What does this line by Walt Whitman mean?

Astonish may be defined as to fill with amazement. So the line may be Whitman asking if you are filled with wonder when everyday like darkness of night is dispelled. It is never late, doesn't fail. It is a part of nature which is consistent.So it is a miracle which happens everyday. God replaces the darkness with light. It is a wonder when you realize how well ordered nature is.