Brightness Of Planets

[Elen Sila]

Here is a photograph I took, about three hours ago. (Disregard how bright the sky looks; that's just light pollution being amplified by the large aperture setting I used. The sky looked black to the naked eye.)



Here is Celestia's simulation of what the sky should have looked like at that time.



Notice that, in real life, Saturn is SIGNIFICANTLY more brilliant than he is portrayed by Celestia. This is an issue not unique to Saturn: Mars and Mercury also suffer from the same problem. Is there a way to fix this by editing the program files; or is this something that would have to be changed in later versions of Celestia?

[selden]

The brightness of an object which is too small to resolve in Celestia (i.e. one which is drawn as a point), is controlled by that object's SSC entry Albedo. You can manually edit Celestia/data/solarsys.ssc (which is a plain text file) to change the Albedo values.

Celestia has three different ways of drawing a point-like object. Type Ctrl-S several times to step through them.

[ajtribick]

There was at one point an attempt being made to add HDR capability to Celestia (which is something we really need given the vast dynamic range we are representing), unfortunately the person responsible left/disappeared before they completed it.

[t00fri]

First of all, this week Saturn has apparent magnitude +0.5 while the nearby Spica (alpha VIR) has +0.98. Thus, Saturn is indeed somewhat brighter than Spica but for the unaided eye it's not all that much.

Next, the star display in Celestia STRONGLY depends on the star style chosen (select with CTRL+S).

You apparently used the "scaled disks" style, which may give a reasonable global star display, but is unsuited for quantitative comparisons. Notably brighter stars (Spica!) will appear too bright (disk getting too large)

The underlying problem is clearly that the narrow dynamic range of computer monitors is generally unable to properly render the LARGE actual brightness differences of celestial objects. Hence one has to use various graphical tricks that are unavoidably more or less bad compromises.

One of the best possibilities is to map the apparent magnitude of a star to the rendered star size in pixel. In order to avoid huge discs (and slow rendering speed) one adds a Gaussian glare beyond a certain threshold brightness. The corresponding star view again depends on several parameters that one needs to adjust carefully etc.

Here is how things look with my shader stars (Celestia.Sci) and with apparent magnitude limit adjusted to 7.0 for naked eye observation:

stars_planets.jpg

The more reddish and somewhat brighter object left of Spica is Saturn. I think it's not all that bad.

This more modern star style is not officially incorporated in Celestia (I use such type of stars in Celestia.Sci though and ChrisL uses it in his new Cosmographia project through the VESTA@ASTOS library). There exists a patch by ChrisL for the Celestia source code, which requires recompilation.

Analogously, planets are inspected under a number of different conditions in Celestia: in the far-away "pointlike" limit, in some kind of telescope mode (-> automag) and from close distance. In the far-away "pointlike" limit one needs to use a similar rendering style as for the stars, in order to be able to compare magnitudes quantitatively. This has not been implemented in the official Celestia version.

Fridger

[duds26]

Hopefully this will be implemented!
Would be nice to have the correct approximation of brightness.