[This SD column is not yet published on this site.]
Well, I’m no astronomer, but it seems to me that the equation r = G * M_sun / v^2 just says that the product of r and v^2 is independent of the sum of M_earth and M_moon. The energy in the earth-moon combination’s orbit about the sun would be E = (1/2) * (M_earth + M_moon) * v^2. If the mass of the moon were taken away, but there was conservation of energy, v, the velocity of the earth in its orbit about the sun, should increase by the square root of M_moon. That, in itself, shouldn’t make the earth’s orbit less stable; however, the less total mass in orbit then would seem to make this orbit less stable in the face of any other objects of a given mass that should happen by, off and on at various angles of attack – presumably to check out what happened to the moon. (I can’t help but think black cats and pumpkins should somehow figure in here, at least if Greg gets the job done before the end of this month.)
Interesting thought. Luna isn’t actually a moon in the strictest sense of the word (don’t recall exactly which of the qualifications it fails). The Earth/Moon pair is actually a binary, i.e., two bodies orbiting around each other in this funky dumbbell shape. But the center of the dumbbell is a couple miles beneath the surface of the earth, so it looks like it orbits around us while we don’t move.
Ah, the things I remember from reading Asimov as a child.
The idea that the earth has less craters on it than the moon because the moon acts as some sort of shield is, well…poopy! I’m not denying that it could be partially responsible, but it goes deeper than that. During the era of bombardment (the name is pretty self-explanitory) early in the earth’s (and moon’s) history, the earth was getting smacked with just as many meteorites as the moon. The difference is that the moon is a “dead” planet, meaning that its internal heat source has pretty much fizzled out. The earth on the other hand is still all nice and gooey inside, and this internal heat is the driving force for plate tectonics. Ah ha, you’re saying, it’s starting to make sense now. That’s right. The reason we see fewer craters on Earth than on the moon is because Earth is continually being re-surfaced. We just got rid of all our craters through plate colisions and the like while the moon, which has no such processes, didn’t. Okay, so maybe this doesn’t have much to do with what would happen if someone obliterated the moon, but don’t you just feel smarter now?
The gravitational pull of Jupiter plays havoc on the rotational components of nearby planets. This was recognized and calculated by scientists in the 19th century. The effect on Earth appears to play a significant (though not the only) role in our climate history.
Part of the effect is that the ellipticity of our orbit varies slightly over a 100ky period. This is coupled with precessional wobble, which changes the orientation of our rotational axis (and thus the dates of the rotational cardinal points, the equinoxes and solstices).
One of the more significant effects is the change in the angle of tilt between the equatorial plane and the ecliptic (currently 23.5 degrees) which accounts for almost all of the seasonality experienced on Earth.
Earth’s tilt varies between 21 & 26 degrees with a period of ~40ky. Mars, a planet without a moon of significant mass, goes through a much more significant change of 15-35 degrees, resulting in much more significant climate swings than we experience here.
I believe that Luna acts as a rotational buffer, a negative feedback to the potentially catastrophic variations in axial tilt.
Yeah. I watched a documentary a while ago that said that if the moon were to disappear, the Earth’s axis would destabilize. In a few years (months?), there would be no more seasons as we know it. There might, however, be seasons like “half a year of everything (including us) melting and boiling”, accompanied by “half a year of temperatures below -100°C”.
Are you sure they were talking about drastic effects like that? All other planets in the solar system have much smaller moons, if any, and their rotation axes seem stable enough to me. I’m sure there will be very small effects and our almanacs would need to be re-calculated, but the seas won’t boil.
At least Cecil is dealing with the big questions. I think we can speculate all we like, but until someone pulls themselves together and blows up the moon we just won’t KNOW.
I’ve never heard that the Moon doesn’t acutally QUALIFY, though some have said it might the Earth/Moon system could be better described as a “double planet” system. The Moon is pretty big for a moon when compared to its planet (second only to Pluto’s moon Charon).
And so, since the Moon is pretty big, it gravitationally tugs on the Earth just as the Earth tugs on it… and so the two bodies sort of orbit around each other. The center of gravity of the Earth/Moon system is not located at the center of the Earth (as it would be if the Moon were tiny.)
This has probably been dealt with before, but our definition of “planet” is somewhat nebulous (no pun intended; well, maybe a little). Basically, planets are those objects which we have historically called “planets.”
Size doesn’t work, because several of the Jovian moons are larger than Mercury and Pluto. Presence of an atmosphere doesn’t work, primarily for the same reason – ability to hold onto an atmosphere depends upon temperature and gravity.
Orbiting the Sun doesn’t really work, because everything orbits the Sun. “Aha, but moons also orbit a primary body,” I hear you say. Well, everything orbits everything else, in truth. All objects orbit around a common center of mass; the Sun is so massive that the system’s center is more-or-less the Sun’s center. For Jupiter and its moons, the center is pretty much Jupiter’s center. For Earth and Luna, the center around which both rotate is about 1/4 to 1/3 the distance from the surface to the Earth’s core.
If you calculate gravitational attraction (proportional to the sum of the masses of the two bodies and inversely proportional to the square of the distance separating their centers of mass) you find that the Sun has twice the pull on Luna as does Earth. This is difficult to picture, but at no point in its orbit around the sun does Luna’s path describe an arc that is convex toward the Sun.
This is why controversy exists about the status of Pluto – It appears to be compositionally similar to other known Kuiper Belt objects, and only marginally larger. But it was the first one discovered, and Clyde Tombaugh was looking for a planet when he found it…
Well, I suppose, if you look everywhere in the universe, you find a continuum of positional and orbital relationships, with say no clear line between a star, a planet a satellite of planet, an asteroid, a metero or a comet, couplets of more than one of any of them or of mixed types, and maybe no clear line between these and small nebulae or galaxies, and all independent of whether any or all of them or their components are solid, liquid or gaseous, etc., etc… . .but. . .I think I’m on solid. . .at least gaseous terra nonfirma in claiming that a planet is a coherent mass that directly orbits a star (however you define that), rather than following an orbit better describable as being about another mass not so orbitting. . .at least I think that was how astronomers (or astrologers or cosmologists or whatever) set about to planet.
Maybe I just missed this point so far, but what about the animals/plants?
It’s not just the dogs baying to Alpha Centauri…
The spawning seasons of many animals depend on the (full) moon. For corals for example it’s important that they all spawn at the same time to make sure at least a few survive.
That time is the full moon in March(? Not sure, maybe February)
When turtles hatch out they move in the direction where the moon is. If the moon’s gone they go anywhere…
A few might find the right way, but for sure not all.
There are 1000s of other examples like that where a missing moon would destroy the way animals do reproduce.
I bet there are other ways that do not depend on the moon, but for the now existing animals it would be probably too late to change.
I guess lighter tides will effect the amount of oxygen available in the sea, that will have a major effect on the animal and plant population there.
All in all I would say a missing moon would not kill life on earth, but it would cause a major change in what kind of animals are living on earth. Many breeds will die, many new will come into being.
All in all I’m very openminded to that situation, but since the human race stands at the end of the food chain I would not bet on McDonalds still selling Chicken McNuggets on a long sight…
People are talking about changes in the tides and interrupting the breeding cycles of frogs.
The point has been mentioned several times that the earth moon system actually rotate around a common point somewhere below the earths surface. No one has mentioned what that fact means as regards the destruction of the moon. If the moon were gone (or even just powdered and made into a planetary ring) the rotational center of the system would shift to the earths center.
Now for a moment consider the effects of such a shift on a body with a molten core and liquid oceans on the surface. Yeah the tide would be gone but the sloshing around the oceans would get as a result of the realignment would be something to see, (maybe from orbit!) And what about the tectonic effects. The thin shell covering the molten core of the Earth would be cracked asunder. A little 8.0 earth quake can cause enough damage to kill thousands. Now imagine a 10.0 that is not localized but happening all over the surface of the Earth at the same time.
I think Cecils answer was simplistic and just plain wrong. The author concentrated on some of the most trifling effects and completely ignored the most catastrophic. I realize this is a purely hypothetical debate. But come on guys. I think maybe we can agree that global catastrophe is just a little more significant than disrupting the breeding habits of frogs.
…and I’d guess that there’d be another significant change related to this: that the molten core of the Earth might eventually begin to cool as well. I have to believe that the oceans aren’t the only part of the Earth’s crust that get sucked up by the pull of the moon’s gravity. Don’t the tectonic plates themselves get yanked up and down? And if so, where do you suppose all of that energy goes, anyway? Betcha it gets directed into the core, which is what keeps it hot and liquid.
No moon = no (or reduced) volcanos/geothermal energy = reduction in the life-carrying capacity of the planet?
Land tides are pretty minor, less than a coupla centimeters IIRC. Most of this is accounted for by movement of surface regolith, which covers the vast majority of the planet’s surface. The fact that the regolith is decoupled from the interior prevents even this tiny amount of energy from being tranferred beneath the surface.
Heat flow from the interior dwarfs this tiny amount of tidal energy. The source of most of this heat is the decay of radioactive elements in the mantle, predominantly uranium (lower mantle) and potassium-40 (upper mantle). The outer core is molten because it hasn’t lost enough of its original heat to fall beneath its crystallization temperature.
Mantle-generated heat keeps the interior warm enough that the temperature gradient between the core and upper mantle is shallow, so heat doesn’t flow across the boundary as quickly as it might otherwise.
If it weren’t for radiodecay the Earth would have cooled completely in a few hundred million years. Note that Venus has no moon, thus no tides, yet there is evidence of (relatively) recent volcanism on its surface. Mars, with no moon of consequence (you can’t count those piddly little rocks, Phobos and Diemos) appears to have produced eruptions as recently as a few hundred million years ago…
