On a new layer I filled with two different colors of gradient - one for day, other for night.
step 2 of 3
I created a layer for stars, using soft brush and smudge tool.
step 3 of 3
I added a layer on the top of all; using elliptical marquee tool, I selectioned a circle, and filled with light grey. With pen tool, first I drew the moon's face, loaded selection and inverted, then I used burn tool to make the details of the face. Put some pink color on cheek and lips. After I did the same for Sun, filling with yellow and using, besides burn tool, liquify tool to make the rays.