O Radiant Light

So, we were going to sing "O Radiant Light" (#184 in Christian Prayer). I chanced to ask "What mode is this in?"

I don't know modes, but it starts and ends on A and has no sharps and flats, so I figured it was in the key of A-minor.  (That would be Aeolian, fyi, according to Thaddeus).

But Thaddeus pipes at as easy as you please and says "D - Dorian."  Which, in fact, is the case. If you play the background chord (drone), it is definitly D-min., which, would ordinarily have a B-flat. But the tune actually has a B. When you raise the sixth in a standard minor key, it becomes Dorian!

The other chants that I looked at all begin and end on the tonic.  Admittedly, I took a very small sampling (3 or 4). Still, it is odd that ORL starts and ends on the fifth and only touches the tonic at the end of the first line.

I have always thought that ORL ended without really ending. Now I know why.