Celestia Forums

Hi,

Completely new to Celestia - just downloaded it for a specific task: I want to create a duplicate Moon on 180 deg out of phase with the real one... so that I can visualise the Earth-Moon relationship (I might have to make the Earth invisible to see through it, but I can live with that... don't ask!)

The problem is that whilst I can create a duplicate Moon with a new .ssc file, I can't change the Epoch and get the file to load... I added half a lunar period to the J2000 date and entered it as "2000 01 15 03:51:36" exactly as I believe it should be (as described at http://en.wikibooks.org/wiki/Celestia/T ... ustomOrbit) but it doesn't seem to work.

Can anyone help out? Where am I going wrong?

TIA

Julian

FWIW, the .ssc file looks like this... (made it Marsy so I can tell the two apart - if I can get them separated!)

"Pseudo-moonA" "Sol/Earth"
{
Texture "mars.*"
BumpMap "moonbump.*"
BumpHeight 3.5
Radius 1737.53
# Oblateness 0.002
InfoURL "http://www.nineplanets.org/luna.html"
CustomOrbit "moon"
EllipticalOrbit {
Period 27.321661
SemiMajorAxis 384400
Eccentricity 0.054900
Inclination 5.15
}
OrbitFrame {
EquatorJ2000 { Center "Sol/Earth" }
}
UniformRotation
{
Inclination 23.45
AscendingNode 0.0
MeridianAngle 38
}
LunarLambert 1.0
Albedo 0.20
}

A CustomOrbit takes priority over an EllipticalOrbit, but a CustomOrbit which specifies an unrecognized name is ignored.

Should the CustomOrbit statement you are using be formatted such that it is as follows?
CustomOrbit <moon>

Honestly, I really do NOT know if this is the problem or not, but it looks (from the documentation you referenced) that this might be a problem.
Remember too, that I'm just some no-nothing Bozo, so please forgive the intrusion if this is not the answer. Just trying to help.

Thanks Bob - the CustomOrbit is copied directly out of the Solar System SSC - I guess the Moon has a custom orbit because it has been VERY well studied... and of course I want my dummy Moon to duplicate it so I left it as it was.

I do actually have a dummy Moon already - but it's right on top of the real one, because I can't shift the Epoch!

Actually, since I was being very precise about only having just downloaded it I'm sure you know more than I do - and all help is gratefully received

BTW - how do you tell Celestia I just want to look UP (at the Zenith)?

Julian I_Do_Stuff wrote:Thanks Bob - the CustomOrbit is copied directly out of the Solar System SSC - I guess the Moon has a custom orbit because it has been VERY well studied... and of course I want my dummy Moon to duplicate it so I left it as it was.

I do actually have a dummy Moon already - but it's right on top of the real one, because I can't shift the Epoch!

Actually, since I was being very precise about only having just downloaded it I'm sure you know more than I do - and all help is gratefully received

BTW - how do you tell Celestia I just want to look UP (at the Zenith)?

Well, first of all you can obviously disregard my advice in the post above. I looked at my Moon SSC file and found that it is listed exactly as you have it, so that ain't the problem. Sorry...

As to your other question though have you tried using the gotolonglat command in a CEL file? An example would be:
gotolonglat {time 4.0 distance 5.0 longitude 90 latitude 0}

I don't know how you can do this in an SSC file, but I know that you can position yourself in almost any fashion you like using this function from within a CEL script.
If you like, you can go to the Celestia Motherlode HERE to download and then view some of the available scripts. Both CEL (for Brain-Dead users like myself) and CELX scripts might give you some better knowledge here.

Ah! CEL files... obviously they should be my next stop for position at least!

Thanks for the tip -

Julian

Julian wrote:Completely new to Celestia - just downloaded it for a specific task: I want to create a duplicate Moon on 180 deg out of phase with the real one... so that I can visualise the Earth-Moon relationship (I might have to make the Earth invisible to see through it, but I can live with that... don't ask!)

I don't know of any way you can persuade Celestia to draw a Moon which is on exactly the opposite side of the Earth from the real one. The Moon's CustomOrbit is an internal routine which uses a very long polynomial to describe the Moon's position for about +/- two thousand years.

If all you want is to look at relationships, though, you could comment out the Moon's CustomOrbit declaration (put a # at the beginning of the line) so that the EllipticalOrbit definition is used. The Moon won't be drawn in the correct position but you could then define an EllipticalOrbit for another Moon that's directly opposite that incorrect position.

If you position your viewpoint inside a planet, Celestia won't draw the planet's surface. You can see out through it as if it weren't there. That might be a reasonable alternative to redefining the Earth as Class "invisible". You'll probably want to tell Celestia not to render the atmosphere or clouds if you do that.

Julian I_Do_Stuff wrote:BTW - how do you tell Celestia I just want to look UP (at the Zenith)?

When you're standing on the object, select it twice. The second selection will pan the view straight down.
When you're looking down, press "*" to look back. Et voil?! you will be looking towards the zenith.

bdm - that's neat... I looked at the Help for keyboard commands but didn't spot that!

Selden - hmmmm... I had hoped to avoid specifying a new orbit because I have no idea how to do it! [hence the hope that I could re-use the internal custom orbit but just offset it in time. I wrote my own gravity simulator a while back and did the orbits but I did it by putting the objects in a specific place with specific velocity - using US Naval Labs orbit data. I don't know how to specify orbits )

I like the idea for making the Earth invisible - but I'd leave the atmosphere on so I could see the relationship...

Much appreciate all the input!

Ah... a bit of (my) stupidity uncovered... in trying to mod the Moon data in the ssc I was changing the Orbit Frame reference "EquatoerJ2000" to another epoch... no wonder it didn't like it!

Might try some scripting/Lua to place a viewpoint at coordinates derived the real moon's position... if I can sort the coordinate math or I'll make my duplicate moon with a custom orbit from JPL Ephemerides with altered dates

Thanks to all for the input.