The writer recalls very vividly the blue aspect of the sky as seen from the top of the San Francisco Mountains in Arizona, which have an altitude of 13,000 feet above the sea; and in an ascent of ...

Night sky photographer Ruslan Merzlyakov captured bright blue-white noctilucent clouds dancing over Nykøbing Mors in Denmark just before dawn on July 1.

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 ...

Night sky photographer Ruslan Merzlyakov captured bright blue-white noctilucent clouds dancing over Nykøbing Mors in Denmark just before dawn on July 1.