Dictionary definition: Sky: region of the atmosphere and outer space seen from earth. Heaven: where various religions regard as where God and angels reside. Where people go when they die etc. But in general when you go outside and look up, that’s the sky. “The sky is so dark today; it’s definitely going to rain.” “The weather calls for clear skies today.” “Look up at the sky, the moon is so bright.” Heaven is used in the religious concept and also we use it to mean paradise or something really nice. “That massage was amazing, I felt like I was in heaven.”

Entiendo por qué preguntas esto, porque tuve el problema opuesto cuando estaba aprendiendo español. Usas "el cielo" para ambas cosas. Esto fue confuso para mí al principio. Pero sí, "the sky" es lo que realmente miras cuando sales de tu casa. Y "heaven" es donde todas las mujeres son hermosas y todas quieren bailar conmigo. (jajaja)

Sky is physically we can see like it's color blue , it change if weather change , like storm it's color gray, heaven is something that we cannot see by our naked eye , say for example when we pass away many say she will be now in heaven

You can see the sky, you can't see heaven. Clouds and birds are in the sky, God is in heaven. cielo vs cielos

One is real one is fake ;)