Tidal locking

[Amoeba]

I would have expected to find an answer to this question via seraches but no such luck... sorry if this quiestion is already answered, I tried to look.

I am not very good at celestia and I really only use it to illustrate fictional star systems. I only know the basic terms illustrated in some tutorials and don't know much about astronomy, physics or mathematics. Could anybody tell me in simple terms how to tidally lock a planet? I set the inclination to 90 degrees but I still have to get one pole somehow following the star.

Oh, also I'm not really fussed which pole ends up at the night-side or day-side because it's a barren rocky planet.

[selden]

The easiest way is to do nothing: delete all rotational specifications from the SSC file.
Tidal locking is the default.

Here's an example. The long axis of the planet "locked" always points toward the Sun.

Code: Select all

"locked" "Sol"
{
   SemiAxes [ 2007.4 197.2 190.7 ]
   Color [ 0 1 0 ]
   EllipticalOrbit { SemiMajorAxis 1 Period 1}
}


[Amoeba]

Ok thanks it worked to take out all rotational details.

I use a different layout for my code than you do:

Code: Select all

{

    Texture "jkeofim2.jpg"
    Color [ 0.75 0.8 0.75 ]
    Radius 3114.2
    Oblateness 0.006
    Albedo 0.1
   
EllipticalOrbit
    {
        Period 76.14
        SemiMajorAxis 0.352     # radius of orbit: multiple of Earth orbit size
    }
}

What does yours mean? Also is there a way to get my locked planet to still spin, but on its side so that one pole always faces the star?

[selden]

Your SSC looks fine.

I used SemiAxes so I could quickly verify that it stayed pointing in the right direction. With one dimension so much bigger than the others, it makes the planet look like a spear. I didn't feel like using a texture, and Coloring it green made it more interesting than plain white.

I think what you describe probably could be done with two coordinate systems -- an invisbile "parent" and the visible planet as its "child". I'll have to try a few things.

[selden]

Using a reference frame which explicitly is oriented toward the Sun seems to be the easiest:
Point its rotational axis toward the sun, and rotate around it.

Code: Select all

"ljkeofim2" "Sol"
{
   Texture "jkeofim2.jpg"
    Color [ 0.75 0.8 0.75 ]
    Radius 3114.2
    Oblateness 0.006
    Albedo 0.1
   
EllipticalOrbit
    {
        Period 76.14
        SemiMajorAxis 0.352 
    }

   BodyFrame {BodyFixed {Center "Sol"}}  # dummy body frame to be replaced by Modify statement
   UniformRotation { Period 24 } # rotational period around rotational axis (z) of reference frame

}

Modify "ljkeofim2" "Sol"
{
   BodyFrame {
    TwoVector {
        Center "Sol/ljkeofim2"
        Primary {
            Axis "z"
            RelativePosition { Target "Sol" }  # point rotational axis (z) of reference frame toward the Sun
        }
        Secondary {
            Axis "x"
            RelativeVelocity { Target "Sol" } # x axis of reference frame points along orbital path
        }
    }
  }
}


See the Celestia WikiBook for more info, although it's rather cryptic.
http://en.wikibooks.org/wiki/Celestia/Reference_Frames

[Amoeba]

Hi thanks for helping me out.

Wonderful, it's working great.

[selden]

You're very welcome.