Well it might not be raw magical might that lets them do it. It's basically their jobs and destinies to control the sun and moon. Even if it is, it pales in comparison. While Bill seems perfectly capable of directly manipulating or f*cking up celestial bodies at his leisure.
He showed Ford what he'd do all of reality if he got loose from Gravity Falls. One of his first acts was scooping a chunk out of the Earth like it was ice cream in a bucket.