Binary orbits and orbital periods: inquiry

[CMDR EfilOne]

I'll give you 1 chance to reply nicely. No one was talking about prograde and retrograde orbits. Someone implied that the planets were moving in opposite directions around the barycenter. I assume you would agree that this is impossible. Planets staring at east and west would move south and have a close approach. Then they would inexplicably move north, as if they were on rails...

Apologies for the hard bump. I'm just triggered when someone comes out of nowhere calling out "dev lololol FDev bad stuff", espetially in a thread whose subject tends to be hard science, where I'd expect tempering, wise thinking, and complete explanations.
To be honest, I'd prefer that you'd school me right now as to why you think it wouldn't work, with proper explanation ...

Reminder that Elite a "simulation", not supposed to depict the real galaxy to its faintest features and kinematics, and when procedurally generating such a complex system, there indeed may be oddities here and there, and even some extremes. That's expected with such a complex system, and I'm simply tired of dev callout, because then I'd like to see the same person come up with a better, more realistic solution that works in realtime as well. Actually if you do have something in mind, because I feel you're one of thoses who know his ways about real-world astronomy/astrophysics, then I'd be very pleased to hear.

 

[CMDR Smior]

Apologies for the hard bump. I'm just triggered when someone comes out of nowhere calling out "dev lololol FDev bad stuff", espetially in a thread whose subject tends to be hard science, where I'd expect tempering, wise thinking, and complete explanations.
To be honest, I'd prefer that you'd school me right now as to why you think it wouldn't work, with proper explanation ...

Reminder that Elite a "simulation", not supposed to depict the real galaxy to its faintest features and kinematics, and when procedurally generating such a complex system, there indeed may be oddities here and there, and even some extremes. That's expected with such a complex system, and I'm simply tired of dev callout, because then I'd like to see the same person come up with a better, more realistic solution that works in realtime as well. Actually if you do have something in mind, because I feel you're one of thoses who know his ways about real-world astronomy/astrophysics, then I'd be very pleased to hear.

Apology accepted. I'm an engineer who studied physics in college. So I forget sometimes to explain things that I take for granted.

Newton's 3 laws of motion: https://www.grc.nasa.gov/www/k-12/airplane/newton.html
I'll restate the problem at hand. Two planets of similar mass in binary orbit around their common barycenter. The barycenter is the center of mass of the system in question. In this case, the two planets. Being of similar mass, the barycenter lies in space between the two planets. No matter the motion of the planets or their eccentricity, the barycenter will always lie in a line connecting the two planets. Restating Newton's first law: every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. The only external forces in this system are the gravity of the 2 planets. (Forces from the gravity of other planets are negligible as they fall of with the square of the distance and Elite doesn't model this anyway.) Therefore, if these planets were moving in opposite directions around the barycenter...
1. The barycenter would move (so it wouldn't be much of a center eh?)
2. If the planets were initially at east and west, they would move south for example. Then after reaching the southern point, they would start moving north. But without an external force, there is no way to explain this change in motion.

https://en.wikipedia.org/wiki/Barycenter

 

[MattG]

I still can't get my head around this. The more I think about it, the more I think you've broken the game (in the first shot anyway). That first screenshot just isn't possible (to my mind) - and yet it exists.

First off - ignoring any other data and whether they're binary orbits or anything, the Journal shows planet 4 was 939.12738 LS from the main star (or "point of arrival") and planet 3 was 940.870483 LS. It's not the most useful piece of data most of the time, but for 2 bodies orbiting the main star you can at least summize the minimum and maximum distances between those 2 bodies - i.e. the shortest distance between the bodies will be if they line up and are on the same side of the star like this (S=star, P=planet):
S.......P...P
In this case, the minimum distance between P1 and P2 is the difference between the entry point distances. Alternatively, the biggest distance between them is if they line up on opposite sides of the star:
P...........S.......P
So here, the distance between the bodies is the sum of the entry point distances. Simple enough, right?

Let's go with the minimum distance - that's 1.743103 LS. In KM, that's ~522568. We know the radius of both planets, but lets go with the slightly larger one - that's ~4840KM, so the diameter is ~9680KM. If we look at both planets face-on from a spot of equal distance from them - as the initial screenshot looks - then there should be some ~54 planet-widths between them. Clearly that's not the case.

FWIW, we can "prove" the actual minimum distance is ~54.9 planet widths, but it's a lot of maths for the same conclusion.

The final piece of evidence is the original picture itself. what is going on with the shadow? Without other influences - of which there are none - those planets should be showing an extremely similar angle of shadow - but they're not even close. I've assumed the journal data is back from Dec, not Feb - if that's the case then I think the game knows the planet should be a lot further away, shadowed it accordingly but for whatever reason it's rendering it in absolutely the wrong place. I scoured the patch notes to see if this was something fixed in the last patch but I can't see anything.

Happy for someone to debunk my thoughts / maths.

 

[Qohen Leth]

I still can't get my head around this. The more I think about it, the more I think you've broken the game (in the first shot anyway). That first screenshot just isn't possible (to my mind) - and yet it exists.

First off - ignoring any other data and whether they're binary orbits or anything, the Journal shows planet 4 was 939.12738 LS from the main star (or "point of arrival") and planet 3 was 940.870483 LS. It's not the most useful piece of data most of the time, but for 2 bodies orbiting the main star you can at least summize the minimum and maximum distances between those 2 bodies - i.e. the shortest distance between the bodies will be if they line up and are on the same side of the star like this (S=star, P=planet):
S.......P...P
In this case, the minimum distance between P1 and P2 is the difference between the entry point distances. Alternatively, the biggest distance between them is if they line up on opposite sides of the star:
P...........S.......P
So here, the distance between the bodies is the sum of the entry point distances. Simple enough, right?

Let's go with the minimum distance - that's 1.743103 LS. In KM, that's ~522568. We know the radius of both planets, but lets go with the slightly larger one - that's ~4840KM, so the diameter is ~9680KM. If we look at both planets face-on from a spot of equal distance from them - as the initial screenshot looks - then there should be some ~54 planet-widths between them. Clearly that's not the case.

FWIW, we can "prove" the actual minimum distance is ~54.9 planet widths, but it's a lot of maths for the same conclusion.

I'm not sure I get the relevancy of your thought. I see the method, but I'm not sure it applies that well to this case. EDIT: I see, I just did the math here too. Oh, and the journal bit is indeed from December.

The final piece of evidence is the original picture itself. what is going on with the shadow? Without other influences - of which there are none - those planets should be showing an extremely similar angle of shadow - but they're not even close. I've assumed the journal data is back from Dec, not Feb - if that's the case then I think the game knows the planet should be a lot further away, shadowed it accordingly but for whatever reason it's rendering it in absolutely the wrong place. I scoured the patch notes to see if this was something fixed in the last patch but I can't see anything.

Happy for someone to debunk my thoughts / maths.

Yeah, the shadow thing has been noted since I saw them. I just checked their radius again, but they're way too close to account for a sort of illusion where one, much bigger, would actually be a lot farther than the other. Besides, I took the pic most probably as I usually do, trying to capture the widest gap between the two, to emphasize, precisely, how close they were. Only thing I could think of about the shadows would be a problem of light source or something...

Btw, someone knows if the Distance From Arrival value is absolute or relative to the drop point, which now varies, depending what system you jump from. Also, does it vary relatively to the position of the body in its orbit too?

 

[marx]

At this point, I'd say the most likely explanation would be a bug. Physics bug, rendering bug (especially with the shadow)... The question is, can the "problematic" screenshot be reproduced? Unfortunately, for that we'll have to wait and see.

 

[Qohen Leth]

Bit more on the math side.

SMA A3 = 300,316,896 m; so orbit diameter = 600,633 km.
SMA A4 = 237,529,840 m; so orbit diameter = 475,059 km.

That's a difference of 125,574 km.

As the orbit of A4 is almost perfectly contained in A3's orbit (from the pic, looks like the barycenter is quasi at the center of both orbits), we can calculate that the distance between the focus of A3 and the focus of A4 on the major axis is equal to 125,574/2 = 62,787 km. That's roughly 6 times the diameter of those planets, seems consistent with the orbit pic. But should A3 and A4 be at their closest, either one would appear 5 or 6 times smaller than the one closer to the ship, or they would appear roughly the same size but 62,787 km appart. Which is, again, not the case in the picture.


DFA A3 = 282,065,874 km
DFA A4 = 281,543,305 km

That's a difference of 522,569 km.

Now I may be mistaken, but the only way to keep this number consistent is to make sure it falls into the largest major axis (600,633 km). It does. But that doesn't work if I'm perpendicular to the line joining the two planets; the DFA would then be the same. So this difference forces us to consider that I drop closely aligned with that joining line, where I can observe a difference nearing the maximum it can be. So, if I haven't smoked too much, that's another element that contradicts the facts: journal says 522,000km, in-game screenshot says roughly 20,000 km.

I'd love to have a FDev take on this.

 

[MattG]

But should A3 and A4 be at their closest

The other problem with this is - they can't be at the closest point at the same time. Their orbits can come that close, but the planets cannot - they remain at opposites. The only thing that could bring them closer would be their eccentricity - but A3 and A4 have only a small degree of eccentricity. From the eccentricity and the SMA, we can work out that at their maximum distance from each other they're ~544567KM apart and at their closest they're ~531125KM - in the grand scheme of things that's pretty much a circular orbit, as your screenshots show. (A1 and A2 have a higher degree of eccentricity, which we can also see from the screenshots, but still nothing like enough to bring them anything like "close").

At this point, I'd say the most likely explanation would be a bug. Physics bug, rendering bug (especially with the shadow)... The question is, can the "problematic" screenshot be reproduced? Unfortunately, for that we'll have to wait and see.

I concur. I think the game generated the objects correctly but for whatever reason has rendered at least one of them in utterly the wrong place. Would've been interesting to see the orbit lines in Dec... I think I'm going to be paying a bit more notice to binary pairs in future though!

 

[Qohen Leth]

Confirmed, they orbit in the same direction. The green HUD is 43 hours after the blue HUD.

Since the Qohen account has beta access, depending where I spawn in the sector I'll precisely know when it was, and maybe catch the two planets halfway between the screenshot and now.

 

[drkaii]

Hey sorry to jump in on this, but planets in binary systems orbit around a barycenter and the distance between them is pretty much fixed. They orbit in the same direction, they can never orbit in opposite directions. The link at the beginning by EfilOne is about the smaller planet orbiting in the opposite direction to the spin of the larger object, not orbital direction.

Anyway, that aside, I think the only explanation is they got reworked due to an update, like happens sometimes. There's no way they were once that close, but now that far apart. I don't think we should get too worked up about this

 

[Qohen Leth]

Anyway, that aside, I think the only explanation is they got reworked due to an update, like happens sometimes. There's no way they were once that close, but now that far apart. I don't think we should get too worked up about this

Why are you sorry?

I don't recall an update between then and now, but that could be it. They were that close, and now they are that far apart, there is no doubting it.

 

[drkaii]

Since then we've had 2.2.03.

 

[Qohen Leth]

Hmm. That's disappointing.

*shrug*

 

[drkaii]

Apology accepted. I'm an engineer who studied physics in college. So I forget sometimes to explain things that I take for granted.

Newton's 3 laws of motion: https://www.grc.nasa.gov/www/k-12/airplane/newton.html
I'll restate the problem at hand. Two planets of similar mass in binary orbit around their common barycenter. The barycenter is the center of mass of the system in question. In this case, the two planets. Being of similar mass, the barycenter lies in space between the two planets. No matter the motion of the planets or their eccentricity, the barycenter will always lie in a line connecting the two planets. Restating Newton's first law: every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. The only external forces in this system are the gravity of the 2 planets. (Forces from the gravity of other planets are negligible as they fall of with the square of the distance and Elite doesn't model this anyway.) Therefore, if these planets were moving in opposite directions around the barycenter...
1. The barycenter would move (so it wouldn't be much of a center eh?)
2. If the planets were initially at east and west, they would move south for example. Then after reaching the southern point, they would start moving north. But without an external force, there is no way to explain this change in motion.

https://en.wikipedia.org/wiki/Barycenter

Another good way to explain it is like this:

If you look at the orbit from the point of view of one of the planets, moving "with it" (i.e. keeping it fixed), you'll see the other planet move around it in a circle or ellipse like this:

So it looks like this:

Center of mass stays put.

If they were moving in opposite directions around the barycenter, the center of mass would constantly change, and that's not allowed!

 

[PanPiper.]

That is a odd variation in distance, although maybe the orbit lines dont show the true orbits. I'm not familiar with the maths and stuff involved but with the Orbital Eccentricity that might account for it.
A & B far higher at 0.1549 compared to B & C with 0.0125 . So A B is far more eccentric. Checking exactly what that refers to quote from wiki on it.
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy.

Some other info on there, but the little gif gives a nice example. Although far higher eccentricity at 0.5

The baycenter is fixed yes, but that isnt visible in SC , depending on the time you visit will show differing distances.
I suspect maybe the orbit lines are not fixed to the planet data exactly but more estimated by the position the planets are in at the time, drawn almost circular and not taking any other data into account.

 

[Qohen Leth]

That is a odd variation in distance, although maybe the orbit lines dont show the true orbits. I'm not familiar with the maths and stuff involved but with the Orbital Eccentricity that might account for it.
A & B far higher at 0.1549 compared to B & C with 0.0125 . So A B is far more eccentric. Checking exactly what that refers to quote from wiki on it.
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptical orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy.

Some other info on there, but the little gif gives a nice example. Although far higher eccentricity at 0.5
https://upload.wikimedia.org/wikipedia/commons/7/7a/Binary_system_orbit_q=3_e=0.5.gif

The baycenter is fixed yes, but that isnt visible in SC , depending on the time you visit will show differing distances.
I suspect maybe the orbit lines are not fixed to the planet data exactly but more estimated by the position the planets are in at the time, drawn almost circular and not taking any other data into account.

That's interesting, now that I think of it I don't remember seeing any very elliptical orbit ever. Or am I wrong?

But then, just what? Why? Why orbit lines if they basically only show actual orbits when those, by luck, coincide with the lines?...

 

[marx]

That's interesting, now that I think of it I don't remember seeing any very elliptical orbit ever. Or am I wrong?

Planets with highly elliptical orbits do exist, but they are very rare. I'd recommend checking the records list on Elite Galaxy Online: while EDDB has more bodies, it doesn't have system map screenshots, which do provide informative context.

Orbit line rendering might be wrong in some cases though. Personally, I have no idea, as I pretty much always have them off. But if you come across some interesting systems, you might want to input their data to EGO and have it render an orrery. Hm, now that I think about it, would you mind doing that with your system?

 

[Qohen Leth]

Argh, another website to register to... I'll try to do it soon.