What if the moon disappeared?

[steve s]

Don't laugh. I was at the beach two weeks ago, walking on the beach on a moonless night with my wife. She remarked on how the sky was FILLED with stars.. and.. why is the moon not out? WHERE IS THE MOON, HONEY?! ITS NOWHERE - NOT EVEN A LITTLE SLIVER!

OK, go ahead and laugh - I sure did.

A friend once asked me why, when the moon is in it's new phase, we don't see a dark disk in the night sky blocking out the stars. It took me half an hour to get him to understand that the moon is often up during the day. He just couldn't wrap his mind around it. Although this is the same guy who couldn't understand that a point on the tip of a fan blade is traveling faster than a point near the hub.

Steve S.

 

[SezMe]

And I notice that none of their talking heads seem to have doctorates.

Actually, not "none". One of the very brief talking heads is Alex Filippenko who likes to brag that he is the most cited astronomer. Actually, he is.

 

[rjh01]

<snip>

Secondly, the earth would loose it's precessional axis. Right now the axis presesses and nutates, but within small bounds. Without the moon to stabilize the oblate bulge, the axis could easily move and point in any direction (over, say, 100,000 years time).
<snip>

Do not think this can be true. The Earth is the ONLY planet that has a moon that is anywhere near the size of the planet. Yet with only one exception all the planets have an axis that is roughly at 90 degrees to the orbit.

If the axis could point in any direction then you would expect every planets' axis to point in random directions.

The one exception I mentioned above they believe was due to a massive object hitting it when the solar system was young and had many massive objects.

Pluto is not a planet.

 

[nathan]

It took me half an hour to get him to understand that the moon is often up during the day.

I remember the first time I noticed the moon during the day (it was a looong time ago). The simple story children are told that the moon comes out at night was a lie! A lie I tells ya!

 

[shadron]

Actually, not "none". One of the very brief talking heads is Alex Filippenko who likes to brag that he is the most cited astronomer. Actually, he is.

Yes, and I see that the lady from Griffith Observatory, Laura Danly, is indeed a doctorate. I guess it has become frowned upon to use their title on the screen; probably some idea on combating perceived elitism from those know-it-all scientists. Of course, that doesn't make them fool proof, and she isn't the one making the "rush to solar" statement. So.

Watched the rest of it today. There are a few gotcha's but they are mostly slips: Williiam K. Hartmann of the Planetary Science Institute speaks of how we would have eight hour days: "We would have four hours of day, four hours of night, and the nights would be totally black." Uhhhhh, no. They'd be no blacker than nights are today when the moon is below the horizon.

One of the heads says the reason that Mars is dessicated is because of its variable axis tilt, but I know of no argument in that direction. I have heard cogent arguments for it being because of either a asteroid collision or because of Mars core turning solid, but I can't see why variable tilt would do that.

They hypothesize that Jupiter pulled Theia out of an Earth Lagrangian point into collision with the earth to create the moon. The theory mentioned in wikipedia, for example, doesn't require Jupiter; it points out that Lagrangian point stability depends on the trojan being much less massive than it's primary, and it becomes unglued if the mess ratio gets to high, as it would for two accreting masses.

Then they do a bunch of speculation about what animals on Earth would be like. Sort of funny.

All in all, I believe that having the moon did make intelligent life possible on Earth, and that it is unlikely to have developed without it. But this video does a really poor problem making that case.

 

[SezMe]

<snip> and it becomes unglued if the mess ratio gets to high, <snip>

Absolutely delightful typo.

 

[shadron]

Do not think this can be true. The Earth is the ONLY planet that has a moon that is anywhere near the size of the planet. Yet with only one exception all the planets have an axis that is roughly at 90 degrees to the orbit.

If the axis could point in any direction then you would expect every planets' axis to point in random directions.

The one exception I mentioned above they believe was due to a massive object hitting it when the solar system was young and had many massive objects.

Pluto is not a planet.

From wiki:

The Earth's axial tilt varies between 22.1° and 24.5°, with a 42,000 year period, and at present, the tilt is decreasing. In addition to this steady decrease, there are also much smaller short term (18.6 years) variations, that is also affected by Sun's gravitation in its depleting angle relative to Earth's, known as nutation.

So, even for Earth, it is not fixed. It is, in fact. one of the famous Milankovitch cycles^WP, the others being periodic changes in the Earth's orbit's ellipticity, two precession figures and the orbital inclination. These effects, all cyclic and all running on different periods, and all having effects upon the amount of solar radiation received by the earth, all occasionally believed to declare a "hot/cold Earth day" and act as a trigger to start the Earth into a climate change.

So the Earth's tilt changes even with the moon's stabilizing influence. As for Mars, see http://www.universetoday.com/guide-to-space/mars/mars-tilt/:

Astronomers know that the current tilt of Mars' axis is just a fluke. Unlike Earth, the planet's tilt has changed dramatically over long periods of time. In fact, astronomers think that the wobble in the tilt might help explain why vast underground reservoirs of water ice have been found at mid-latitudes, and not just around the planet's poles.

The wiki article on Axial Tilt^WP says this:

Other planets may have a variable obliquity too, for example on Mars the range is believed to be between 11° and 49°, as a result of gravitational perturbations from other planets[3]. The relatively small range for the Earth is due to the stabilizing influence of the Moon, but it will not remain so. According to Ward, the orbit of the Moon (which is continuously increasing due to tidal effects) will have gone from the current 60 to approximately 66.5 Earth radii in about 1.5 billion years. Once this occurs, a resonance from planetary effects will follow, causing swings of the obliquity between 22° and 38°. Further, in approximately 2 billion years, when the Moon reaches a distance of 68 Earth radii, another resonance will cause even greater oscillations, between 27° and 60°. This would have extreme effects on climate. Tentative evidence has recently emerged for extreme (> 50°) variations in terrestrial axial tilt.[4]

 

[shadron]

Absolutely delightful typo.

Yes, and appropriate for my current place in life as well.

 

[rjh01]

<snip>



The wiki article on Axial tilt^WP says this:

Interesting link. Your link did not work, but I fixed it.

 

[G O R T]

Why has no one mentioned that the disappearance of the Moon would change the Earths orbital radius?

The Earth-Moon system currently circles the Earth-Moon barycenter. Disappearance of the Moon would impart the offset part of the Earth's mass moment (1.23%) at the time of disappearance into the orbital velocity. The Earth would speed up or slow down a bit depending upon the vector angle.

Anyone feel like calculating?

 

[MRC_Hans]

I think this sort of question is meaningless without any understanding of the process by which the moon suddenly ceases to exist. If it just is there one moment and isn't the next, then the law of conservation of mass-energy has gone with it, and even 50' tsunamis on all coasts is pretty small beer compared to the fact that we can never again make any valid predictions whatsoever about any feature of the physical universe. If it's blown to small fragments, then the mass is still there, and there will be some transition between the initial state, of a single compact mass, and the final state, of a whole load of small rocks spread out to a radius greater than the moon's current orbit, which in themselves could be as interesting to study as the nature of the final state. If it's pulled away intact, how fast does it move? It's no longer a sudden change, so there's time for the system to react. It's impossible to make predictions until you've got some idea of the process involved.

Dave

Thouroughly agreed. If the moon suddenly, magically disappeared, i assume Earth would drop nearly half its diameter to the former common gravity centre, and no doubt chaos would ensue.

However, the scenario is rather uninteresting, since we have no reason to think it is possible.

Hans

 

[shadron]

The whole thing is a flight of fancy, and just gives The Universe's writers an excuse to talk about the Theia hypothesis and some other odds and ends, and add to current time paranoia, thereby selling more of their own and other commercial products.

 

[Dr. Trintignant]

That is incorrect. First, the core is solid. Second, the earth's core rotates slightly faster than the crust. The moon's gravity is acting to slow down the crust (through tides), and hence is putting a small frictional drag on the core.

Hmm. But couldn't that be the reason why the core is spinning faster than the crust? In other words, perhaps the crust was spinning as fast as the core in the distant past, but the tides have slowed the crust since then, and the core lags behind since it does not have a solid coupling with the crust. Thus, today it spins more quickly, even if it is still slowing down due to friction.

- Dr. Trintignant

 

[Ziggurat]

Hmm. But couldn't that be the reason why the core is spinning faster than the crust? In other words, perhaps the crust was spinning as fast as the core in the distant past, but the tides have slowed the crust since then, and the core lags behind since it does not have a solid coupling with the crust. Thus, today it spins more quickly, even if it is still slowing down due to friction.

That's exactly what is happening, which was my point: the moon is slowing the rotation of both the crust and the core, but the former directly and the latter indirectly.

 

[Dr. Trintignant]

That's exactly what is happening, which was my point: the moon is slowing the rotation of both the crust and the core, but the former directly and the latter indirectly.

Right, but what I'm saying is that there would be no differential at all without that. It would just reach a steady-state eventually without the tidal forces. So what madurobob said...:

that the moon's gravity helps keep the Earth's molten iron core spinning.

...could be taken as true, if one is very charitable (ignoring the molten bit, of course). Yes, the tides slow down the core, but they slow down the crust first, and thus we could say they are responsible for the net spin.

- Dr. Trintignant