Yes, I know what a virgin is.  And yes, I know that "yon" is short for younder "over there,"  that is, the lyric author is seeing the manger scene from a distance and sees Mary over there.   But what in the world is "round"?   

My college students in class yesterday really had no idea, but they assumed she was indeed round because of being very pregnant.   Not so fast!  The next lines are  "...mother and child, holy infant so tender and mild..."   Ok, so she cannot still be pregnant, since her baby (Jesus) is described as holy, tender, and mild.

So again, one asks, what in the world does "round" mean.  I posit it is an adjective describing Mary.  And behold, in the American College Dictionary there are 20 different adjectival uses of this word, and #15 is "complete" and #18 is "plain."  So thus, non-poetically, the author is saying, in essence,  

"Silent Night, its a holy night, quite calm and surprisingly bright (considering its night time), over there is the plain and now complete Mary, who has given birth to the Son of God, no less."

Now of course I may be wrong on all this, and some you with PhD's in sociomusicology may be aghast at my ignorance, but hey, I'm satisfied that my interpretation is plausible.

All, may you often think of the AWE of what took place that holy night!.

AuthorLeonard Kageler