Spectators were also holding “watch-the-bomb parties” in Hawaii, as the countdown was broadcast over shortwave radio. Photographers aimed their lenses toward the horizon and debated the best camera settings for capturing a thermonuclear explosion in outer space. It turned out that the blast—a 1.4 megaton bomb, 500 times as powerful as the one that fell on Hiroshima—was not subtle. “When that nuclear weapon went off, the whole sky lit up in every direction. It looked like noon,” “It looked as though the heavens had belched forth a new sun that flared briefly, but long enough to set the sky on...