Phobos and Deimos

[Yorick]

Why are Phobos' and Deimos' orbits retrograd in Celesetia? Is it the case in the reality?

[selden]

They're certainly prograde in my copy of Celestia: travelling counterclockwise when seen from above the north pole of Mars.

Perhaps you were accidentally watching from the south celestial pole?

[Yorick]

They're certainly prograde in my copy of Celestia: travelling counterclockwise when seen from above the north pole of Mars.

Perhaps you were accidentally watching from the south celestial pole?

Mars rotates counterclockwise and moons orbit clockwise. I watch it moretimes.

[Yorick]

It seems it's only my problem.

[selden]

Then someone has changed the entries for their orbits in your solarsys.ssc

Below I've included the definitions that are in my copy. Note that "Inclination" is a small positive value. That's what makes them prograde. If they were retrograde, the inclination would be between 90 and 270 degrees. See the orbit of Neptune's moon Triton for an example of a retrograde orbit.

One easy way to verify this is to change your viewpoint so you are looking at the south celestial pole. Turn on "Label Moons", "Orbits" and "Celestial Grid" in the "Render" "View Options" menu. Center the planet inside the -75 circle. Back off so you can see the orbits of the moons. Type "l" several times so that time is running fast.

Here's a Cel:// url for watching Phobos and Deimos:

cel://Follow/Sol:Mars/2003-03-25T22:36:16.01?x=LSQHZnIAGrGyDA&y=6lOYC2kaTNT//////////w&z=TVU0PmMLlFUV&ow=0.296725&ox=0.464934&oy=0.699557&oz=0.454325&select=Sol:Mars&fov=45.000000&ts=10000.000000&rf=38499&lm=4

And here's one for Triton:
cel://Follow/Sol:Neptune/2003-03-26T23:03:50.07?x=AMECY7BHfUv1DQ&y=CpUmaCGChlA&z=VZC4jKoWafZmAQ&ow=0.421356&ox=0.629573&oy=0.550387&oz=0.350959&select=Sol:Neptune&fov=45.000000&ts=10000.000000&rf=38499&lm=4


I hope this helps.

Code: Select all

"Phobos" "Sol/Mars"
{
   Mesh "phobos.3ds"
   Texture "phobos.jpg"
   # BumpMap "phobosbump.jpg"
   # BumpHeight 5
   Radius 11

   EllipticalOrbit
   {
   Period         0.318910
   SemiMajorAxis  9377.2
   Eccentricity   0.0151
   Inclination    1.075
   AscendingNode  128.694   # J2000
   ArgOfPericenter 213.804  # J2000
   MeanAnomaly    191.771   # J2000
   }

   Obliquity              1.075
   EquatorAscendingNode 128.694
   RotationOffset        45.575

   Albedo         0.07
}

"Deimos" "Sol/Mars"
{
   Mesh "deimos.3ds"
   Texture "deimos.jpg"
   Radius 6

   EllipticalOrbit
   {
   Period            1.262441
   SemiMajorAxis     23463.2
   Eccentricity      0.00033
   Inclination       1.793
   AscendingNode    25.229
   ArgOfPericenter 208.213
   MeanAnomaly     344.128
   }

   Obliquity              1.791
   EquatorAscendingNode  25.229
   RotationOffset       192.341

   Albedo         0.08
}


[Yorick]

Thank you. I will try it if I go home.