Adding to the night sky

I used this composite in my recent Photoshop workshop that I conducted in my home to demonstrate how to add the moon, fireworks, or lightning into a twilight or night sky.  It is easier than you would think.  There is no need to precisely cut out any of these objects.  Simply use the lasso tool to make a rough selection around any of them, including the original dark sky, and then when you paste (Edit > paste) the object(s) into the sky choose the 'lighten' blend mode in the layers palette.  The dark sky of the moon, fireworks, or lightning disappears and you've retained all of the detail you need to make the composite look believable.  This is a shot of the government buildings in London with the Tower Bridge in the distance.