Add lagrange points L4 and L5 #3090

snumlautoken · 2023-03-08T22:11:34Z

Description

This code adds the option to enable markings of L4 and L5 lagrange points between the current body and the solar object.

As points L1, L2 and L3 are all analogue to solar and antisolar points, I saw no reason to add them.

No additional dependencies are required.

Fixes #1111

Screenshots (if appropriate):

Type of change

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to change)
This change requires a documentation update

How Has This Been Tested?

Honestly, I have just been eyeballing the feature in action and debugging to see that my math is correct. The math is not particularly hard, but I am not sure how this could be tested automatically. Let me know if you want me to add anything.

Test Configuration:

Operating system: Arch-linux
Graphics Card: Not really applicable

Checklist:

My code follows the code style of this project.
I have performed a self-review of my own code
I have commented my code, particularly in hard-to-understand areas
I have made corresponding changes to the documentation (header file)
I have updated the respective chapter in the Stellarium User Guide
My changes generate no new warnings
I have added tests that prove my fix is effective or that my feature works
New and existing unit tests pass locally with my changes
Any dependent changes have been merged and published in downstream modules

github-actions · 2023-03-08T22:12:29Z

Great PR! Please pay attention to the following items before merging:

Files matching src/**/*.cpp:

Are possibly unused includes removed?

This is an automatically generated QA checklist based on modified files.

github-actions

Thanks for adding your first pull request to Stellarium. If you have questions, please do not hesitate to contact us.

alex-w · 2023-03-09T03:25:19Z

OK, I see 2 Lagrange points for Earth-Sun system and this is good, but OP in the issue #1111 want to see L4 & L5 points for Earth-Moon system.

To avoid misunderstanding I propose rename "L4" into "L4 (Sun-planet)" and "L5" into "L5 (Sun-planet)" in the GridLinesMgr.cpp file for current L-points. What about L-points for the Moon?

Atque · 2023-03-09T09:59:30Z

As points L1, L2 and L3 are all analogue to solar and antisolar points, I saw no reason to add them.

Wouldn't those be good though, for educational reasons? And to understand the numbering of the lagrange points.

snumlautoken · 2023-03-09T10:31:51Z

@alex-w
I have renamed the existing lagrange-points and added the L4 and L5 points for the earth-moon system

@Atque
Thought about it, but wouldn't it be cluttered? Especially seeing as how L1 and L3 occupy the same point (from the observer).
Would be easy to add if requested though.

snumlautoken · 2023-03-09T10:53:44Z

Realized that my implementation does not take into account if the common plane between the two bodies is different from the ecliptical plane.
Will fix that.

gzotti · 2023-03-09T11:03:17Z

src/core/modules/GridLinesMgr.cpp

+		{
+			// Takes the positional vector of the planet from the solar object
+			Vec3d pos= core->getCurrentObserver()->getHomePlanet()->getHeliocentricEclipticPos();
+			// Creates points by adding/subtracting 60 degrees to the vector along the ecliptic plane


This approach may work for circular orbits. How does it appear for e.g. Mars or Mercury? Its distance from the sun will place it far off the orbit distance at that point.

See commit 78d81e7
I am thinking it should account for deviations from orbit, but I am not really an astronomer.

alex-w · 2023-03-09T11:04:29Z

Earth-Moon lagrange points should be visible from Earth only

src/core/modules/GridLinesMgr.cpp

gzotti · 2023-03-09T11:38:27Z

L1, 2, 3 would be more clearly visible when observed from the associated "planet observer". Also, if they are computed with the planetocenter, an observer on the surface will see at least the distinction of L1 from L3 from parallax shift, even if only a few arcseconds. (But L3 is hidden behind the sun...)

snumlautoken · 2023-03-09T11:59:25Z

Earth-Moon lagrange points should be visible from Earth only

This has been fixed

snumlautoken · 2023-03-09T12:02:44Z

L1, 2, 3 would be more clearly visible when observed from the associated "planet observer". Also, if they are computed with the planetocenter, an observer on the surface will see at least the distinction of L1 from L3 from parallax shift, even if only a few arcseconds. (But L3 is hidden behind the sun...)

If that is the case, wouldn't we need the masses of the bodies to calculate the distances? As far as I can tell the stellar objects classes have no members for mass, so this would not work for the general case.
Please correct me if I am wrong.

alex-w

Well, we don't have masses of the SS bodies in our INI files, I think other LP should be implemented in separate PR if it critical. The current PR need couple cosmetic fixes, but it can be fixed after merge.

Atque · 2023-03-09T14:53:28Z

I think the labels should be "Sun-< name of current planet >" instead of "Sun-Planet".

gzotti · 2023-03-09T22:21:31Z

When observing from Earth Observer, I expect to find L4, L5 somewhere close to Earth's orbit line. However, they are at no location that appear to make sense.

snumlautoken · 2023-03-10T00:22:05Z

When observing from Earth Observer, I expect to find L4, L5 somewhere close to Earth's orbit line. However, they are at no location that appear to make sense.

Yes, I just awoke to the realization that I am calculating the common plane incorrectly, again...
It has been ages since I last touched linear algebra.
I will try to resolve it soon.

snumlautoken · 2023-03-10T00:55:56Z

@gzotti
If you would please have a look at 2e838ba.
Been sure to make my testing more extensive and can at the very least confirm that the vectors (in the triangles) are the same length.
Sorry for all the trouble.

alex-w

OK, some cosmetic fixes (like: “L4(Sun-planet)” -> “L4 (Sun-planet)”) may be fixes before merging.

snumlautoken · 2023-03-11T10:04:18Z

If you then move away from the earth (Alt+End), you will see this.

Gotcha! This has been fixed!

snumlautoken · 2023-03-11T10:05:23Z

OK, some cosmetic fixes (like: “L4(Sun-planet)” -> “L4 (Sun-planet)”) may be fixes before merging.

Also fixed this in the commit, though it is not visible in the screenshot as I had not added it at the time that the screenshot was taken.

gzotti · 2023-03-11T10:25:23Z

OK, seems we are getting somewhere. Now in this view, L1,2,3 would be a welcome addition. So, maybe show L1,2,3 if observed from an observer location?

The other thing is the label. IMO it is too much screen clutter. "L<n> (<planetname>)" or even "L<n>_{<planetsymbol>}" should be enough.

The next question is of course the actual reason for showing L points. OP wanted to see the location of the elusive Kordylewski clouds in the Moon's L4,5. AS soon as we have that, other people will become interested in seeing L4,5 of the other planets in search of what are Trojans for Jupiter. This would mean, 8x2 extra points (or 8x5 if observed from an "observer") over the whole sky if they are plotted for every planet. Or a fine-grained selection of which planet's L points the users are interested.

snumlautoken · 2023-03-11T11:35:18Z

Now in this view, L1,2,3 would be a welcome addition

Is there a way to get my hands on the mass of the planet without hard-coding it? As far as I can tell it will be necessary for the calculation of the points.

This would mean, 82 extra points (or 85 if observed from an "observer") over the whole sky if they are plotted for every planet

I could imagine adding a drop-down menu for other planets in the SS, so while the current planet would be the default the user would have the option to choose whatever other planet in the SS. Does that sound reasonable?

gzotti · 2023-03-11T11:50:35Z

We could introduce an optional mass_kg entry (default 0) to the ssystem_*.ini files, massKg instance variable and getMass() function to the Planet class.

alex-w · 2023-03-11T14:30:30Z

The other thing is the label. IMO it is too much screen clutter. "L<n> (<planetname>)" or even "L<n>``<planetsymbol>" should be enough.

Bad idea for performance

The next question is of course the actual reason for showing L points. OP wanted to see the location of the elusive Kordylewski clouds in the Moon's L4,5. AS soon as we have that, other people will become interested in seeing L4,5 of the other planets in search of what are Trojans for Jupiter. This would mean, 8x2 extra points (or 8x5 if observed from an "observer") over the whole sky if they are plotted for every planet. Or a fine-grained selection of which planet's L points the users are interested.

Showing L-points for selected planet or for current planet when any planet is not selected?

gzotti · 2023-03-11T15:00:01Z

The other thing is the label. IMO it is too much screen clutter. "L<n> (<planetname>)" or even "L<n>_{<planetsymbol>}" should be enough.

Bad idea for performance

What is so costly here? String creation in SkyPoint::updateLabel() is done once whenever language changes. String display? Is a short but complex string so much slower to draw than a long one? How much? If the sub is costly, we can omit it. But short labels should in any case be used to avoid screen clutter.

The next question is of course the actual reason for showing L points. OP wanted to see the location of the elusive Kordylewski clouds in the Moon's L4,5. AS soon as we have that, other people will become interested in seeing L4,5 of the other planets in search of what are Trojans for Jupiter. This would mean, 8x2 extra points (or 8x5 if observed from an "observer") over the whole sky if they are plotted for every planet. Or a fine-grained selection of which planet's L points the users are interested.

Showing L-points for selected planet or for current planet when any planet is not selected?

A choice of up to 8 planets plus Earth's Moon for which L points could be made available. This would call for a new sub-dialog available from the Gridline viewconfig page. It should be possible to display L4,5 for Jupiter and Saturn while having selection on some other object.

However, this all comes not in time for the upcoming release. Please let us discuss this further after 23.1 is out, there are still tasks to do.

alex-w · 2023-03-12T03:50:10Z

What is so costly here? String creation in SkyPoint::updateLabel() is done once whenever language changes. String display? Is a short but complex string so much slower to draw than a long one? How much? If the sub is costly, we can omit it. But short labels should in any case be used to avoid screen clutter.

You need call SolarSystem/Planet classes every frame to get the localised name of the planets.

gzotti · 2023-03-12T09:10:58Z

What is so costly here? String creation in SkyPoint::updateLabel() is done once whenever language changes. String display? Is a short but complex string so much slower to draw than a long one? How much? If the sub is costly, we can omit it. But short labels should in any case be used to avoid screen clutter.

You need call SolarSystem/Planet classes every frame to get the localised name of the planets.

updateLabels() is called when language changes, not in every frame. This of course assumes there are L points for each planet. If there is only a set of max. 5 L points for the current planet (or observed planet if user is on an observer-type planet), labels have to be recreated also if observed planet changes.

snumlautoken · 2023-03-14T15:10:16Z

Things currently seem to be in limbo, so what exactly should we do to move forward with the PR?

gzotti · 2023-03-14T15:44:43Z

Sorry, we are busy with real-world jobs and bugfixes for 23.1.

I wonder whether it makes sense to have up to all 8x5 L points for all planets and maybe even some moons of the gas giants which have other moons in their L4/5 points visualized in SolarSystemObserver view. This would require a dedicated selection GUI.

If you need planet mass, you could find and insert mass data for the planets in ssystem_major.ini. Just add lines like

mass_kg = 5.972168e24

snumlautoken · 2023-03-14T17:02:37Z

I was thinking about just adding a dropdown-menu to select the body whose lagrange-points to show; or alternatively to show those of the current cursor-selected body. Wouldn't that suffice?
Thinking it would be a little wonky with a dedicated selection GUI.

alex-w · 2023-03-20T08:44:00Z

Masses of the Sun and major planets in the master already

alex-w · 2023-05-19T13:18:45Z

Any news?

snumlautoken · 2023-05-20T13:00:57Z

Any news?

Ah, sorry, got busy with other work, so no progress has been made since
If I find time for it I could get it done for 23.2

luzpaz · 2023-09-25T19:02:36Z

soft bump, hoping this gets over the finish line soon 🤞

github-actions · 2023-10-31T03:10:14Z

This pull request has conflicts, please resolve those before we can evaluate the pull request.

gzotti · 2024-03-06T15:02:17Z

@snumlautoken Any updates here?

snumlautoken added 3 commits March 8, 2023 21:34

Add basic prototype of lagrange

c006395

Add changes to UI

b1c7310

Add comments

b453e12

github-actions bot requested review from 10110111, alex-w and gzotti March 8, 2023 22:11

github-actions bot assigned snumlautoken Mar 8, 2023

github-actions bot reviewed Mar 8, 2023

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alex-w added this to the 23.1 milestone Mar 9, 2023

Rename to Lagrange Solar

c7561ce

Add earth-moon lagrange points

8bd341f

gzotti reviewed Mar 9, 2023

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src/core/modules/GridLinesMgr.cpp Outdated Show resolved Hide resolved

Fix common plane

78d81e7

Fix visibility

e4cc0ea

alex-w approved these changes Mar 9, 2023

View reviewed changes

Fix common plane, again

2e838ba

alex-w requested changes Mar 11, 2023

View reviewed changes

Fix points for observers

2a43850

alex-w added this to Needs triage in Visualization via automation Mar 12, 2023

alex-w added this to Needs triage in Solar System via automation Mar 12, 2023

alex-w modified the milestones: 23.2, 23.3 Jun 15, 2023

alex-w modified the milestones: 23.3, 23.4 Sep 10, 2023

github-actions bot added the has conflicts The pull request has conflicts label Oct 31, 2023

alex-w modified the milestones: 23.4, 24.1 Dec 5, 2023

alex-w removed this from the 24.1 milestone Mar 11, 2024

Add lagrange points L4 and L5 #3090

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Add lagrange points L4 and L5 #3090

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snumlautoken commented Mar 8, 2023 • edited

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