Author Topic: Episode #688  (Read 6161 times)

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Offline Bytor

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Re: Episode #688
« Reply #30 on: September 18, 2018, 08:41:14 PM »
Light Sails

I wonder if any of the rogues I've ever read any Robert L. Forward? He was a hard SF Sutter with incredibly detailed explanations of the technology in his books (and he was also a physics professor). One of his books "Flight of the Dragonfly" is about a human mission to Barnards Star on a laser-propelled light sail vehicle.

Offline CarbShark

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Re: Episode #688
« Reply #31 on: September 18, 2018, 08:57:40 PM »

Ceres was originally called a planet when it was discovered.  One of the reasons why it was downgraded was when other bodies were discovered at similar distances from the Sun, and if they were were called planets too, then the numbers of planets would soon assume very large numbers - so defining planets in such a way as to avoid unmanageable numbers made sense.

The same happened with Pluto.


This is astronomy. They deal with large numbers all the time. If they can manage 100+ Messier objects they can handle a couple dozen planets. (Not to mention the number of galaxies and stars)



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Having a clear definition of a planet is important - it’s necessary for it to be large enough to be spherical. 

And directly orbits the sun (is not a satellite)

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There should be a definition of planetary moons that excludes every tiny chunk of ice or rock, regardless of whether it’s metres or kilometres in maximum size, otherwise the number of moons will expand enormously.  Is there any point in listing (and naming) 79 moons of Jupiter, most of which are tiny?

In the taxonomy of Astronomy I don't think there's a separate criteria for moon and satellite. Moon is an informal way of saying satellite.

The Moon is the name of Earth's satellite. Whether or not there should be a different definition for satellites that are massive enough to be spherical is a separate issue.

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You don’t seem to appreciate the situation with the Moon.  Currently the Moon is a moon.  But in a few billion years, it will become a planet.  If we every discover an almost identical twin of the Earth-Moon system orbiting another star, it doesn’t make much sense to call the Moon-twin a moon if it’s orbiting close to its planet and a planet if it’s orbiting a little further out.

Well, first, the fact that it has been receding does not mean it will forever. It may reach an equilibrium between its orbital speed and average distance.

Second, if it does continue to recede, until the time when it is no longer orbiting earth it will be a satellite. When it's not orbiting earth (and is directly orbiting the sun) it will be a planet.

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And what would you call the moons if they were kicked out of their planetary orbits to orbit the Sun independently? 
A planet, if they meet the criteria of a planet. An asteroid if they don't.
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The Moon would be called a planet.  Many of the current Jovian moons, if it happened to them, would be called comets or asteroids.  The same with Phobos or Deimos.

None would be called comets. They are not comets. If they were directly orbiting the sun they would all be planets or asteroids.
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Offline The Latinist

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Re: Episode #688
« Reply #32 on: September 18, 2018, 09:23:27 PM »
Here’s the thing, though: it’s not really ever accurate to say that one object orbits another; always the two objects orbit the center of mass of their combined system (called its barycenter).  Depending on their respective masses, this can fall almost anywhere between the centers of the two objects.  For particularly lopsided relationships, that barycenter may lie inside one of the objects; but it’s a bit arbitrary to make that point the be-all-end-all of the definition of a moon. Indeed; it’s not really accurate to say that the Earth currently orbits the sun while the moon orbits the Earth; more accurately, they both orbit their barycenter and that barycenter orbits the sun.

So it’s not really a question of the Moon someday ceasing to orbit the Earth and beginning to orbit the sun.  Both the Earth and Moon will continue to orbit their barycenter, and that barycenter will drift toward the surface of the earth until it eventually no longer lies inside the Earth.  For a time it will be right at the surface, then it will move off into the atmosphere and eventually into space.  But both will always still orbit their barycenter, and that barycenter will continue to orbit the sun.
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Offline arthwollipot

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Re: Episode #688
« Reply #33 on: September 18, 2018, 09:42:57 PM »
more accurately, they both orbit their barycenter and that barycenter orbits the sun.

Even more accurately, that barycenter and the sun have a barycenter that both orbit.

Gravity is complicated.
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Offline CarbShark

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Episode #688
« Reply #34 on: September 18, 2018, 09:52:45 PM »
Add to that the  orbits are elliptical and in some cases can be influenced by other satellites and planets.

It is entirely possible for the moon to stop orbiting earth.

AFAIK, Pluto and Charon are the only case in the solar system where it’s not perfectly clear who’s orbiting whom, but even then the consensus is that Charon is a satellite.




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Offline arthwollipot

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Re: Episode #688
« Reply #35 on: September 18, 2018, 10:38:17 PM »
Add to that the  orbits are elliptical and in some cases can be influenced by other satellites and planets.

If by "in some cases" you mean "always", then yes.

It is entirely possible for the moon to stop orbiting earth.

What?

AFAIK, Pluto and Charon are the only case in the solar system where it’s not perfectly clear who’s orbiting whom, but even then the consensus is that Charon is a satellite.

While this may be the case now, there is a probability that we will find other cases.
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Online gebobs

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Re: Episode #688
« Reply #36 on: September 19, 2018, 11:28:23 AM »
How about make the criteria for a moon be if the barycenter lies outside a certain percentage of the distance between the centers of two objects. The Moon-Earth barycenter is about 98% of the distance between them. For Charon-Pluto, it's about 90%.

Offline The Latinist

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Re: Episode #688
« Reply #37 on: September 19, 2018, 01:24:40 PM »
It is entirely possible for the moon to stop orbiting earth.

I'd love to hear you explain this further.
I would like to propose...that...it is undesirable to believe in a proposition when there is no ground whatever for supposing it true. — Bertrand Russell

Offline CarbShark

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Re: Episode #688
« Reply #38 on: September 19, 2018, 01:29:16 PM »
Add to that the  orbits are elliptical and in some cases can be influenced by other satellites and planets.

If by "in some cases" you mean "always", then yes.

Well, yes, but in some cases influenced enough to dynamically change the orbits.
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It is entirely possible for the moon to stop orbiting earth.

What?

If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.


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AFAIK, Pluto and Charon are the only case in the solar system where it’s not perfectly clear who’s orbiting whom, but even then the consensus is that Charon is a satellite.

While this may be the case now, there is a probability that we will find other cases.

Sure, and if and when that happens we (astronomers) would make a determination as to which, if either, would be considered a satellite.

But if they're roughly equal in size then I imagine that they would both be considered planets.

Again, how is that a problem for science?

It's only what Metzger considers "Cultural" considerations that make people uncomfortable with, say, dozens of planets, and are the motive for coming up with definitions that limit the number.

There's no scientific reason to limit the number of planets.

(I was neutral on this question until I found Metzger's blog and the full text of the paper. At this point I agree with Metzger.)
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Offline CarbShark

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Re: Episode #688
« Reply #39 on: September 19, 2018, 01:32:10 PM »
How about make the criteria for a moon be if the barycenter lies outside a certain percentage of the distance between the centers of two objects. 

Why?

What's the scientific reason and purpose for introducing this arbitrary criteria?
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Online 2397

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Re: Episode #688
« Reply #40 on: September 19, 2018, 01:46:33 PM »
Listening to the planet segment, I agree with Bob for the most part. Because he brings up the things I was thinking of. Such as doesn't a planet's ability to gravitate itself into a sphere depend on what it's made of?

In a way, Titan and Ganymede are more like planets than Saturn and Jupiter are, if planets are something broadly similar to Earth. I was fine with Pluto not being a planet, it doesn't seem to fit among the rest, although it could be argued it's on the other extreme where Earth is somewhere in the middle.

A definition of planets that includes Charon and Jupiter, but not the largest moons, there is something about that that doesn't seem right to me.

Offline The Latinist

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Re: Episode #688
« Reply #41 on: September 19, 2018, 01:59:28 PM »
Here's another wrinkle: the barycenter of the Sun-Jupiter system is actually outside the radius of the sun.  Should we call Jupiter something other than a planet?

If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.

The sun has already 'captured' the moon in the sense that it and the earth both orbit the sun.  The question is whether the moon can cease to orbit the earth, which is entirely a function of its orbital distance and its velocity relative to earth, and I'm pretty sure (though it's been a long time since I read about it in detail) that the earth and moon will become doubly-tidally-locked before the moon reaches escape velocity for its orbital distance from the earth.  At that point the moon will cease to recede, and we will be stuck together.

So, no, it's not possible, barring some sort of solar-system transforming collision with another star, for the moon to stop orbiting the Earth.
I would like to propose...that...it is undesirable to believe in a proposition when there is no ground whatever for supposing it true. — Bertrand Russell

Offline CarbShark

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Re: Episode #688
« Reply #42 on: September 19, 2018, 02:39:48 PM »
Here's another wrinkle: the barycenter of the Sun-Jupiter system is actually outside the radius of the sun.  Should we call Jupiter something other than a planet?

If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.

The sun has already 'captured' the moon in the sense that it and the earth both orbit the sun.  The question is whether the moon can cease to orbit the earth, which is entirely a function of its orbital distance and its velocity relative to earth, and I'm pretty sure (though it's been a long time since I read about it in detail) that the earth and moon will become doubly-tidally-locked before the moon reaches escape velocity for its orbital distance from the earth.  At that point the moon will cease to recede, and we will be stuck together.

So, no, it's not possible, barring some sort of solar-system transforming collision with another star, for the moon to stop orbiting the Earth.

Capturing the Moon would entail having a stronger pull on it directly than the earth does. This could have the effect of preventing it's return from apogee or increasing its speed significantly on approach to perigee. Either could send it on a different trajectory. If this happened the most likely trajectory would be new orbit around the sun. Other trajectories could slingshot it out of the solar system; put it into orbit around venus, say, or even crash it into the sun or the earth.

To say it's "not possible" is false, but I agree it's not likely.
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Offline The Latinist

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Re: Episode #688
« Reply #43 on: September 19, 2018, 02:59:45 PM »
Dude, the sun already has significantly
Here's another wrinkle: the barycenter of the Sun-Jupiter system is actually outside the radius of the sun.  Should we call Jupiter something other than a planet?

If the Moon keeps receding eventually it will be far enough away at its apogee that the gravitational attraction to Earth would be less than the pull of the Sun, or Venus, or even Mars or Jupiter. If that were to happen the Moon could be captured by the Sun, another planet or could even be expelled from the solar system.

The sun has already 'captured' the moon in the sense that it and the earth both orbit the sun.  The question is whether the moon can cease to orbit the earth, which is entirely a function of its orbital distance and its velocity relative to earth, and I'm pretty sure (though it's been a long time since I read about it in detail) that the earth and moon will become doubly-tidally-locked before the moon reaches escape velocity for its orbital distance from the earth.  At that point the moon will cease to recede, and we will be stuck together.

So, no, it's not possible, barring some sort of solar-system transforming collision with another star, for the moon to stop orbiting the Earth.

Capturing the Moon would entail having a stronger pull on it directly than the earth does. This could have the effect of preventing it's return from apogee or increasing its speed significantly on approach to perigee. Either could send it on a different trajectory. If this happened the most likely trajectory would be new orbit around the sun. Other trajectories could slingshot it out of the solar system; put it into orbit around venus, say, or even crash it into the sun or the earth.

To say it's "not possible" is false, but I agree it's not likely.

That’s not how it works.  The earth and the moon are gravitationally bound.  As long as that is true, the pull of the sun acts as though it is pulling on the barycenter of the earth-moon system, and any changes in the effects of the sun are felt by the system; they cannot be felt more by the moon than by the earth—the two will accelerate or decelerate as a pair.  The sun can’t just reach out and touch only the moon.

The only way for that to change is for the moon to achieve escape velocity, and that’s not something the sun can impart to it.  It just isn’t.

Incidentally, the sun’s pull on the moon already exceeds that of the earth by more than double.  But that’s irrelevant to it pulling the two apart because *all* that matters is the relative velocities ofbthe two bodies, and there’s no way for the sun to differentially accelerate the moon as long as it’s gravitationally bound to the earth.  And the recession of of the moon due to tidal forces is just not adequate for it to achieve escape velocity before tidal locking.  Barring, as I said, a colission or something else from *outside* the earth-moon-sun system that can impart new velocity to the moon, it simply cannot escape the earth.
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Offline tralfaz

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Re: Episode #688
« Reply #44 on: September 19, 2018, 03:30:59 PM »
Beets.  Oxalic acid.  I guess I'm the only one here who prepares beets and the beet greens.