270 at 12:26 PM on July 19, 2010
(pretty sure, anyhow)
posted by jon1270 at 12:26 PM on July 19, 2010
Worth noting that the moon has an elliptical orbit and the solar and lunar discs don't always exactly line up, which results in annular eclipses, where some portion of the edge of the solar disc is still visible even at the maximum coverage. In the far future, as the moon slowly gets farther from the Earth, annular eclipses showing ever more sun around the edge will become the norm.
posted by aught at 12:29 PM on July 19, 2010
posted by aught at 12:29 PM on July 19, 2010
Best answer: "Can anyone cook up a statistical probability? "
Probably not (heh), because we don't know enough about other systems outside our own, so we don't really know how many suns have planets, how many planets have moons, and how often the apparent size of a star or moon are equivalent.
"Even the moons of Jupiter can cause eclipses."
True - but the moons of Jupiter don't cover the entire planet in eclipse. I'm somewhat sure that the earth/moon combo is unique in that regard, for our system.
posted by kanewai at 12:31 PM on July 19, 2010
Probably not (heh), because we don't know enough about other systems outside our own, so we don't really know how many suns have planets, how many planets have moons, and how often the apparent size of a star or moon are equivalent.
"Even the moons of Jupiter can cause eclipses."
True - but the moons of Jupiter don't cover the entire planet in eclipse. I'm somewhat sure that the earth/moon combo is unique in that regard, for our system.
posted by kanewai at 12:31 PM on July 19, 2010
oh wait ... let me rethink that ... if the moons of Jupiter are casting a shadow on the surface, then I suppose that to an observer on the surface of Jupiter it would appear to be a total eclipse.
If Jupiter had a surface.
- But even then, Jupiter's moons might have a larger apparent disc size than the sun, so it wouldn't be the same as our total eclipses ... I don't know if you'd see the corona.
posted by kanewai at 12:37 PM on July 19, 2010
If Jupiter had a surface.
- But even then, Jupiter's moons might have a larger apparent disc size than the sun, so it wouldn't be the same as our total eclipses ... I don't know if you'd see the corona.
posted by kanewai at 12:37 PM on July 19, 2010
Also important to note that the moon only has the same apparent size right now-- it's moving away from the earth and thus at some point in the future will be as good at providing total eclipses as Jupiter's moons.
posted by shakespeherian at 12:38 PM on July 19, 2010
posted by shakespeherian at 12:38 PM on July 19, 2010
By chance, our Moon is a size and distance that makes it pretty good at covering the sun for eclipses. We are supremely lucky, and live on a planet full of wonders.
posted by theora55 at 12:44 PM on July 19, 2010 [9 favorites]
posted by theora55 at 12:44 PM on July 19, 2010 [9 favorites]
Due to the transfer of angular momentum, the distance from the Earth to the moon is gradually increasing, corresponding to a gradual increase in the rotational period of the Earth. If you looked a few hundred million years in the past or a few hundred million years in the future, it would not appear to cover the Sun as it does now, so in that sense it's an extreme coincidence that we happen to be around to witness it.
posted by Rhomboid at 12:48 PM on July 19, 2010
posted by Rhomboid at 12:48 PM on July 19, 2010
I seem to recall that Sir Fred Hoyle, in a book the title of which I can't just now remember, suggested that eclipses for the dinosaurs must have been much more frequent and more spectacular than for us. BTW, the moon is one-quarter the diameter of the earth, not one-quarter of its mass; the ratio of masses is more like 1 to 80. It's correct that the moon and earth are closer in size than any other satellite and its primary (except for Pluto and Charon, which don't count).
posted by Logophiliac at 12:56 PM on July 19, 2010
posted by Logophiliac at 12:56 PM on July 19, 2010
BTW, the moon is one-quarter the diameter of the earth, not one-quarter of its mass; the ratio of masses is more like 1 to 80
Opps... I should check Wikipedia before posting :)
posted by sbutler at 1:01 PM on July 19, 2010
Opps... I should check Wikipedia before posting :)
posted by sbutler at 1:01 PM on July 19, 2010
I don't know if you'd see the corona.
You almost certainly would, given that the corona extends twenty solar radii out from the sun. That makes it something like fourteen million miles across. That's pretty freaking huge, and even as far away as Jupiter you'd need something pretty large to block out an "object" that size.
posted by valkyryn at 1:17 PM on July 19, 2010
You almost certainly would, given that the corona extends twenty solar radii out from the sun. That makes it something like fourteen million miles across. That's pretty freaking huge, and even as far away as Jupiter you'd need something pretty large to block out an "object" that size.
posted by valkyryn at 1:17 PM on July 19, 2010
Consider this: tens (hundreds) of millions of years ago the moon was much closer to the earth! I bet the solar eclipses looked different then.
Indeed! When I was a kid, Qfwfw and I would often row out in a boat, and when we were underneath, prop a ladder against her and scramble up.
posted by yeti at 1:59 PM on July 19, 2010 [2 favorites]
Indeed! When I was a kid, Qfwfw and I would often row out in a boat, and when we were underneath, prop a ladder against her and scramble up.
posted by yeti at 1:59 PM on July 19, 2010 [2 favorites]
Saturn's moon Rhea eclipses Dione
The Moon was only about 1% closer to the Earth in the time of the dinosaurs, so eclipses should have been the same for the dinosaurs as they are for us, except that dinosaurs didn't arrange cruises to see them as far as we know.
posted by lukemeister at 2:02 PM on July 19, 2010
The Moon was only about 1% closer to the Earth in the time of the dinosaurs, so eclipses should have been the same for the dinosaurs as they are for us, except that dinosaurs didn't arrange cruises to see them as far as we know.
posted by lukemeister at 2:02 PM on July 19, 2010
Thank you, Yeti, for bringing up Calvino's book. I had heard of him off and on-never read any of his writings-and now I have a wonderful place to begin.
posted by girdyerloins at 2:46 PM on July 19, 2010
posted by girdyerloins at 2:46 PM on July 19, 2010
Dumb luck, that's all. Some people have argued that the sizes are so conveniently close and, because this ended up helping our early civilizations so much with astronomical calculations that it must be by some greater design. 'Course, now you're talking Velikovsky-Venus-was-a-moon-of-Jupter crazy-talk.
OR IS IT?
Yes.
posted by Civil_Disobedient at 4:30 PM on July 19, 2010 [2 favorites]
OR IS IT?
Yes.
posted by Civil_Disobedient at 4:30 PM on July 19, 2010 [2 favorites]
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posted by dabug at 12:13 PM on July 19, 2010