Description: All about writing scripts for Celestia in Lua and the .cel system
Malenfant wrote:I guess one possible limitation is whether or not there's a way for a script to return the Phase Angle between the observer-sun-object.
29/10/2005 => -2.3 mag
29/10/2005 => -1.9 mag
cpotting wrote:Who would have thought it would be so complicated. I would have thought the variables would simply be a) the absolute magnitude, b) the distance from the sun, c) the phase angle and d) the distance from the planet.
I'm still not sure why there are different phase functions for each of the planets. A reflecting ball is a reflecting ball is a reflecting ball.
The absolute magnitude should account for the albedo, any differences in colour, reflectivity, etc.
The only other factors I could see coming into it are the planetography - different amounts of light reflected from one area as opposed to another, and the oblateness. Both of these should have very minor effects for the major planets though.
(Personally, I think these scientists just complicate things to keep us laypeople on the outside. Come on! Who's with me for simplifying pi to 3.0, using circular orbits, and changing all these trigonometric curves to linear functions? Think about it - if we did, we could hack so much extraneous code out of Celestia that it could be loaded as a Java applet and run on your cell phone! )
Did your contact give a reason for the need for separate phase functions? Why doesn't the equation for Mercury work for Venus, etc? I would be interested in knowing.
Okay. I see that. I'm just surprised that there is that much difference between the photometry of the planets as to actually affect the calculation to such a degree. I figured it might account for a .01 magnitude error or thereabouts.Malenfant wrote:Hate to tell you this, but nope.
This all goes back to photometry - the way an object/surface reflects and scatters light.
I never really noticed before... amazing, isn't it - the things you can stare at for years and years and never realise or question?Celestia doesn't handle this realistically at all - to illustrate this have a look at the full moon in Celestia. You'll notice that it's brightest at the subsolar point and dims around the edges? Now compare that with how the full moon really looks - you'll see that the real moon doesn't get dimmer around the edges, and that actually (surface variation notwithstanding) it's the same brightness throughout the lit hemisphere. The brightness of the moon doesn't change with distance from the sub-solar point.
Again, I knew they were factors, I just grossly underestimated their magnitude as factors.There's about two pages of text explaining about oblateness in the chapter from the Explanatory Supplement that I'm using for all this - it changes the size of the disk seen from Earth. Surface variations also do change the visible magnitude - the surface markings on Mars (and dust storms) for example can cause a variation of up to 0.25 magnitudes.
*Slapslapslap* Get a grip man, science is supposed to be complicated!(Personally, I think these scientists just complicate things to keep us laypeople on the outside. Come on! Who's with me for simplifying pi to 3.0, using circular orbits, and changing all these trigonometric curves to linear functions? Think about it - if we did, we could hack so much extraneous code out of Celestia that it could be loaded as a Java applet and run on your cell phone! )
cpotting wrote:Thanks for all the info. There's a fair amount of new-to-me material here for me to mull over. BTW: were you aware of how many worms were going to crawl out from under this rock when you started? (I know I had no idea how much rotations and quaternions were going hurt my head when I started doing the Guide)
A year or two ago I raised the question of planet/satellite magnitudes in this forum and (I think) with Grant Hutchison. I mentioned that I have some FORTRAN code put together that incorporates many of the sources mentioned in this thread, plus some others. It is a work in progress and has most of the planets and satellites in the solar system. It has observed (or reasonably assumed) phase functions for most of them. Perhaps a look at this code (if I can email it or post it when I'm back in my office) would be useful for Celestia script development?