Fix computation of signed phase angle to determine apparent magnitude #3306

10110111 · 2023-07-05T19:30:33Z

Description

Current computation of apparent magnitude uses two similar quantities: phase angle and something that approximates it, adding a sign to indicate whether the it's a waxing or waning phase.

As I understand, the only reason of determining the sign is to make magnitude depend on what half of the Moon is illuminated, due to asymmetry in the albedo of the visible side of the Moon. So the quantity should actually be equal to the phase angle, up to a sign.

The unsigned phase angle is computed correctly, taking into account positions of the observer and the Moon. But the way the signed quantity is currently computed has some problems:

It has some daily ripples compared to the phase angle that shouldn't be there (possibly the shift of the observer from planet center isn't taken into account);
It switches sign not at the point of 0 or 180° phase, but a few dozens of minutes away from it.

This PR implements the correct computation of whether the phase angle is increasing or decreasing (this is done in Planet::isWaning()). This computation uses heliocentric position and velocity of the observer (the velocity is computed in StelCore::getObserverHeliocentricEclipticVelocity()).

~~I spent some time time trying to reverse-engineer orientation of the axes of the AltAz frame, so the first commit here documents my findings. Please check that it's actually correct.~~

Also, it appeared that the computation to determine the phase angle tendency is quite nontrivial (the simple elongation-based trick mentioned in #2916 won't yield precise results), so the third commit here exports a flag to the scripting API that indicates whether the planet is in the waning phase, i.e. whether its phase angle is increasing.

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?

Test Configuration:

Operating system: Ubuntu 20.04
Graphics Card: Intel UHD Graphics 620

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 · 2023-07-05T19:31:30Z

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.

src/core/StelCore.hpp

gzotti · 2023-07-08T10:55:29Z

src/core/StelCore.hpp

@@ -305,6 +305,9 @@ class StelCore : public QObject

 	//! Return the observer heliocentric ecliptic position (GZ: presumably J2000)
 	Vec3d getObserverHeliocentricEclipticPos() const;
+	//! Return the observer heliocentric ecliptic velocity. This includes orbital and diurnal motion;


If you include diurnal changes in velocity, this should be added to the aberration correction of the observer in SolarSystem::computePositions().

I suppose then observerPlanet->getHeliocentricEclipticPos() should also be replaced with core->getObserverHeliocentricEclipticPos(), right? Because currently that computation takes "observer" to be in the center of the planet.

Yes, I also think that's all that has to be changed. When I did aberration correction, I said we won't include the diurnal aberration, but when you are doing this computation now, it's even one more step. Not sure about the magnitude, though, it may introduce an observable milliarcsecond wobble. Of course, switching off topographic correction will also disable that.

Do you think it should go right into this PR? This seems quite unrelated.

It will be an obvious next change then. Probably we can even make a switch aberration: none/annual/diurnal (for didactic purposes).

Why do you only say that it should be applied in SolarSystem? Shouldn't it equally affect everything?

alex-w

Thanks!

10110111 · 2023-07-16T18:20:09Z

@gzotti What is your verdict on this PR?

gzotti · 2023-07-16T18:33:58Z

Just having a look into its differences, but answering other duties at the same time.

Why is the Moon about 0.1-0.2mag less bright? It may not matter a lot in reality, but still it's a notable difference between my model and your change in computing one of the required angles.

10110111 · 2023-07-16T20:54:06Z

The change in magnitude reflects the change in the phase angle that's fed into the algorithm that computes the magnitude. A totally expected result.

It proved to be quite challenging to properly determine whether phase angle is increasing or decreasing. So it's worth providing a flag for this in the scripting API.

10110111 · 2023-07-16T21:28:39Z

The difference between the old angle and the new one is that the old angle was computed between projected positions of the observer and the Moon, while the new one is between the actual 3D positions.

As we take into account different z components of the positions, we should expect the angle between the objects to increase, which will reduce the visible lit part.

gzotti

OK, the original paper was in terms of phase angle, not dLong, so it should be fine.

This change will introduce localized heliocentric velocity, so despite previous reservation, diurnal aberration should come soon.

github-actions · 2023-07-25T12:18:45Z

Hello @10110111!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

github-actions · 2023-09-26T03:01:53Z

Hello @10110111!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest

github-actions bot requested review from alex-w and gzotti July 5, 2023 19:30

github-actions bot assigned 10110111 Jul 5, 2023

gzotti reviewed Jul 5, 2023

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10110111 force-pushed the signed-phase-angle branch 3 times, most recently from a0f65e3 to 934f403 Compare July 7, 2023 18:43

gzotti reviewed Jul 8, 2023

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src/core/StelCore.hpp Outdated Show resolved Hide resolved

gzotti reviewed Jul 8, 2023

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alex-w added this to the 23.3 milestone Jul 8, 2023

alex-w approved these changes Jul 15, 2023

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10110111 added 2 commits July 17, 2023 01:02

Fix computation of signed phase angle of a planet

04aba79

Add is-waning to planet info map

9e8316c

It proved to be quite challenging to properly determine whether phase angle is increasing or decreasing. So it's worth providing a flag for this in the scripting API.

10110111 force-pushed the signed-phase-angle branch from 934f403 to 9e8316c Compare July 16, 2023 21:07

gzotti approved these changes Jul 19, 2023

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10110111 merged commit 7a2d667 into Stellarium:master Jul 19, 2023
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10110111 deleted the signed-phase-angle branch July 19, 2023 14:56

alex-w added the state: published The fix has been published for testing in weekly binary package label Jul 25, 2023

alex-w removed the state: published The fix has been published for testing in weekly binary package label Sep 26, 2023

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Fix computation of signed phase angle to determine apparent magnitude #3306

Fix computation of signed phase angle to determine apparent magnitude #3306

10110111 commented Jul 5, 2023 •

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Fix computation of signed phase angle to determine apparent magnitude #3306

Fix computation of signed phase angle to determine apparent magnitude #3306

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10110111 commented Jul 5, 2023 • edited

Description

Type of change

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Checklist:

github-actions bot commented Jul 5, 2023

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