Good try
Gareth
But Celestia uses a separate cloud texture that overlays and moves indipendantly of the Earth texture.
Celestia can use up to five separate textures
1:
Earth
2:
Specular map (Water reflections)
3:
Bump / Normal map (Raised terain)
4:
Night Lights (Earth City night lights)
5: Cloud map (you remember this one)
Have you tried Celestia yet ?
Something tells me you have only seen screen shots.