It's power that's strongly aspected to attractive forces.

And that gap in the cave ceiling is the path for it...

Hmph.

It's just like you said.

This makes the whole thing self-contained.

How did I not notice earlier?

..."You cannot choose the blue sky."

Right.

As energy goes, it's stronger, but...

Regulating the power of sunlight is much harder.

It's almost got a mind of its own.

In comparison, the moon's more stable and less willful.

It's a bit weaker, but if you've got a choice, you'd definitely go with that.