Remco65 wrote:Hello,i have celestia for a short time and it is a fantastic program,because you can travel to ervery object,planet,star,galaxy you want. But my question is how real is celestia?
Realistic enough, but not actually realistic at all.
The thing with Celestia is that to the first order it's pretty realistic. Planets and moons and asteroids and comets are where they ought to be in our solar system, stars are where they ought to be, and there's a lot of galaxies and binary systems in there now. That's realistic enough for most scenarios.
However, go more in depth and you'll realise that it's not actually that realistic in the detail. The obvious thing to realise is that it can theoretically only be as realistic as what we actually know about the universe, and in practice it's only as realistic as the astronomical catalogues that are entered into it. Fridger's done a sterling job incorporating the galaxies and binary systems so far, but this has barely scratched the surface of what's out there (e.g. triple systems and systems with more stars aren't in there yet, and the galaxies aren't necessarily oriented correctly - they're in the right place and are the right type, but the spirals may go in opposite directions to reality, or they may have more or less arms than in reality). There are a lot of catalogues out there, and a lot of data to incorporate, and there's also limitations of time and computing power too. So at best you can say that Celestia is a decent approximation to the Universe (better in some parts than others), but certainly not a complete representation of the universe.
My impression is that it looks (very) real,it looks like how it really is ,if you are in the real space.And the illumination of the sun on the planets on celestia,is that how you approximatily it can see through a telescope or if you are in space? And for example,is asteroid Ceres shown more sunilluminated than Uranus? and (less sunilluminated than Venus?
This is another place where Celestia IMO spectacularly fails to be realistic (and something that I gripe about all the time
).
The problem is that Celestia renders planets supposedly as if the human eye has adapted to the light levels at that distance from the sun/star. This is largely due to the limitation of representing things on a monitor that is itself emitting light, being viewed by a human eye that is adapting to that (and the light levels in the room). For one thing, I'm not convinced that this is being done accurately at all - first of all, what the human eye sees is highly subjective, being dependent on how good a person's vision is, the distribution of rods and cones in the eye, how their brain interprets the image on the retina, how the eye adapts to bright light against the dark background of space, etc etc. In Celestia (as best as I can understand it), all objects (illuminated by a single star) are normalised and rendered at the same brightness - so Pluto looks as bright as Mercury, despite the fact that the actual light levels are vastly different at those locations.
Furthermore, the planets themselves are not treated as light sources - the light they reflect does not illuminate nearby objects (this is known as 'radiosity'), so you can't accurately see the effects of reflected light on the dark sides of their moons. You can fudge this by tinkering with ambient light levels, but that's not remotely realistic. This also means that distant stars are visible around the edges of lit planets (and stars, for that matter) when they should actually be invisible, since the light reflected from the planet should overwhelm the eye and you won't be able to see the small points of light - basically, your dark-adapted vision would be screwed up by the light source. But despite the fact that Celestia supposedly simulates the human eye, it doesn't account for this somewhat important effect at all.
Plus of course, stars in Celestia don't look at all like they should to the human eye (which would in most cases be blinded or dazzled at the very least), largely due to the fact that monitors can't put out that much light. There are however ways to simulate this (see the EVE Online computer game for example, which does a dazzling star effect rather well).
The illumination of objects rendered by more than one star is also badly flawed and not remotely realistic. See
this thread for full details.
Unfortunately, the solution to these illumination problems is not very clear. It may be that there just ISN'T a better way to illuminate objects on a computer monitor in a realtime 3D environment right now (e.g. some aspects might require actual raytracing, which would greatly slow things down). But right now things seem to be somewhat inconsistent or incompletely-implemented.
Another big illumination problem is that of photometry - while the textures of the planets are generally accurate (though naturally the gas giants are only broadly similar to what they really look like right now, since the clouds in their atmospheres are constantly changing as they rotate), in Celestia they don't actually reflect light as they should in reality. The best example of this is the Earth's Moon - in reality the illuminated part of the moon does not get dimmer towards the limb of the body, but in Celestia it does because the photometric function - the way that the object reflects light - is incorrect. Again, this is theoretically something that can be corrected in Celestia by allowing the definition of these parameters/functions for each planet in the ssc, but implementation of this is likely to be a long time coming.
Most people wouldn't even notice these issues, which is why I'd say that Celestia is 'realistic enough' for most purposes, but I think it's important to be aware of its limitations. That said though, it's still a remarkably good piece of software and I'm not knocking the efforts of Chris or Fridger or anyone else who has worked on what is there so far. It's just important to realise that there is still a lot more work to do (especially in the illumination department)...
And are the positions of the stars and planets really of how it is at the moment? Can you answer this questions? Big thanks. Remco.
Generally, yes - at least in the solar system. Outside it gets rather less accurate since we usually don't know the orbits of other planets that well. Also, I'm not sure over what timespan the orbits are most accurate - I presume they get less accurate as one moves far back or forward in time.
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