<p>Use the initial Center Of Mass (COM) as an origin.</p>

<p>It's trivial to show that the COM of a closed N-body system (in Newtonian physics) moves with a constant velocity.  This is a relatively easy system to work with and transforming to other coordinate systems, such as the heliocentric one you mention, is <em>relatively</em> trivial compared to computing the 3D N-body system with good accuracy.  A word of advice : if the math of converting between coordinate systems is a problem for you, the mathematics and algorithms of an accurate simulation will be likely much worse.  I'd suggest trying out such conversion mathematics as a warm up exercise and if you're comfortable with that then dive into the simulation.</p>

<p>COM can be shown to move at constant velocity because it's a closed system (or can be treated as such over the timescale you mention), and so net force on the COM is zero, meaning it has to move with constant velocity.</p>

<p>Adding a small body like an extrasolar asteroid makes next to no difference to the system as a whole.  It's a minute change.  You can treat such objects as special cases, ignoring their effect on the system and just calculating the effect of the system on them.  If you're planning to add very large bodies and see what happens then you need to include them in the general system all the way through the simulation.</p>

<p>Note there are a lot of existing simulations out there that implement a wide range of approaches.  This is a problem that has been extensively studied and a search online would yield a myriad of approaches.</p>

<p>The motion of the Barycenter is complex.  I'd avoid this complication in what is already gong to be a messy mathematical system to simulate.</p>

<p>Your main priorities should be gathering accurate data for initial starting values (non trivial) and choice of algorithms.  Choice or ordinate system is comparatively minor.</p>

CC BY-SA 4.0
2020-03-13T14:46:16.240Z
2020-03-13T14:46:16.240Z
StephenG - Help Ukraine
<p>For the purpose of a project, I require the components that describe the inertial frame to be as independent as possible from solar system bodies itself. Since the solar system barycenter is itself determined by the positions and masses of the solar system bodies, it will not be a good choice.</p>

<p>I was thinking that the frame that I want could be described by some nearby star that does not have planets of its own or else the galactic centre itself. </p>

<p>If such an inertial frame exists, I would like the same to be expressed in heliocentric cartesian coordinates.</p>

<p>Any relevant thoughts would be much appreciated!</p>

CC BY-SA 4.0
2020-03-13T06:21:37.523Z
How about a combination of the barycentre and a bunch of quasars? https://en.wikipedia.org/wiki/International_Celestial_Reference_System
PM 2Ring
CC BY-SA 4.0
2020-03-13T09:54:57.457Z
@PM2Ring I did check out the ICRS, but here too, I would be defining an inertial system whose origin is at a point that is inherently determined by the planets themselves (the solar system barycenter). The directions of the pole and RA origin for the ICRS are maintained fixed relative to the bunch of quasars you were talking about. The role of these distant quasars are but to act as "fixed" points in space that the axes of the ICRS point to (currently ICRF3).
Siddharth Bhatnagar
CC BY-SA 4.0
2020-03-13T10:06:46.970Z
It will be helpful if you tell us what you want to use this coordinate system for, including the time span it needs to cover. Any celestial coordinate system has its pitfalls because everything's moving, and we don't have perfect knowledge of the location & motion of astronomical bodies, even nearby stars.
PM 2Ring
CC BY-SA 4.0
2020-03-13T10:34:58.140Z
@PM2Ring So the coordinate system is being used to describe the state vectors of each planet in the solar system, over about 250 years.
Siddharth Bhatnagar
CC BY-SA 4.0
2020-03-13T10:47:39.500Z
In that case, I think you *should* use the barycentre. Conservation of momentum tells us that the barycentre of an isolated system isn't affected by the motions of the components of the system. And the solar system is pretty well isolated, apart from occasional small visitors, like ʻOumuamua.
PM 2Ring
CC BY-SA 4.0
2020-03-13T11:30:39.420Z
That's true, but the barycenter is affected by the masses present in the system. Let's assume that our solar system is just comprised of the inner planets, the position of the barycenter significantly changes then. That is somewhat where the essence of this project lies. Therefore, what I would like is a frame centred at an origin that is either stationary in space relative to the solar system or a frame that is moving at a constant velocity wrt the solar system - essentially a point that is undisturbed even if changes akin to the above are made to the solar system.
Siddharth Bhatnagar
<solar-system><frame-of-reference>
What would be the best inertial reference frame to describe objects in the solar system (other than the solar system's barycenter)?
Siddharth Bhatnagar
# What would be the best inertial reference frame to describe objects in the solar system (other than the solar system's barycenter)?

(Asked by: username_0 on 2020-03-13)

For the purpose of a project, I require the components that describe the inertial frame to be as independent as possible from solar system bodies itself. Since the solar system barycenter is itself determined by the positions and masses of the solar system bodies, it will not be a good choice.

I was thinking that the frame that I want could be described by some nearby star that does not have planets of its own or else the galactic centre itself.

If such an inertial frame exists, I would like the same to be expressed in heliocentric cartesian coordinates.

Any relevant thoughts would be much appreciated!

**Tags:** solar-system, frame-of-reference

> **Comments:**
> **username_1**: How about a combination of the barycentre and a bunch of quasars? https://en.wikipedia.org/wiki/International_Celestial_Reference_System  
> **username_0**: @PM2Ring I did check out the ICRS, but here too, I would be defining an inertial system whose origin is at a point that is inherently determined by the planets themselves (the solar system barycenter). The directions of the pole and RA origin for the ICRS are maintained fixed relative to the bunch of quasars you were talking about. The role of these distant quasars are but to act as "fixed" points in space that the axes of the ICRS point to (currently ICRF3).  
> **username_1**: It will be helpful if you tell us what you want to use this coordinate system for, including the time span it needs to cover. Any celestial coordinate system has its pitfalls because everything's moving, and we don't have perfect knowledge of the location & motion of astronomical bodies, even nearby stars.  
> **username_0**: @PM2Ring So the coordinate system is being used to describe the state vectors of each planet in the solar system, over about 250 years.  
> **username_1**: In that case, I think you *should* use the barycentre. Conservation of momentum tells us that the barycentre of an isolated system isn't affected by the motions of the components of the system. And the solar system is pretty well isolated, apart from occasional small visitors, like ʻOumuamua.  
> **username_0**: That's true, but the barycenter is affected by the masses present in the system. Let's assume that our solar system is just comprised of the inner planets, the position of the barycenter significantly changes then. That is somewhat where the essence of this project lies. Therefore, what I would like is a frame centred at an origin that is either stationary in space relative to the solar system or a frame that is moving at a constant velocity wrt the solar system - essentially a point that is undisturbed even if changes akin to the above are made to the solar system.  

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## 1 answers

### Answer by username_2 on 2020-03-13. Score: 5

Use the initial Center Of Mass (COM) as an origin.

It's trivial to show that the COM of a closed N-body system (in Newtonian physics) moves with a constant velocity. This is a relatively easy system to work with and transforming to other coordinate systems, such as the heliocentric one you mention, is *relatively* trivial compared to computing the 3D N-body system with good accuracy. A word of advice : if the math of converting between coordinate systems is a problem for you, the mathematics and algorithms of an accurate simulation will be likely much worse. I'd suggest trying out such conversion mathematics as a warm up exercise and if you're comfortable with that then dive into the simulation.

COM can be shown to move at constant velocity because it's a closed system (or can be treated as such over the timescale you mention), and so net force on the COM is zero, meaning it has to move with constant velocity.

Adding a small body like an extrasolar asteroid makes next to no difference to the system as a whole. It's a minute change. You can treat such objects as special cases, ignoring their effect on the system and just calculating the effect of the system on them. If you're planning to add very large bodies and see what happens then you need to include them in the general system all the way through the simulation.

Note there are a lot of existing simulations out there that implement a wide range of approaches. This is a problem that has been extensively studied and a search online would yield a myriad of approaches.

The motion of the Barycenter is complex. I'd avoid this complication in what is already gong to be a messy mathematical system to simulate.

Your main priorities should be gathering accurate data for initial starting values (non trivial) and choice of algorithms. Choice or ordinate system is comparatively minor.


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