Fixing normals on projected texture.

I am trying to use the following code in my sky shader to project a moon texture on to my sky dome mesh. It's working perfectly except for the normals. I have no idea how to transform the normals to match the new orientation of the texture.

             float4 moonVector = mul(_MOON_MATRIX,IN.vertPos*1);
             uv = moonVector.xy + .5;
             float4 moon = tex2D (_MoonTex,uv) * saturate(-moonVector.z);
             float3 moonNorm = UnpackNormal(tex2D (_MoonNormal,uv));

If anybody could give me any advice I would greatly appreciate it, I'm completely stuck with this one!