if the moon doubled in size

[Third Eye Open]

What would be the effects on the earth if the moon were to suddenly (magically) double in mass? Earth quakes? Tidal waves? Would we weigh less because of the increased pull from the moon? Would it be noticeable at all, and if not, how much mass would the moon have to gain for us to notice?

[gambling_cruiser]

This magic double mass moon would change the movement of the earth (moon and earth orbit the common barycenter, the position fo the barycenter would move nearer to the moon), and the tides would get greatly increased (about doubled).

[Denver]

If the moon doubled in mass, but did not rebalance its orbit, it would either crash into the earth, or its current momentum would take it out of orbit (I'm not sure which, but I suspect the former).

[Monketey Ghost]

Bad Things. Tidal waves, yes, but I think a good portion of our atmosphere would be skimmed away too, so high winds all over the planet as the air flows toward the sky...

[Lanzy]

There was recently a TV movie of the week about this (kinda). Basically lots of destruction until we come up with a last minute effort to shrink it back down.

[Tumbleweed]

What would be the effects on the earth if the moon were to suddenly (magically) double in mass? Earth quakes? Tidal waves? Would we weigh less because of the increased pull from the moon? Would it be noticeable at all, and if not, how much mass would the moon have to gain for us to notice?
I think you pretty much have it covered. I most definitely think that doubling the size would produce a noticeable effect
On a related note: Am I lighter when the moon is on the other side of the Earth from where I am standing. If so, my guess is that doubling the moons size would double that effect And wouldn't the moon then have to seek a different orbit and in doing so go flinging away or come crashing down?

[Tumbleweed]

I think you pretty much have it covered. I most definitely think that doubling the size would produce a noticeable effect
On a related note: Am I lighter when the moon is on the other side of the Earth from where I am standing. If so, my guess is that doubling the moons size would double that effect And wouldn't the moon then have to seek a different orbit and in doing so go flinging away or come crashing down?
Oops, I meant am I heavier when the moon is on the other side of the Earth

[Stray Cat]

There would be twice as many lunatics* around on the full moon?

*(if there were any verifiable evidence to suggest that an increase in insanity had ever been recorded in connection to the moon phase)

[Dilb]

If the moon suddenly doubled in mass, but kept the same speed, it would mostly stay in the same orbit. There would be changes on earth; huge tides, and probably a lot of earthquakes as existing faults suddenly have greater strain on them, but otherwise the earth shouldn't be too affected. The earth is still much more massive than the moon, so the mass of the moon doesn't have a huge effect on it's orbit. You would still need some very sensitive instruments to detect the change in your weight due to the moon.

If the additional mass slowed the moon down, like if a moon sized chunk of the earth were flung straight up at it (with negligible speed away from earth by the time of the collision), then the moon would enter into an elliptical orbit, with it's apogee at the point where it was hit. I'm pretty sure it wouldn't hit the earth, but the moon getting that much closer to earth would cause some really big tides. Anyone feel like figuring out the change in distance of the moons orbit if it suddenly had it's velocity divided by 2?

[~enigma~]

There was recently a TV movie of the week about this (kinda). Basically lots of destruction until we come up with a last minute effort to shrink it back down.
Impact was the name and Natasha Henstridge looked quite attractive in it.

http://forums.randi.org/imagehosting/thum_132574a85b0cb8d024.jpg (http://forums.randi.org/vbimghost.php?do=displayimg&imgid=17270)

[Darth Rotor]

Wait a minute:

If moon at present range doubled in size (do you mean radius doubles, or mass? ) then at twice (or about eight times, see below) the present mass (assuming density of moon material does not change in this transformation), my back of the napkin math shows the mass to mass attraction from gravity pulling moon toward earth.

My stubby pencil needs better than that to decide if Moon misses earth as the force pulls it toward the larger body, or if it does as a lot of satellites to, and slingshots past and to re-establish a new, larger radius orbit.

Anyone have a bit of software that calculates such things, in terms of collision or "missed and flies by" dynamics?

Moon is roughly a sphere.

sphere volume = (4/3) pi r ^ 3
Current volume is r = 1. (1 being radius or moon) Mass and volume are proportional.
Expanded volume with r =2 is 4/3 pi 2^3 or roughly eight times the current mass of the moon.

DR

[Ziggurat]

The barycenter would shift slightly. This would make increase the moon's elipticity, but not a hell of a lot, and the earth and moon would remain in orbit around this barycenter. Tides would increase, you might get some earthquakes, but nothing too dramatic right away.

But there would be a huge and devastating effect. An instant change in the mass of the moon would mean a change in the velocity of the earth-moon barycenter about the sun. This orbit is pretty circular right now, but would become noticeably more eliptical. Think AGW is a problem? It wouldn't compare.

[Towlie]

Wait a minute:

If moon at present range doubled in size (do you mean radius doubles, or mass? ) then at twice (or about eight times, see below) the present mass (assuming density of moon material does not change in this transformation), my back of the napkin math shows the mass to mass attraction from gravity pulling moon toward earth.

My stubby pencil needs better than that to decide if Moon misses earth as the force pulls it toward the larger body...Think about the geostationary orbit where television relay satellites can seemingly hang in space so that satellite dishes here on Earth can be permanently aimed at them without any need for tracking mechanisms. That orbit is directly above the Equator and about 22,000 miles above sea level.

Now, does it matter what the mass of the satellite is? If a satellite is twice as massive as another satellite, does it have a different geostationary orbit at a different altitude? No. The altitude of the geostationary orbit can be calculated without knowing the mass of the satellite, and it has one specific value. Similarly, for any other orbital period, there's one specific distance where a satellite orbits with that period and that distance is independent of the mass of the satellite.

[Towlie]

The barycenter would shift slightly.According to an Isaac Asimov article I read a few decades ago, the barycenter of the Earth-Moon system is located about 1000 miles underground. I would guess that in the scenario we're discussing, it would move to somewhere above ground. I guess.

[Ziggurat]

Now, does it matter what the mass of the satellite is? If a satellite is twice as massive as another satellite, does it have a different geostationary orbit at a different altitude? No.

This is only true to the extent that we can approximate the location of the barycenter as being unaffected by the mass of the satellite (ie, at the center of the earth). That is not a good approximation in the case of something as large as the moon.

[Towlie]

This is only true to the extent that we can approximate the location of the barycenter as being unaffected by the mass of the satellite (ie, at the center of the earth). That is not a good approximation in the case of something as large as the moon.I know that, but it should be enough to convince Darth Rotor that his napkin math is wrong. By his logic, if the mass of a communications satellite was increased then its orbit would change.

[Ziggurat]

According to an Isaac Asimov article I read a few decades ago, the barycenter of the Earth-Moon system is located about 1000 miles underground.

Well, let's figure it out. The earth is roughly 5.98x1024 kg, the moon is roughly 7.36x1022 kg, and the total distance between them (center to center) is roughly 3.84x108 m. So the earth-barycenter distance should be 7.36x1022/(7.36x1022 + 5.98x1024) * 3.84x108 m = 4.67x106 m. The radius of the earth is about 6.37x106 m, so the barycenter should be about 1.7x106 m below ground, which is about 1060 miles. So yup, that's about right.

I would guess that in the scenario we're discussing, it would move to somewhere above ground. I guess.

Why guess when you can know? If we double the mass of the moon, that puts the barycenter around 9.23x106 m from the center of the earth, or about 1770 miles above the surface of the earth. The fractional change in moon-earth barycenter distance would still be fairly small, so the moon would definitely stay in orbit. It would become more elliptical, but probably not terribly so.

[Jimbo07]

What would be the effects on the earth if the moon were to suddenly (magically) double in mass?


etc. etc. etc.


Probably wouldn't happen. In a situation where it did, it would almost certainly mean that magic existed. If magic existed, then it would become very difficult to reliably predict anything (by using concepts such as mass and velocity, rather than magic).

:D

[Subduction Zone]

Just a side note, early in the history of the Moon it was much closer to the Earth. Tidally its effects then were much greater than it would be now even if the Moon was twice as large. As Jimbo just pointed out, short of magic, there is now way you could double the size of the Moon. And as any creationist will tell you when magic is allowed you can throw all the rules out of the window.

[Thabiguy]

It would become more elliptical...

... or less elliptical. ;)

[Third Eye Open]

Ok, how much mass would the moon have to gain for us to weigh noticeably less here on earth, or does it even work like that?

[MikeSun5]

Interesting stuff. I would guess massive flooding in places and receding shorelines in others. Maybe our orbit around the sun would be affected, lengthening/shortening summers/winters. Does the moon's pull have any effect on the Earth's rotation? Would our days be longer or shorter?

...not to derail the thread, but I wonder what would happen if the moon broke up into smaller pieces (http://members.cox.net/renegade_sith/miscjunk/mooncrash.jpg)...

[rjh01]

Ok, how much mass would the moon have to gain for us to weigh noticeably less here on earth, or does it even work like that?

That depends purely on how you define 'noticeably less'. Actually you weigh slightly less when the moon is either directly overhead or below your feet (ie at high tide). You weigh the most when the moon is at right angles to the vertical (at low tide).

All the other effects on the orbits mentioned depend on the velocity of the new mass with respect to the moon. If that is zero then about the only effect of the orbit would be the centre of gravity would be changed as per the above post.

I do not see earthquakes increasing as the forces are too small.

One effect not yet mentioned is that satellites would need their orbits adjusting
a little more often as the moon will have a greater pull on them.

[ponderingturtle]

Bad Things. Tidal waves, yes, but I think a good portion of our atmosphere would be skimmed away too, so high winds all over the planet as the air flows toward the sky...

Why? This didn't happen when the moon was much closer to the earth after all.

[Towlie]

Probably wouldn't happen. In a situation where it did, it would almost certainly mean that magic existed.After reading the post by ~enigma~ yesterday, I watched the first half of Impact (http://www.imdb.com/title/tt1227637/). It's not really such a bad story so far. Yes, it's full of scientific implausibilities, but at least the scientist characters in the movie don't pretend that science can explain it all. They are thoroughly baffled, as they well should be.

In my opinion, movies are much worse when scientists spout nonsense like "Einstein proved that time travel is possible" or "Einstein proved that faster-than-light travel is possible". As long as the writer doesn't pretend that his far-fetched plot actually makes scientific sense, I'm willing to allow myself the luxury of suspension of disbelief and let myself enjoy the show.

Incidentally, A science fiction writer sacrifices a lot of potential for interesting stories when he commits himself to passing scientific muster. That's something that Arthur C. Clark could pull off, but such authors are rare.

Anyway, in Impact, the moon gains mass when a stray chunk from a brown dwarf buries itself inside. That seems reasonably plausible and doesn't require magic, although there's no escaping the fact that the moon's orbit would have to change as it absorbed the kinetic energy of the object that hit it. In fact, this is what happens in the movie.

Now to see what the second half is like.

[~enigma~]

After reading the post by ~enigma~ yesterday, I watched the first half of Impact (http://www.imdb.com/title/tt1227637/). It's not really such a bad story so far. Yes, it's full of scientific implausibilities, but at least the scientist characters in the movie don't pretend that science can explain it all. They are thoroughly baffled, as they well should be.

In my opinion, movies are much worse when scientists spout nonsense like "Einstein proved that time travel is possible" or "Einstein proved that faster-than-light travel is possible". As long as the writer doesn't pretend that his far-fetched plot actually makes scientific sense, I'm willing to allow myself the luxury of suspension of disbelief and let myself enjoy the show.

Incidentally, A science fiction writer sacrifices a lot of potential for interesting stories when he commits himself to passing scientific muster. That's something that Arthur C. Clark could pull off, but such authors are rare.

Anyway, in Impact, the moon gains mass when a stray chunk from a brown dwarf buries itself inside. That seems reasonably plausible and doesn't require magic, although there's no escaping the fact that the moon's orbit would have to change as it absorbed the kinetic energy of the object that hit it. In fact, this is what happens in the movie.

Now to see what the second half is like.
But did you find Natasha sexy?

[stilicho]

How would the effects of a same mass moon half the distance of where it is now compare? My thumbnail calcs say it's twice the effect but I could be in error. And, as I recall, it was the Moon nearing the earth that was the plot device in that Natasha Henstridge made-for-TV movie but I don't really remember because I only watched it to see Natasha Henstridge and quickly lost interest when they tried to gussy it up with dialogue.

IIRC, the gravitational force is doubled by increasing mass by two and quadrupled by reducing distance by half.

How about another exercise where the mass of the earth magically doubles but the distance and mass of the moon remains the same?

[Towlie]

But did you find Natasha sexy?Not as sexy as she was in Species (http://www.imdb.com/title/tt0114508/).

[Towlie]

If the moon has all this tremendous gravity that's causing chaos down on Earth, why don't those chunks of debris fall back to the surface instead of just floating there?

http://www.smacaw.com/randi/impact.jpg

Oh well...

[rjh01]

How would the effects of a same mass moon half the distance of where it is now compare? My thumbnail calcs say it's twice the effect but I could be in error. And, as I recall, it was the Moon nearing the earth that was the plot device in that Natasha Henstridge made-for-TV movie but I don't really remember because I only watched it to see Natasha Henstridge and quickly lost interest when they tried to gussy it up with dialogue.

IIRC, the gravitational force is doubled by increasing mass by two and quadrupled by reducing distance by half.

How about another exercise where the mass of the earth magically doubles but the distance and mass of the moon remains the same?


Actually we are talking about tidal effects, not gravity effects of the moon. I believe the effect would be a lot bigger. Redo the calculations and work out what the difference in gravity of the moon is over a certain distance when measured from
a. The Earth
b. Half way to the moon.

I think the answer will be a lot more than what you expect.

[Piggy]

If the moon were to double in size, it would hit your eye like a big pizza pie.

I have it on good authority.

[Roboramma]

Probably wouldn't happen. In a situation where it did, it would almost certainly mean that magic existed. If magic existed, then it would become very difficult to reliably predict anything (by using concepts such as mass and velocity, rather than magic).

:D

Well, since we've been able to reliably predict everything except that one magic event with such concepts as mass and velocity, even after that one event, I would still put quite a bit of trust in calculations based on modern physics. Maybe less than now, but still quite a bit.

[jasonpatterson]

That depends purely on how you define 'noticeably less'. Actually you weigh slightly less when the moon is either directly overhead or below your feet (ie at high tide). You weigh the most when the moon is at right angles to the vertical (at low tide).


It would seem you're imagining a human tide, of a sort. (I'm not saying you believe such a thing, only that that is how your statement reads.) The changes in the relative positions of the moon-person system throughout a day doesn't have much of an effect on the strength of the force of gravity between them. It would tend to be farther when it was on the far side of Earth (below your feet) and closer when it was above your head, but that difference is small enough that the force would barely vary in magnitude between the two.

More imporant in this case is the direction of that force. In the case of the moon being directly overhead, its (tiny) force of attraction would pull you upward, decreasing your weight slightly. When it was directly below your feet, it would pull you downward, increasing your weight slightly. When the person-Earth-moon system formed a right angle, the moon's attraction would be pulling you sideways, it wouldn't alter your weight at all.

[Towlie]

When it (the moon) was directly below your feet, it would pull you downward, increasing your weight slightly.The tide goes through roughly two complete cycles per day, changing from one extreme to the other every six hours, and high tides correspond (roughly) to when the moon is overhead and then 12 hours later when it's below your feet. If you're on a ship in the ocean and the moon is directly below your feet, there's a high tide at your location. How then can you be experiencing extra weight when the water around you has risen?

[rjh01]

It would seem you're imagining a human tide, of a sort. (I'm not saying you believe such a thing, only that that is how your statement reads.) The changes in the relative positions of the moon-person system throughout a day doesn't have much of an effect on the strength of the force of gravity between them. It would tend to be farther when it was on the far side of Earth (below your feet) and closer when it was above your head, but that difference is small enough that the force would barely vary in magnitude between the two.

More imporant in this case is the direction of that force. In the case of the moon being directly overhead, its (tiny) force of attraction would pull you upward, decreasing your weight slightly. When it was directly below your feet, it would pull you downward, increasing your weight slightly. When the person-Earth-moon system formed a right angle, the moon's attraction would be pulling you sideways, it wouldn't alter your weight at all.

Actually no. Remember there are two high tides per day, not one.

When the moon is directly below there is a high tide, not a low tide as implied in your post, making tow high tides per day. Can you explain that? Once you have then you should understand my logic.

[Jimbo07]

Well, since we've been able to reliably predict everything except that one magic event with such concepts as mass and velocity, even after that one event, I would still put quite a bit of trust in calculations based on modern physics. Maybe less than now, but still quite a bit.

We might not be keen to throw out the entire idea of science, based on a single incident...

... but seriously...

...the existence of something like magic would call into question everything we knew. We wouldn't be able to reliably say that all of the previous successes of scientific inquiry weren't just consequence of, "having the wool pulled over our eyes." The same goes for other Matrix scenarios and the like.

In fact, science would be fine for discovering the internal rules that a simulation plays by, but if it were possible to bend or manipulate the rules, or even radically change the nature of the simulation itself, it would lead one to question the utility of scientific inquiry.

Fortunately, for getting out of bed in the morning, that doesn't seem to happen. :cool:

[Monketey Ghost]

Why? This didn't happen when the moon was much closer to the earth after all.

I'm imagining that increasing the size again would add quite a lot of mass, increasing the gravity gradient... depends on how much mass the Moon gains. Maybe someone who knows more than I could calculate the amount of mass the Moon would require to do what I speculated.

[stilicho]

Actually we are talking about tidal effects, not gravity effects of the moon. I believe the effect would be a lot bigger. Redo the calculations and work out what the difference in gravity of the moon is over a certain distance when measured from
a. The Earth
b. Half way to the moon.

I think the answer will be a lot more than what you expect.

I was hoping someone would do those calculations for me, actually.

But, regarding mere tidal effects, I had thought those were mostly under the sway of the sun and simply magnified or modified by the Moon. Does a 2x mass Moon really do that much?

I guess I'm imagining that a 2x mass Sun would have a much greater effect than a 2x mass Moon, given that the one is several magnitudes larger already.

---

ETA: Actually we are talking about tidal effects, not gravity effects of the moon.

Wait a second. Don't the Moon's gravitation effects operate as tidal effects?

Please be kind here. I am working with pencils and paper and a vague recollection of high school science.

[rjh01]

The Earth and the Moon orbit each other around a common point. Forget about the Moon going around the Earth. Just think about the part about the Earth going around the moon. If the Earth was a point there would be no tides. This point would still go around the moon once every month or so. The point of the Earth that is closest to the moon wants to orbit faster than the Earth. Solution is to try to get closer to the Moon. For a large body of water that produces a high tide.

Now go to the other side of the Earth. The amount of time for an object to go around the moon at that distance is less than what it is at the centre of the Earth. Solution is to go further away from the Moon. Again this produces a high tide.

I hope this answers your question in ETA.


When I do the calculations for tidal effects I get the 'wrong' answer. So now I am asking the question how does the tidal effects change as we get near a body?


Edit. Delete a part of the post.

[Darth Rotor]

The Earth and the Moon orbit each other around a common point.
Thanks to Towlie, Zig, and rj. I had my frame of reference wrong. ;)

DR

[rjh01]

Just done a search. Found this

Thus, the tidal force depends not on the strength of the gravitational field of the Moon, but on its gradient (which falls off approximately as the inverse cube of the distance to the originating gravitational body
Bolding mine.
Ref http://en.wikipedia.org/wiki/Tide

Now I know the 'right' answer, I need to work out how to get there.

[Dorian Gray]

Okay, here's a question for you: how come the moon's gravity can move billions of gallons of water such that it raises the water level by several feet, but doesn't cause animate objects to fly up in the air by even a millimeter?

[jasonpatterson]

The tide goes through roughly two complete cycles per day, changing from one extreme to the other every six hours, and high tides correspond (roughly) to when the moon is overhead and then 12 hours later when it's below your feet. If you're on a ship in the ocean and the moon is directly below your feet, there's a high tide at your location. How then can you be experiencing extra weight when the water around you has risen?


Actually no. Remember there are two high tides per day, not one.

When the moon is directly below there is a high tide, not a low tide as implied in your post, making tow high tides per day. Can you explain that? Once you have then you should understand my logic.

I am well aware that there are two tides in a day, and there is no implication that there is a low tide opposite the moon. You have to understand WHY there is a high tide opposite the moon to understand why your interpretations of my statement are in error.

Essentially, on one side of the planet the ocean is deeper because the moon is pulling it upward (decreasing the total gravitational force.) This part is easy to understand. The harder part is the far side of the planet. Because it is farther from the Moon, the force that the moon exerts on it is less than the near side and the water is not pulled downward as forcefully, thus there are two bulges.

This has almost nothing to do with the person example though. The reason that it works with an ocean is that the ocean covers most of the planet. There aren't tides to speak of even in a body of water as large as the Great Lakes because its parts are not sufficiently spread out compared to their distance to the moon.

A conceptual explanation for why my earlier statement was correct -
Suppose the system were arranged like so:
Me - Earth - Moon
The force between Me and Earth would be to the right. The force between Me and the Moon would be to the right. Those forces, when added, would reinforce one another, their sum would be larger than either.

Suppose the system were arranged like so:
Moon - Me - Earth
The force between Me and Earth would be to the right and of the same strength as before. The force between the Moon and Me would be to the left and slightly stronger than before. The sum of the two forces would be to the right and less in magnitude than the force between Earth and Me.

Mathematically (Newton's gravity):
The force of attraction between Earth and me -
Fme-Earth = (6.67 x 10-11 x 100 x 5.98 x 1024)/(6.38x106)2 = 980. N toward Earth's center

(This ignores Earth's rotation and variation in the diameter/composition of Earth that play a larger role in determining weight than the Moon's position. In any case, these factors would not change based on the Moon's position.)

The force of attraction between the Moon and me when I am on the near side of the planet.
Fme-Moon-near = (6.67 x 10-11 x 100 x 7.36 x 1022)/(3.79x108)2 = 0.0034 N away from Earth's center

The force of attraction between the Moon and me when I am on the far side of the planet.
Fme-Moon-far = (6.67 x 10-11 x 100 x 7.36 x 1022)/(3.91x108)2 = 0.0032 N toward Earth's center

My weight when I am on the near side (sig figs be darned.)
980 - 0.0034 = 979.997

My weight when I am on the far side (Sig figs? Sig figs? We don't need no stinking sig figs!)
980 + 0.0032 = 980.003

For a person, the tidal effects would be almost impossible to measure (Please note, this is NOT what I just did a quick calculation to show.) You'd basically be trying to figure out the difference in the force of gravity between your head and your feet due to the Moon. There is none to speak of. The only place tides of significant strength would exist for something so small would be a black hole, or perhaps very near the surface of a neutron star. (Yay for spaghettification!)

[Ziggurat]

Okay, here's a question for you: how come the moon's gravity can move billions of gallons of water such that it raises the water level by several feet, but doesn't cause animate objects to fly up in the air by even a millimeter?

If you leave a glass of water on a table, will it experience tides? No, it won't. Tides happen not only because of the moon, but also because of all the other water in the oceans. It takes an ocean of water being pulled on by the moon to raise water levels a few feet.

[Thabiguy]

Mathematically (Newton's gravity):
The force of attraction between Earth and me -
Fme-Earth = (6.67 x 10-11 x 100 x 5.98 x 1024)/(6.38x106)2 = 980. N toward Earth's center

(This ignores Earth's rotation and variation in the diameter/composition of Earth that play a larger role in determining weight than the Moon's position. In any case, these factors would not change based on the Moon's position.)

The force of attraction between the Moon and me when I am on the near side of the planet.
Fme-Moon-near = (6.67 x 10-11 x 100 x 7.36 x 1022)/(3.79x108)2 = 0.0034 N away from Earth's center

The force of attraction between the Moon and me when I am on the far side of the planet.
Fme-Moon-far = (6.67 x 10-11 x 100 x 7.36 x 1022)/(3.91x108)2 = 0.0032 N toward Earth's center

Up to this point, the calculation is correct.

My weight when I am on the near side (sig figs be darned.)
980 - 0.0034 = 979.997

My weight when I am on the far side (Sig figs? Sig figs? We don't need no stinking sig figs!)
980 + 0.0032 = 980.003

I'm afraid this is not correct. You calculated the force of attraction between two pairs of the three bodies, but neglected to calculate the force of attraction between the third pair - the Earth and the Moon.

Using your figures, Moon's gravitational attraction accelerates the Earth by
aEarth-Moon = (6.67 x 10-11 x 7.36 x 1022)/(3.85x108)2 = 0.000033 m x s-2 towards the Moon.

Therefore, your weight when you're on the near side is
100 x (9.8 - 0.000034 + 0.000033) = 979.9999 N

and your weight when you're on the far side is
100 x (9.8 + 0.000032 - 0.000033) = 979.9999 N

You weigh less both when the Moon is above your head and below your feet.

It's important to keep in mind that one's weight on Earth is the force between oneself and the Earth - not with respect to some arbitrary "absolute" reference.

To further illustrate, the Moon's gravitational attraction is almost insignificant in this regard: the force of attraction between a person and the Sun is more than a hundred times greater than that between the person and the Moon, and yet we don't see daily oscillation of 100 kg mass' weight by 0.6 N - because what matters is the difference in Sun-caused acceleration of the person and the Earth, and that is even smaller than in case of the Moon.

For a person, the tidal effects would be almost impossible to measure (Please note, this is NOT what I just did a quick calculation to show.) You'd basically be trying to figure out the difference in the force of gravity between your head and your feet due to the Moon.

This is something different still, and has nothing to do with one's weight. Just to clarify: the tides are not caused by the difference of seawater weight between the surface and the bottom. They happen because water in oceans is (more or less) free to flow from places where it weighs more (where the Moon is on the horizon) to places where it weighs less (where the Moon is overhead or underfeet). In smaller water bodies, water weighs about the same all across (even though that weight changes a little during the day), so it has little reason to flow anywhere.

[jasonpatterson]

Poop. I was imagining myself floating in space, essentially, rather than standing on the planet's surface. Conceded on the varied weight question.

I know that tides aren't caused by the difference between the force of gravity between the top and bottom of the ocean, they are caused by a difference in the force of gravity (from the Moon and Sun) over the surface of the planet. However, for a person to experience a tidal force, that is, a difference in the force between his head and feet, those two positions would have to be sufficiently spread apart. What you wrote about tides in the last paragraph is basically what I had already written, including the portion about lakes and oceans.

[Monketey Ghost]

Okay, here's a question for you: how come the moon's gravity can move billions of gallons of water such that it raises the water level by several feet, but doesn't cause animate objects to fly up in the air by even a millimeter?

Simplest answer is that the earth's gravity is having a much greater effect. Tides have a strong effect on the matter nearest to them( ocean), and a much weaker effect on what's farther away (ocean on other side of large planet)

Tides are the difference in gravity over distance.

[Thabiguy]

Okay, here's a question for you: how come the moon's gravity can move billions of gallons of water such that it raises the water level by several feet, but doesn't cause animate objects to fly up in the air by even a millimeter?

Imagine a wide container half-filled with water. By tilting the container, you can easily make the water depth increase or decrease by the edge of the container, but it's not going to cause anything resting on the bottom of an empty container to fly up in the air.

The effect of Moon's gravity is somewhat similar to very slight tilting over very large distances (thousands of miles). Using that inexact analogy, it's rather obvious how it can cause water level in a sufficiently wide container (like the ocean) to raise at one point and fall at another, yet cannot make anything fly up in the air.

[Roboramma]

We might not be keen to throw out the entire idea of science, based on a single incident...

... but seriously...

...the existence of something like magic would call into question everything we knew. We wouldn't be able to reliably say that all of the previous successes of scientific inquiry weren't just consequence of, "having the wool pulled over our eyes." The same goes for other Matrix scenarios and the like.

In fact, science would be fine for discovering the internal rules that a simulation plays by, but if it were possible to bend or manipulate the rules, or even radically change the nature of the simulation itself, it would lead one to question the utility of scientific inquiry.

Fortunately, for getting out of bed in the morning, that doesn't seem to happen. :cool:

Well, it seems to me that we would know that something else was going on that what we had already figured out, but that doesn't mean that the things we've already figured out don't in general apply.

It would certainly be something to start asking big questions about, and might open up a whole new field of science, and might also have philosophical implications regarding what our current theories mean. But it wouldn't destroy their usefulness: we'd still know that we landed men on the moon with newtonian physics, for instance. And I don't think there'd be any reason to cancel the next launch.

Now, if this sort of incident started happening more and more frequently... well, then we might have to start to worry. :P

[stilicho]

Okay, here's a question for you: how come the moon's gravity can move billions of gallons of water such that it raises the water level by several feet, but doesn't cause animate objects to fly up in the air by even a millimeter?

But it would have an effect on our mass too. What would happen if all humans coalesced into a single body in, say, Central Park in New York City?

[stilicho]

...cannot make anything fly up in the air.

I wasn't suggesting that, either, by my last question.

But would certain atmospheric conditions on earth be considerably altered if we all coalesced as a wriggling mass of flesh in the middle of Manhattan? Given another equation, could that affect the wobble of the Earth?

[rjh01]

When I was in school one of the things our science teacher told us was that if he had a matchbox full of neutrons (as in a neutron star) it would alter the rotation of the earth. This would happen because it would alter the centre of gravity of the earth.

The suggestions above would not work very well as they do not have much mass compared to the mass of the earth.

The moon can influence the water in the oceans not just because of its gravity but the difference in gravity between the centre of the earth and the surface of the earth. If you want to fly off the earth one thing you can do is increase the size of the earth until Earth's gravity equals the moons gravity. That would be huge earth. You are not allowed to increase the mass of the earth either.

[Thabiguy]

But would certain atmospheric conditions on earth be considerably altered if we all coalesced as a wriggling mass of flesh in the middle of Manhattan? Given another equation, could that affect the wobble of the Earth?

Absolutely not. The total mass of all humans is far too tiny for any of that to happen.

Even if we gratuitously overestimate the average mass of a human at 100 kg and their number at 7 billion, the total mass would be 700 million tons. That's only about the mass of a small mountain. The same mass of water flows through the Hudson river every two weeks.

Humans commonly create bigger accumulations of mass; for example, there are many dams in the world that hold greater mass of water than that.

Concentrating all people at one place could disrupt the local ecosystem and possibly have a mild effect on local weather, but there would be no global effect at all.

[Corsair 115]

Absolutely not. The total mass of all humans is far too tiny for any of that to happen.

Even if we gratuitously overestimate the average mass of a human at 100 kg and their number at 7 billion, the total mass would be 700 million tons. That's only about the mass of a small mountain. The same mass of water flows through the Hudson river every two weeks.

Humans commonly create bigger accumulations of mass; for example, there are many dams in the world that hold greater mass of water than that.

Concentrating all people at one place could disrupt the local ecosystem and possibly have a mild effect on local weather, but there would be no global effect at all.


Hrrrmphhh!

*throws down prototype of human teleporter-compactor device*

So much for that idea to destroy the Earth!

*goes back to drawing board*