See simple effect of lighting on complicated material? So I was trying to follow Andrew Price's tutorial on how to make the earth, per here.

So far, I have this: 
and while perhaps someone was trying to do that, this is not my goal.  I was trying to go for a distinct break between night and day - or at least, a more exaggerated one.



like this.  The problem is, I have no way of seeing the effects of my work except in rendered mode, and as you can see, that would take a lot of processing power.  All I can do now is put an even bigger sphere over the earth and render it in textured mode that way, but it still takes quite a bit of power in order to check it.  Is there a better way?
Where the output of the normal node is connected to the mix factor you can add a ColorRamp node. By bringing the white and black points close together you will reduce the amount of blending between the two. The location of the points will determine where the change happens.

You can also try different interpolations with B-Spline being a slow transition and constant being a very sudden change.



You can try using the material option for viewport shading to get a fast preview, or you could use simple diffuse nodes like shown here until you get the position right and then connect the rest of your nodes into the mix shader once you have it.