No rule for this one
No ending or word group predicts the article of Himmel. Learn it as a pair and say it out loud: der Himmel.
der Himmel
It's der Himmel. Himmel is masculine, plural die Himmel.
Der Himmel ist blau.
The sky is blue.
No ending or word group predicts the article of Himmel. Learn it as a pair and say it out loud: der Himmel.