Wasn't it Oleguer Vilella who wrote:
>I'm trying to move this object along the z axis, but when I write "translate 
><5, 0, 9>", it also moves along the y axis.
>I've rotated on the z and y axis. Could be there the problem?
>
>One thing I could do is if I translate it along the z axis 9 units, move it 
>in the same time -9 units along the y axis.

The object is moving along the actual z axis. The only odd thing is that
you've placed the camera high overhead, so you're getting an overhead
view, and the z axis is up the screen.

If that's not clear to you, try temporarily adding a visual
representation of the three axes to the scene
  cylinder {-x*15,x*15 0.3 pigment {rgb <1,0,0>}}
  cone     {x*15,0.6, x*18,0 pigment {rgb <1,0,0>}}
  cylinder {-y*15,y*15 0.3 pigment {rgb <0,1,0>}}
  cone     {y*15,0.6, y*18,0 pigment {rgb <0,1,0>}}
  cylinder {-z*15,z*15 0.3 pigment {rgb <0,0,1>}}
  cone     {z*15,0.6, z*18,0 pigment {rgb <0,0,1>}}
and you'll see that a motion along the z axis is in the direction of the
z-axis arrow (blue) which is in a sort of nort-north-west direction on
the screen.


If you change your camera position to something more usual, like 
  camera { location  <7, 3, -10> look_at   <5, 3, 0> }
then motion in the z direction moves away from the camera.


If you want to retain your current camera position and move the object
away from the camera then you can calculate the camera view direction
like this

  #declare Direction =  vnormalize(<5, 3.0,  0.0> - <7, 25, -10>); 

And then add a movement in that direction to the translate.

  translate <5, 0, 0> + Direction*9

-- 
Mike Williams
Gentleman of Leisure

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