John 1,51

"You will see the sky opened and the angels of God ascending and descending on the Son of Man."

Teaching

Our visible cosmos is akin to the material appendix of an immense intangible world, writes Father Jean-Michel Garrigues, O.P.

"In the beginning, God created the heavens and the earth," states the Bible in Genesis 1,1. The heavens are not the sky we see, which is created only the second day (Genesis 1,6-8). The heavens are the "all things invisible" in the Creed, the realm of the ethereal spirits we name "angels," as they are God's envoys to mankind (in Greek, Angelos means "messenger").

Biblical authors mention a nine-folds celestial hierarchy, from the lowest to the highest:  angels, archangels, principalities, powers, virtues, dominions, thrones, cherubim, and seraphim. 

Pseudo-Dionysius, a fifth-century theologian, ordered the classes of angels into three Hierarchies, Spheres or Triads, with each Hierarchy containing three Orders or Choirs. Those Hierarchies constitute a spiritual ladder, which is used by the Divine Light to reach mankind, and by the human soul to attain God. If the Church never officially validated this classification, the Bible clearly hints at the fact that the celestial world is not uniform: it is composed of a myriad of angels, to each its function. 

Prayer

Ineffable Creator,
Who, from the treasures of Your wisdom,
have established three hierarchies of angels,
have arrayed them in marvelous order
above the fiery heavens,
and have marshaled the regions
of the universe with such artful skill,

You are proclaimed
the true font of light and wisdom,
and the primal origin
raised high beyond all things.

Pour forth a ray of Your brightness
into the darkened places of my mind;

Saint Thomas of Aquinas.