What does «cerner» mean in Spanish?

To sift, to separate with a sieve the finer parts of a thing from the coarse ones.
Observe, examine.

Examples of use in Spanish: "cerner el horizonte".
(cernirse) Keeping birds and planes in the air.
(cernirse) Threaten.

Examples of use in Spanish: "se cernía el infortunio sobre la familia".