Is schauen considered a verb of direction/motion?
You didn’t mention Clozemaster’s suggested English translation, which is “He looked to the heavens”. It’s the “to the” in English that requires accusative case here. It’s similar to saying “he walked INTO the room” (er ging ins Zimmer, accusative case, because he’s entering the room) as opposed to “he walked INSIDE OF the room” (er ging im Zimmer, dative case, because he’s neither entering nor leaving the room).
I suppose it might make sense in some context to say “er schaute im Himmel”, meaning something like “he looked around inside of heaven”, but I’m not 100% sure that that would make sense to a German.
But generally, yes, when you’re looking at or towards or to something in English, that will be “schauen auf etwas”, accusative case, or “schauen in etwas”, also accusative case, in German.
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Er schaute zum Himmel. - He looked towards the sky/heavens.
That means he is standing on earth, and his gaze is sent upwards in the direction of the sky. It is kind of a superficial look. It also means that you are not necessarily looking at the sky itself, just upwards in its direction.
Er schaute auf den Himmel. - He looked at(/upon?) the sky/heavens.
This does not really work, because “auf” kind of means “to look at the top surface from above”, which is hard to do when you’re below on earth. “Gott schaute auf die Erde (herab)” works of course 
Er schaute in den Himmel. - He looked at/to/into the sky/heavens.
“in den Himmel” implies that “Himmel” is like a box that has an inside into which you can look from outside.
Even though it does not seem to make much sense, it is often used. Probably because you can think of it as your gaze piercing through the surface and looking at the things that are “in” the sky, such as clouds and stars, and not really at the sky itself as a whole.
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