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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
apenpaapDate: Tuesday, 19.01.2016, 17:16 | Message # 421
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Aren't they? They're both pretty close to perfectly rounded. Iapetus has that mountain ridge, but I can't find any sources that consider it major enough to consider Iapetus not in hydrostatic equilibrium.




I occasionally stream at http://www.twitch.tv/magistermystax. Sometimes SE, sometimes other games.
 
Wicker1MDate: Tuesday, 19.01.2016, 17:43 | Message # 422
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Yeah, that's what puzzles me. According to Wikipedia, Dione, Iapetus and Tethys aren't in hydrostatic equilibrium. But they are large enough to be round.
 
WatsisnameDate: Tuesday, 19.01.2016, 18:53 | Message # 423
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Could you link the article?




 
Wicker1MDate: Tuesday, 19.01.2016, 23:41 | Message # 424
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https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size

The list says that the moons that I mentioned aren't in hydrostatic equilibrium.
 
WatsisnameDate: Wednesday, 20.01.2016, 02:13 | Message # 425
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Okay. It looks like the wikipedia page is citing this paper in Icarus. Surprising, but yes, these bodies are not quite in hydrostatic equilibrium in a rigorous sense.




 
NikolaAnicic007Date: Monday, 01.02.2016, 15:59 | Message # 426
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Why are Dione and Iapetus not in hydrostatic equilibrium? They are big enough to be in hydrostatic equilibrium for icy objects of that size.

I'd think that it's because no object is perfectly homogeneous, thus the percentage of materials inside of them can have an impact on the exact mass needed to achieve hydrostatic equilibrium.
 
JackDoleDate: Sunday, 07.02.2016, 11:01 | Message # 427
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I have created a barycenter for the Sol-Jupiter system. The barycenter is slightly inside the sun. I think this would be about right.



My question now is; must be the orbits of the other objects of the system around the sun or around the barycenter?
How do I calculate the correct barycenter for the entire system?
Is that even possible?



Should not actually a barycenter be defined for any two bodies, and then again for two barycenter, and so on? sad tongue

Attachments: 7909777.jpg(286Kb)





Don't forget to look here.



Edited by JackDole - Sunday, 07.02.2016, 11:14
 
WatsisnameDate: Sunday, 07.02.2016, 12:26 | Message # 428
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The barycenter is the center of mass. All bodies (more specifically, all barycenters) orbit the collective barycenter. So the Earth and Moon both orbit the Earth-Moon barycenter, and the Earth-moon barycenter orbits the solar system barycenter.

To find the barycenter of a group of masses, choose a coordinate system (placing the origin at the center of the most massive object is convenient). Then sum the moments (each mass times its distance from the origin), and divide by the sum of the masses.



Example for barycenter of Sun + Jupiter + Saturn, showing that if Saturn and Jupiter are aligned, then the barycenter is about half a solar radius outside of the Sun. If Saturn and Jupiter are on opposite sides (give Saturn a negative 9AU location), then the barycenter is about half a solar radius inside the Sun.

If you're in two or three dimensions, adopt the same formula for each coordinate axis. It's not hard, it's just a lot of calculation.





 
JackDoleDate: Sunday, 07.02.2016, 13:24 | Message # 429
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Quote Watsisname ()
It's not hard, it's just a lot of calculation.

That's the problem! sad wacko





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SpaceEngineerDate: Monday, 08.02.2016, 13:12 | Message # 430
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It's not possible to make planets rotate around common barycenter: they orbits will no longer be ellipses (but close to them). Earth-Moon barycenter system is approximation, more or less accurate. Sun-Jupiter barycenter is less accurate approximation. If you add it, then inner planets must rotate around Sun, while outer ones must rotate around this barycenter. You may add a Saturn - (Sun-Jupiter) bartcenter by the same way, but it will be even less accurate.




 
JackDoleDate: Monday, 08.02.2016, 13:36 | Message # 431
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Quote SpaceEngineer ()
If you add it, then inner planets must rotate around Sun, while outer ones must rotate around this barycenter.

This I will try. Thanks for the answer.



My Sol-System now looks like this. FOV is 0.5 degrees. biggrin


Attachments: 2573774.jpg(206Kb)





Don't forget to look here.



Edited by JackDole - Monday, 08.02.2016, 15:35
 
YantraDate: Saturday, 27.02.2016, 01:08 | Message # 432
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I'm a total noob for astronomy and physics, so there is plenty of the program which I do not know the meaning of. One of those things are the "vectors" setting that puts these arrows on the planets. By a bit of "educated guessing", I assume the longest one pretty much describes where is the North and where is the South of the planet (as it pierces it's poles). Then again, which end is the North? The end with the arrowhead or the blunt end?

I realized, as well, that the other arrows mean the vector for translation and rotation respectively (mainly deduced this because the translation arrow is red, like the color of orbits when switched on). However, in a few planets I have seen a "fourth" arrow (colored light blue, if my memory serves me right). What does this arrow means?
 
WatsisnameDate: Saturday, 27.02.2016, 01:52 | Message # 433
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Yeah, you have a piercing white vector which defines North and South, and another white vector sticking out of the planet and rotating with it which defines zero longitude and latitude. So white vectors are basically describing the planet orientation relative to its spin axis.

The colored vectors describe how the planet is moving. The blue one shows its velocity (which direction is it moving), and the red is the acceleration (which direction is it being pulled). For Earth, this can be confusing since the acceleration is based on the barycenter, so the red vector actually points towards the Moon instead of the Sun.





 
YantraDate: Monday, 29.02.2016, 16:33 | Message # 434
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Quote Watsisname ()
Yeah, you have a piercing white vector which defines North and South, and another white vector sticking out of the planet and rotating with it which defines zero longitude and latitude. So white vectors are basically describing the planet orientation relative to its spin axis.

The colored vectors describe how the planet is moving. The blue one shows its velocity (which direction is it moving), and the red is the acceleration (which direction is it being pulled). For Earth, this can be confusing since the acceleration is based on the barycenter, so the red vector actually points towards the Moon instead of the Sun.

Thank you so much, Watsisname. That helps a lot. I have also noticed that several planets on the program have the very exact same orbital period and rotation period. Is this normal or is simply the program "guessing" under the lack of data of a given celestial body?
 
WatsisnameDate: Monday, 29.02.2016, 23:43 | Message # 435
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This is normal -- they are tidally locked or "synchronously rotating" planets. smile Think of the Moon orbiting the Earth, always presenting the same side to us. It is spinning at the same rate that it orbits. This is a common result of the tidal evolution of moons orbiting planets, and of planets orbiting close to stars.

You see it a lot for planets around low mass stars because then the system is more compact and the habitable zone planets tend to get tidally locked fairly quickly (the time to become tidally locked increases quickly with orbital distance). SE calculates this with a formula when generating them.





 
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