Blue. Out of the impossibly blue sky, the unthinkable came. A generation has grown up since the events of Sept. 11, 2001 — 22 Septembers ago — but those who can cast their mind back to that ...

So the sky isn’t blue, at least not always. It is a glow-in-the-dark night sky too, colored a mix of green, yellow, and red. Its colors result from scattered sunlight, oxygen, and sodium from ...