5 For what fault have they to find with the vast creation of God?— they, who ought to have been struck with amazement on beholding the vaultings of the heavens: they, who ought to have worshipped Him who reared the sky as a dome, who out of the fluid nature of the waters formed the stable substance of the heaven. For God said, Let there be a firmament in the midst of the water. God spoke once for all, and it stands fast, and falls not. The heaven is water, and the orbs therein, sun, moon, and stars are of fire: and how do the orbs of fire run their course in the water? But if any one disputes this because of the opposite natures of fire and water, let him remember the fire which in the time of Moses in Egypt flamed amid the hail, and observe the all-wise workmanship of God. For since there was need of water, because the earth was to be tilled, He made the heaven above of water that when the region of the earth should need watering by showers, the heaven might from its nature be ready for this purpose.
Source: Catechetical Lectures (New Advent)