The easiest way to get a night scene is, of course, to go out at night. But just in case you have a shot you can't go out and reproduce at night, here's how to do it in Photoshop
The day shot:
Step 1: Create an adjustment layer
After you've opened your image click on the black&white circle in the adjustment layer and select 'Solid Color'. When a new dialogue box opens select a dark blue that's almost black and click OK.
Step 2: Bring your scene back
We now have a dark sky but the problem is that our scene is now a little too well hidden. To bring it back try changing the blending mode to multiply and/or reduce the opacity of the layer. If you still think it's a little too dark you can brush some of the original layer back in. To do this make sure the layer mask is highlighted, select your Paint Brush tool, make sure the foreground colour is set to black, reduce the opacity of the brush to around 20% and brush away.
Step 3: The moon is out
Take your image of the moon and cut it out of the background it sits on. The easiest way to do this is with the Quick Selection tool then go to Edit>Cut. Open your night scene, create a new layer (Layer>NewLayer) and paste the moon onto it (Edit>Paste or CTRL+P).
Step 4: Resize the moon
If the moon's a little on the large size go to Edit>Free Transform and adjust the corner anchors until you're happy then hit the enter key.
It's standing out a little too much so reduce the opacity as we did with the night sky. To give the moon a little more shape double click on the layer to open the Layer Stle box and find inner shadow. We set the angle to 30 degrees and opacity to 75% but this isn's set in stone. Close the box once you've made your changes.
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