Add parallax effect to stars

[henrysky]

As discussed in #3982, this PR will add support for modelling parallax effect caused by the difference between the Earth's heliocentric location at the star catalog epoch and the current observer's location. Meaning the parallax effect here should be correctly modelled on different planets/moons too. This PR depends on the work of PR #3992 to be merged first which is work-in-progress, I am opening this PR now to show a work-in-progress preview.

This PR also adds support to update parallax of stars displayed in info string when on different planet/moon.

Fixes # (issue)

Screenshots (if appropriate):

I have added an option to enable/disable Parallax effect, and to exaggerate parallax effect from 1x to 10000x.

<img width="823" alt="Screenshot 2024-12-24 at 1 38 04 AM" src="https://github.com/user-attachments/assets/f99c0cdb-a288-4097-aeda-bdd945df5730" />

To sanity check for now, you can stay on Earth or go to a different planet, enable parallax but disable aberration (so only parallax effect is shown), display ecliptic plane line. Stars near the ecliptic plane should move in 1d motion parallel to the ecliptic plane while stars farthest from the ecliptic plane should move in circular motion. The period of the motion of stars should match the orbital period of the planet. Keeping the same exaggeration factor, stars should have greater motion caused by parallax when you are on an outer planet like Uranus.

Here are video showing the parallax effect:

On earth near ecliptic plane (exaggerate 1000x):

1.mov

On earth near ecliptic pole (exaggerate 1000x):

2.mov

On Uranus near ecliptic plane (exaggerate 1000x):

3.mov

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
• Housekeeping

How Has This Been Tested?

Test Configuration:

• Operating system: <Name, version number>
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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]

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

Files matching src/**/*.cpp:

• Are possibly unused includes removed?

Files matching guide/**:

• Did you remember to update screenshots to match new updates?
• Did you remember to grammar check in changed part of documentation?

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

[henrysky]

A real world check with Proxima Cen. NASA New Horizon's observation where the parallax computed here match what New Horizon has observed "near" Pluto in Apr 2020 (I emphasize "near" because it has passed Pluto in 2015 so long gone from Pluto in 2020 but it is the closest object I can select in Stellarium). Ref: https://www.nasa.gov/solar-system/nasas-new-horizons-conducts-the-first-interstellar-parallax-experiment/

This is the images NASA took, I have added a few anchors to help you navigate:

This is from this PR:

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[gzotti]

The placement of the marker ring is off (should be simple to fix), but the result is impressive!

[Atque]

The placement of the marker ring is off (should be simple to fix), but the result is impressive!

Stars don't connect to the constellation lines either, perhaps the same reason.

EDIT: Nevermind, I was using yesterday's build.

[alex-w]

Awesome feature! When multiplier is 1000 - I think it can be very impressive demo for real planetariums too

[gzotti]

This is fabulous. Watching Proxima Cen or Barnard's star with all effects (nutation, aberration, parallax) together with equatorial grid is a somewhat dizzying experience, showing the difficulties in actually measuring position. And greatly funny to see a two-color rendition of Barnard's star, Proxima, Wolf 359 or Groombridge 1830 in DSS - you can date the plates!

Yes, at 1000x it reminds me of one special projector in Vienna's old Zeiss Model IV which had a wobbling Sirius to demonstrate parallax, probably at 1000x. Not many bright stars can be said to have notable parallax, though.

[Atque]

Hmm, standing on another planet/moon and pressing "Return to default location" crashes the program for me.

[gzotti]

Uh, yes, selecting Jupiter and pressing GOTO (Ctrl-G) also crashes. IIRC the same call.

[gzotti]

Not sure if I understood you. The parallax shift measured by Gaia (moving with the earth around the sun, so in ~1AU distance) enables astronomers to compute a star's distance. In the catalog, it is enough to store one to compute the other. Obviously, parallax from Mars (say, 1.6AU) should be "parallax measured at earth's distance" times 1.6. The star's distance will not change by more than a few light minutes, though (negligible). Max Parallax is non constant per object, but depends on the current observer planet's orbit orientation around the sun (see Hale-Bopp issue above). Of course, the direction and magnitude of offset from some average position depends on current heliocentric position of the observer in relation to the sun-star axis, i.e., stars near the observer planet's orbital poles show circles (or ellipses in shape of the planet's orbit), stars along the observer planet's orbital plane just linear shifts.

I would be OK to have proper motion without tweaks, but allow exaggeration of parallax shift. Else, if you want to freeze proper motion to separate the effects, I'd immediately want to "freeze PM at some configurable epoch". In this case Infostring should include a note "PM Epoch: XXXX" or so if it is not epoch of date.

Separate infostrings: Not sure if this does not clutter the screen too much. At least for aberration I did not do that. It is applied to the J2000 coordinates after PM but before transforming to Jdate. I think most users are happy to read arcseconds and are not too concerned about sub-arcsecond or mas shifts. And when exaggerating effects for demonstrations, are the numbers of any value?

In case you know/can find out why exaggerating aberration fails for factors above 5, also this could be improved to 100x or 1000x.

[henrysky]

@gzotti Responding to the first part of your respond. I was confused too thats why I've proposed to change the parallax. If you look at the algorithm of Gaia astrometric solution (https://arxiv.org/pdf/1112.4139) where in Section 3.2 equation 4, relevant terms are; $\bar{u(t)}$ is the direction to the star from observer at epoch $t$, $\varpi$ is parallax, $b_G(t)$ is the barycentric position of the observer at epoch $t$ and $A_u$ is the astronomical unit.

The astrometric soluiton is asking which parallax the stars need to be to produce the motion of the stars you've observed. Because the equation have taken observer barycentric position at epoch $t$ into account, the astrometric solution would produce parallax consistent with the definition of 1 / 1 arcsec $\equiv$ 1 parsec no matter where and whatever orbit you are. You would only compute a difference in a star's parallax from Mars and from Earth if you neglect the observer's barycentric position (e.g., by simply measuring the annual shift of the star relative to the background as you've mentioned).

We are displaying parallax computed from these algorithms by Gaia and Hipparcos so we better to keep the parallax consistent with these algorithms too. But we can also add another field called apparant annual parallax or something like that.

[henrysky]

I've added barycentric position and velocity function for DE43* and DE44*, otherwise the Sun is assumed to be at the barycenter of the solar system. I've switched parallax and aberration to use barycentric position and velocity.

I think a high aberration will also mess up the "zoning" of stars but outside the scope of this PR. @alex-w @gzotti @10110111 this PR is ready to be reviewed

[10110111]

Please rebase this against the current master, this should fix your build problems on Windows/Qt5.

[gzotti]

Selecting Mars and pressing Ctrl-G for "goto" still crashes :-(

[alex-w]

Selecting Mars and pressing Ctrl-G for "goto" still crashes :-(

Yes, I can confirm crash when parallax is enabled only

[henrysky]

I think the root cause is planet to planet transition, now I have set to use cached parallaxdiff until the transition is completed. Seems to have fixed the problem.

[alex-w]

Thanks!

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[alex-w]

Oops… @henrysky please resolve conflict

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