I was at a Welsh Heritage Week once, and John Albert told us that the "night" of Ar Hyd y Nos refers to old age. He papaphrased the song as

The stars shine on the sea.
The stars shine on their sister, the earth.
Like the stars, I will be with you all through the night.

Beautiful, don't you think?

(I forget John Albert's last name, but he teaches Welsh music and culture and is from Wales.)