さえわたる夜半のけしきに深山辺の雪のふかさを空にしるかな

saewataru yowa no keshiki ni miyamabe no
yuki no fukasa wo sora ni shiru kana

Fujiwara no Suemichi, Senzai Wakashu 447.

"As the chill of midnight enfolds everything,
I know in my heart the deep snow of the mountains."

sora ni shiru means 'indirectly/implicitly know'. I wonder where this expression comes from...

(translation by Zoltan Barczikay...)