I’m trying to make the Earth like in the Blender Guru tutorial (but in cycles).
As you can see it’s not working how it should. It should be that on the light side it displays normal colour and on the dark side it displays the night time colours.
How can I fix this?
Thanks
The Earth-Lights image comes with way too much ambient light, you have to get rid of the blue of the sea/land and just have the city lights.
To do that, stick an RGB curves node between the city lights texture and it’s emission shader. Then play around with the curve until you get only the brightest city lights.
You’re taking the wrong route. Normally you can’t just take BI techniques and apply them directly in Cycles. For example, you can’t take BSDF output and stick it into Color input and expect it to work 
You need to rethink your approach and find another way of determining shadowed parts of your sphere. Possibly even rendering twice with each texture and then compositing.
You could try an “Add Shader” node instead of a “Mix Shader” node.
Also, it appears you are using a diffuse BSDF output to drive the mix factor. Use an RGB or Value node from the Input category instead. You are forcing a lot of extra calculations instead of just looking up the value/colour.
The noodle dot socket colour indicates the type of data they are passing. Some will automatically do a conversion when joined to another type (yellow rgb dots can be connected directly to grey value dots for example) but as a general rule they should be connected to other sockets of the same colour.
