In version 5.x the layer file tree on the left has "Primary Database" as the main layer that all other layers are appended to. Click on "Primary Database" and use the transparency scroll bar. It will dim until you get to the end when it will just disapear.
In version 6.2.0 in the layer file tree go to Primary Database>>>Satellite>>>Radar something like that. I don't remember exactly but if you see "Radar" or 'Satellite" you are on the right track. It might be under "Weather." Whatever it is it is the layer that controls the radar imagery that shows clouds. You get to that layer and click on the icon that looks like a cylinder just above the layer file tree. Then a scroll bar appears and you can use it to dim until the earth disapears. In version 6.2.0 it is highly unstable so if you are using for your own purposes at home I would suggest using version 5.x.