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See note [4] on the first sentence of 13:2 , which seems to have a similar meaning.
The signs of the heaven include the galaxies, planets, stars, etc., all of which have an orbit. Our planet orbits the sun, the sun orbits the Milky Way Galaxy, and the Milky Way Galaxy orbits the Virgo Supercluster, etc.
Canopy well guarded: the heavens form a canopy that is secure from falling down: they also form a sublime spectacle and a Mystery that man can only faintly reach. Cf. also xv. 17.