Two worlds or two realms co-existing and occasionally interacting with each other. The natural and the supernatural. What does that interaction look like? More to the point, what did it look like at that first Christmas?
Consider Joseph and Mary. (Matt. 2 & 3) Here was an engaged couple looking forward to their marriage and life together as husband and wife. What could be more natural than that scenario? But that natural setting is suddenly invaded by a divine emissary from the supernatural realm.
Gabriel is sent by Almighty God with a message for Mary. The next thing we know is that Mary is a pregnant virgin! Now what could be more supernatural than this turn of events? Joseph's natural world is repeatedly invaded by dreams in which he gets directions from the supernatural realm - from another divine emissary, "an angel of the Lord". Nothing is 'ordinary' anymore.
Consider a group of shepherds on nightshift protecting their sheep (Luke 2). Such a scene would hardly ever attract a second glance. Their world was (I think) dull, boring and predictable. Because they were looking after sheep, every day was pretty much the same.
Suddenly, their world was invaded by the members of another world. It was full on. However the invading army was made up of angels and, assuming they would live through this invasion, the former world of the shepherds would be gone forever. This time the invasion was in the form of a musical extravaganza!!
It began as a solo number but developed into a massive, divine choir. The idea or concept of another kingdom running 'alongside' ours and that there is (or can be) some measure or awareness of interaction between the two kingdoms explains so much.
Consider a small group of astrologers pursuing their life-long passion as they gaze at the stars and the immensity of the Universe. Nothing unusual here. Until one of them called to the others and asked something like, "Come and have a look at this. What do you make of it?"
It was a question that set them on a journey to a distant land where, according to their ancient writings, a king was to be born. King Herod was furious that there might be a threat to his authority but controlled himself well enough to enlist these astrologers on a mission to find this new king and bring him news so that he might worship him, too. Yeah, right!!
But, yet again, a divinely-initiated dream from the unseen kingdom warned these eastern visitors not to go back to Herod. Heaven and earth are interacting again. It's what happens when two worlds collide.