## 0681: "Gravity Wells"

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Vnend
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### Re: "Gravity Wells" Discussion

Win.

Poster? Even bigger win.

Ubik
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### Re: "Gravity Wells" Discussion

jrock91 wrote:It's basically impossible to have a diagram that shows planetary sizes and interplanetary distances to the same scale. If you scale the Earth to one cm diameter, the Sun-Earth distance is roughly 10 m, and the Sun-Neptune distance is hundreds of meters.

Or did you mean a diagram that shows interplanetary distances to scale but does NOT show planetary sizes?
I was suspecting that a diagram, without any deformation to make the interplanetary distances smaller to exaggerate the planets themselves, would mostly contain a large curve representing gravity well of Sun and some dents on it caused by larger planets, and possibly some really hard to see dents caused by smaller planets. It would probably be mostly interesting to see it to get some idea of the scale even if it meant that rocky planets are hard to see in the picture.

gravityhomer
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### Re: "Gravity Wells" Discussion

I have a question on the peaks between the planets.

These peaks vary in height on the poster from jupiter to the left (ie, it is easier to go to venus, than return). But they are all at the same height from jupiter to the right. It seems like this variation is due to the sun's gravity well. Is the sun's gravity well really relatively flat on the right half of the comic (from jupiter to saturn to urnaus to neptune), incomparison to the the left?

The force of sun's gravity goes down with 1/ (r^2) so I guess it could be pretty flat out past jupiter? I'll believe it, but I was wondering if it was drawn this way just to fit it in a good size comic. because if saturn, urnanus and netpune kept going up to the right, there would be a lot more white space in a rectangular comic.

oh and by the way, I think it is an awesome comic. I love these large blow up ones.
Last edited by gravityhomer on Mon Dec 28, 2009 4:01 pm UTC, edited 1 time in total.

Fluid_Dynamic
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### Re: "Gravity Wells" Discussion

oz1cz wrote:
freespace wrote:Earth's gravity well depth has 3 different figures: 5478, 6000, 6379km. The last one should be the correct one.

That's what my calculations say as well. The 6000 is obviously an approximation, but why 5478? Is it a simple error, or is there something more subtle behind it?

--
Claus

It looks to be just an error, specifically the number for [url=http://www.google.com/search?hl=en&safe=off&q=G*mass+of+venus%2F%28radius+of+venus*9.81m%2Fs^2%29]Venus[/url]

Semiraghe
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### Re: "Gravity Wells" Discussion

oz1cz wrote:
freespace wrote:Earth's gravity well depth has 3 different figures: 5478, 6000, 6379km. The last one should be the correct one.

That's what my calculations say as well. The 6000 is obviously an approximation, but why 5478? Is it a simple error, or is there something more subtle behind it?

--
Claus

Having not done the calculations myself I can't be positive but I believe that 5478 comes from the affect you get from the earth's rotational velocity. Essentially it’s like getting a head start if you launch in the same direction as the rotation of the planet.

Ragnarokio
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### Re: "Gravity Wells" Discussion

am i reading this wrong, or are you still in mars's gravity well while on phobos's or deimos's gravity wells?

Schumi
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### Re: "Gravity Wells" Discussion

Ragnarokio wrote:am i reading this wrong, or are you still in mars's gravity well while on phobos's or deimos's gravity wells?

Yes. They wouldn't be in orbit if they weren't.

toomuchtodo
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### Re: "Gravity Wells" Discussion

Any chance we could see this in the Store as a poster? I want this framed on my office wall =)

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### Re: "Gravity Wells" Discussion

I was disappointed there wasn't a Uranus joke.

thicknavyrain
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### Re: "Gravity Wells" Discussion

Evadman wrote:I was disappointed there wasn't a Uranus joke.

Hey Uranus is big enough without needing to draw more attention to it...
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Princess Marzipan
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### Re: "Gravity Wells" Discussion

axilog14 wrote:
sje46 wrote:
Ferahgo wrote:
sje46 wrote:The things making noise on Titan...is that a Vonnegut reference?

That was my thought also. If it is, then well done, as that's one of my favorite books ever.

One of mine as well. Amazing science-fiction. This is what I'm talking about, if anyone's wondering: http://en.wikipedia.org/wiki/Sirens_of_titan

The little weeoooeeoooeeeooo was my favoritest part of the comic, and I've never even heard of that book. Now I want to read it! Thanks Randall!

I thought that was a Kung Pow: Enter the Fist reference.
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Ragnarokio
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### Re: "Gravity Wells" Discussion

Schumi wrote:
Ragnarokio wrote:am i reading this wrong, or are you still in mars's gravity well while on phobos's or deimos's gravity wells?

Yes. They wouldn't be in orbit if they weren't.

well obviously they aren't actually in it, that just wouldn't make sense, i was more asking if the chart <i>implied</i> that they were in mars's gravity well.

santy22
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### Re: "Gravity Wells" Discussion

The next poster print, anyone?
Also "A thrown baseball could escape Deimos."
That's the way to learn about gravity! Screw mr. Lazaro!

MrMiyamoto
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### Re: "Gravity Wells" Discussion

I also think that Pluto is more than cool enough for inclusion. Ha, 44 Kelvin. Anyways, don't be a Pluto hater.

The Old Wolf
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### Re: "Gravity Wells" Discussion

Could someone explain why my "There are NINE planets" post was deleted? Did it contravene a rule about images or some such?
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SirMustapha
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### Re: "Gravity Wells" Discussion

sdague wrote:My only question: Can we get this one as a poster?

I believe that question was already answered even before Randall did the first sketches.

Beacons!
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### Re: "Gravity Wells" Discussion

toomuchtodo wrote:Any chance we could see this in the Store as a poster? I want this framed on my office wall =)

I wouldn't buy a poster with an incorrect quote on it.
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Coorsmackio
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### Re: "Gravity Wells" Discussion

So where does the line to buy this as a poster start?
Witty comment pending...

rollo
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### Re: "Gravity Wells" Discussion

Okay, I love this, I really do, but YOU FORGOT PLUTO (forum readers: ..."but wait I must explain!" ). No matter how many people claim that its not a planet, I refuse to accept this denial. It just makes sense, 9 planets, mathematically and aesthetically is logical. The problem is a combination of bureaucratic and artistically lacking scientist. Pluto was already considered a planet, it was awesome, had a dog named after it, god of the underworld, etc... It also orbits closer than Neptune every so often. 9 planets has a square root of 3, and can be broken into perfect symmetry, plus, adding the sun makes 10, the ideal multiple. I could probably go on all day with this, but please, remember Pluto, it feels rejected

But awesome diagram, one of my favorites (I do support a request for a reprint with pluto, and a Uranus joke). Also the comment, I believe that is somewhat unnecessary (correct me otherwise), in the fact that orbital motion need not be added simply, do to the fact that it is a separate factor from the initial gravity, in other words, another variable that would instead add to the propulsion of the object attempting to reach escape velocity, as motion is specific to the object rather than the mass of the planet.

TheNextCaesar
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### Re: "Gravity Wells" Discussion

atimholt wrote:
The Old Wolf wrote:Picard: There are 9 planets!

black chupaqueso!

Anyways, I hate to be the inevitable guy asking for the explanation to the joke, but whats the 'an even more glorious dawn awaits' thing? Apparently something to do with those Carl Sagan autotune videos. I guess I could just watch them, but... um... its 4 in the morning, and... um... how many are there? More than one? One?
Fine. Don't answer my question (unless you want to ). I'll scrounge Youtube when the time of day is more sane. Maybe. I might forget to, it's not like it's that important, or even vital to this comic, or to the grand scheme of life. Really when we get right down to it, does any of this matter? Or are we just dust blowing in the wind?
It's 4 in the morning.

Somebody get the super soaker.
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ZerothRoot
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### Re: "Gravity Wells" Discussion

rollo wrote:10, the ideal multiple.

Oh you.

But yeah, cool comic.

snow93
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### Re: "Gravity Wells" Discussion

rollo wrote:Okay, I love this, I really do, but YOU FORGOT PLUTO (forum readers: ..."but wait I must explain!" ). No matter how many people claim that its not a planet, I refuse to accept this denial. It just makes sense, 9 planets, mathematically and aesthetically is logical.

Hmmm hmm hmmm what is it with these Pluto nutters.

On another note, this is of course another comic that Randall has put up in order to make infinite monies from selling it as a poster -- a brilliant ploy, because it *would* make a wonderful poster. It's just the version on the website feels a little like a first draft, what with slightly incorrect quotes, not all heights filled in, not quite every centimetre filled with jokes/other facts of interest.
On the other hand, I do fully support more and more issues of XKCD being epic poster-style thingummys.

Popliteal
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### Re: "Gravity Wells" Discussion

Both neat and informative, Kudos!

XbHW_TestEngr
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### Re: "Gravity Wells" Discussion

Awesome work there Randall.
I wonder how many professors are going to use this in their lectures. (They had better give you credit.)

I know my dad (EE-PhD), my son (working on BSEE) and I will have a fun time with the chart.
My daughter (BA-Music) will just sit in the corner an pout.
... and there will be cake.

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benbald72 wrote:I feel connected to the author and therefore appreciate the comic, regardless of whether or not I understand the joke ....

corkiethedog
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### Re: "Gravity Wells" Discussion

Great strip!
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westrim
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### Re: "Gravity Wells" Discussion

skeptical scientist wrote:
westrim wrote:Someone needs to hurry up and integrate the threads.

Integrating the threads is not in the moderators' job descriptions, so far as I know, but replying only to the first thread posted is in yours.

I only said "someone" (as in someone with more thread skill than me), not "the moderators". Don't assume, as that can lead to being the first three letters. And anyway, the moderators general job so far as I know, without checking anywhere, is to keep the threads and boards clean. I'd guess that does include getting rid of duplicate threads.

Maybe you didn't pick it up from
But I'll concede defeat (this time, Gadget!) and kill mine.
, but I DID create the FIRST thread. So it was your responsibility (and that of everyone else) this time, not mine, and you failed to live up to it. This was the SECOND thread, and Eco's was the third. So shove off Mr. High-and-Mighty.

I'm not mad you understand, just annoyed at your misunderstanding. I'm fine with my thread not becoming the main one, as the inclusion of the Gadget line should have indicated.
Last edited by westrim on Mon Dec 28, 2009 7:21 pm UTC, edited 1 time in total.

sableye22
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### Re: "Gravity Wells" Discussion

Aww, the graph-y ones are some of my favorites.

This is why little stick figures tend to show up in my notebooks. Concepts come so much easier when tiny stick people are saying them to me as crazy as that sounds.
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Arvedui
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### Re: "Gravity Wells" Discussion

I'd just like to point out that I am incredibly overjoyed to see that Pluto is not represented. It might have been considered a planet when we thought it was the only thing out there, but like Ceres before it, it must now be considered just a small plutoid.

metaphysicist
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### Re: "Gravity Wells" Discussion

gravityhomer wrote:I have a question on the peaks between the planets.

These peaks vary in height on the poster from jupiter to the left (ie, it is easier to go to venus, than return). But they are all at the same height from jupiter to the right. It seems like this variation is due to the sun's gravity well. Is the sun's gravity well really relatively flat on the right half of the comic (from jupiter to saturn to urnaus to neptune), incomparison to the the left?

The force of sun's gravity goes down with 1/ (r^2) so I guess it could be pretty flat out past jupiter? I'll believe it, but I was wondering if it was drawn this way just to fit it in a good size comic. because if saturn, urnanus and netpune kept going up to the right, there would be a lot more white space in a rectangular comic.

oh and by the way, I think it is an awesome comic. I love these large blow up ones.

Kind of... I believe what you're noticing is mostly the big gap between the terrestrial planets and the gas giants. There isn't a property of the gravity well that incidentally makes it get much flatter past jupiter, it's just the large amount of space between Mars and Jupiter, and then all the space between the different gas giants that makes it appear that way.
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Pfhorrest
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### Re: "Gravity Wells" Discussion

rollo wrote:Okay, I love this, I really do, but YOU FORGOT PLUTO (forum readers: ..."but wait I must explain!" ). No matter how many people claim that its not a planet, I refuse to accept this denial. It just makes sense, 9 planets, mathematically and aesthetically is logical. The problem is a combination of bureaucratic and artistically lacking scientist.

If you wanted to do things on the grounds of numeric aesthetics, they should have considered "dwarf planets" a type of planet still (as the name would imply), which would have left us with four rocky inner planets (Mercury, Venus, Earth, Mars), four gas giants (Jupiter, Saturn, Neptune, and Uranus), and four dwarf planets (Pluto, Charon, Eris, and Ceres), for twelve planets evenly divisible into three groups of four.

Of course, then Makemake and Haumea had to go and be discovered and Charon discounted from the dwarf planets leaving us with an awkward count of five dwarves...
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Random832
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### Re: "Gravity Wells" Discussion

DragonHawk wrote:
philip1201 wrote:You won't be able to escape Deimos with a bike and a ramp: the bike accelerates because of friction with the surface. You get the friction because gravity pulls you to the ground.

Kerist, it's the airplane-on-a-treadmill all over again.

Well if you had a jet-powered bike then there would be no problem. Similarly, no-one's saying a _car_ would be able to move forward on a treadmill.

fenrir_darkwolf
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### Re: "Gravity Wells" Discussion

Sooo... please tell me we're getting a poster version of this!
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Möbius
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### Re: "Gravity Wells" Discussion

Awesome graphic! I did something similar last year, but as usual, XKCD wins on style. Way to go!

Fwiw, here's my discussion of gravitational potential within the solar system:
http://www.typnet.net/Essays/GravPot.htm

--
Möbius

sab39
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### Re: "Gravity Wells" Discussion

philip1201 wrote:You won't be able to escape Deimos with a bike and a ramp: the bike accelerates because of friction with the surface. You get the friction because gravity pulls you to the ground. WIth as little gravity as on Deimos, every little bump you hit will launch you into the air, so you won't be able to accelerate properly before you reach the ramp. If you do manage to get close to escape velocity, you could be launched into a stable orbit by a pebble-sized bump. The baseball is pretty doable though.

I specifically came into this thread to see if someone would make that point

Random832
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### Re: "Gravity Wells" Discussion

Möbius wrote:Awesome graphic! I did something similar last year, but as usual, XKCD wins on style. Way to go!

Fwiw, here's my discussion of gravitational potential within the solar system:
http://www.typnet.net/Essays/GravPot.htm

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Möbius

Interesting how different it looks with everything to scale

Ubik
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### Re: "Gravity Wells" Discussion

Möbius wrote:Awesome graphic! I did something similar last year, but as usual, XKCD wins on style. Way to go!

Fwiw, here's my discussion of gravitational potential within the solar system:
http://www.typnet.net/Essays/GravPot.htm

--
Möbius
Thanks for the link and thanks for the content behind the link too!

JCM
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### Re: "Gravity Wells" Discussion

Wah! Huge image is huge! My poor little dial-up and 800x600 screen resolution can't handle this.

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### Re: "Gravity Wells" Discussion

Well it does say this is under constant Earth surface gravity. So I guess he means the amount of energy it takes to launch a bike off a ramp on Earth* would be enough to escape the gravitational pull on Deimos, not that you could actually accelerate a bike properly under Deimos' gravity. Heck, if we're going to get all technical we'd better factor in the weight of the rider's space suit and how difficult it would be to ride a bike while wearing one. Then, for that matter, what kind of bike, and what length/angle are the ramp? How strong is the rider and how fast is he going when he hits the ramp? Is he pedalling at full strength? etc.

*I am ignoring the fact that as the bike goes up the ramp, it's no longer on the surface and thus no longer surface gravity. Call it rounding error.

I am a bit curious why the full version links to an HTML page with nothing but an embedded image, instead of directly to the image. O.o This prevents Firefox from being able to scale it to the window size.

Also I'm having trouble thinking of how to explain this in layman's terms, since it requires some hypotheticals. "If you were in a place where gravity does not decrease as you go upward, the amount of energy it would take to go 6000km (or whatever the proper figure is) straight up there is the same amount it takes to break Earth's gravitational pull." It might work, but I think some people would be confused by the theoretical place.
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BrBilly
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### Re: "Gravity Wells" Discussion

...OK... sorry about that. I've got it now.
Last edited by BrBilly on Wed Dec 30, 2009 5:16 pm UTC, edited 1 time in total.

DVC
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### Re: "Gravity Wells" Discussion

Pit wrote:This comic made me so glad that I took Astronomy 101. I wish he had put Ceres and Eris though... it would have been a good excuse to stick Pluto in.

This is precisely why the IAU's definition of a planet is wrong. By default we are excluding significant solar system bodies from relevant discussions.

My Definition:
Planet
1) Approximately round under its own gravity.
and either
2a) Orbits a star without also orbiting another body (i.e. no other body's gravitational influence results in the candidate body having a barycentre outside it.)
or
2b) If two or more bodies orbit each other (and meet condition 1), i.e. the barycentre of the system is not within any of the bodies, they are a binary/trinary/etc. planet system.

Yes, Ceres and Pluto and some other things become planets under that definition, Pluto and Charon become a binary planet system, but who cares. Why should a planet be so special? Recently we've discovered there are lots of planets in the solar system, that should be exciting but the insular IAU freaks out and says 'nope not planets they can't be, there would be too many of them!'

As an added bonus the definition I suggest then tends to make it easier to define a moon, and has some commonalities with the way we define binary and trinary star systems.