"The news came like a clap of thunder, reverberating around the world," read the caption that accompanied this picture in the Nov. 29, 1963, issue of LIFE magazine. "In New York, at the moment she heard it, a woman recoils with universal dismay and disbelief."
Dismay. Disbelief. Two small words. Four short syllables. And yet what galaxies of emotion they encompass. All these years after President Kennedy's assassination, it's difficult for anyone who was not alive on that Friday afternoon to grasp what it felt like, what emotions were unleashed, upon first hearing news of the shooting. "The nation really did get turned inside out when Kennedy was killed," Adam Gopnik recently wrote in the New Yorker, and that deceptively simple image is as close to the reality of what happened — to men, women, kids, communities — as any that anyone is ever likely to craft. The world was not merely upended when JFK was shot; it was gutted. Transformed. Unhinged.
Pearl Harbor. The Challenger explosion. 9/11. Some moments stop time. There is before, and there is after, and the instant in between seems to last forever -- because, in a very real sense, it does. The instant is relived again and again by those who witnessed it, or those who survived it. It is commemorated by subsequent generations who were not there. It is a touchstone that changes everything, and everyone.
— Ben Cosgrove is the Editor of LIFE.com