This is what we will be making -
This tutorial was inspired the excellent Blender Earth tut by Andrew Price - http://www.blenderguru.com/create-a-realistic-earth/
The textures I used are from his tut , although they are freely available from NASA`s website. I did reduce them further in size to 4000x2000 to make them more manageable in PDN.
As the textures are so large , if you have a less powerful PC you might want to reduce them in size even further to ease the strain . Just make