Did you notice how the “roof” radical 宀 factors into all those characters pertaining to the heavens? Here they are again:
空 (sora: sky)
宇 (U, ie: roof, heaven)
宙 (CHŪ, hiro: air, space, sky)
In A Guide to Remembering Japanese Characters, Kenneth Henshall says that 空 originally referred to a hole that would later be covered by a roof, forming a primitive dwelling. The concept of the covered space eventually extended to mean “sky.”
Henshall says 宇 originally meant “roof that completely covers.” This meaning then led to “eaves” and eventually to “heaven.”
Similarly, according to Henshall, 宙 first meant “that contained under a roof” (i.e., eaves). People later applied 宙 figuratively to “heaven” and by extension “space” and “sky.” Then the figurative meaning took over, and people no longer use 宙 to mean “eaves.”
In English, too, we speak of the “roof of heaven.” Also, gasoline prices can both go through the roof and go sky-high.
So I suppose the sky appears to be a sort of roof. Plus, one definition of “roof” is “the highest point.”
Then again, if it weren’t for cold, wet things dropping from the sky, we wouldn’t need roofs. So I’m not sure the sky is much of a roof after all!
