Post pictures if possible and why you feel it sums up the last century.
I couldn't pick just one so I'm doing two.
"Taken on August 23, 1966, this grainy image of the Earth - captured as the Lunar Orbiter 1 began to dip behind the moon - was humanity's first glimpse of our planet's place in the universe." I always felt awed at how Earth was floating in the inky blackness of space. It's just so removed from our everyday experiences down on the ground. Beautiful.
Against that, when we learned to unleash the power of the atom, July 16, 1945.