December 21

1807: Joseph Fourier's memoir "On the Propagation of Heat in Solid Bodies," was read to the Paris Institute, and contained what we now call Fourier series, which he had worked upon since around 1804. 1878: Jan Łukasiewicz, who invented Reverse Polish Notation (RPN), was born. 1898: Scientists Pierre and Marie Curie discovered the radioactive element radium. 1968: Apollo 8, the first manned voyage to the moon, was launched. 1988: A Libyan terrorist's bomb exploded aboard Pan Am Flight 103, a Boeing 747, over Lockerbie, Scotland, killing 270 people. 1990: American aeronautical engineer Kelly Johnson, who founded the Lockheed Skunk Works, died. 2012: The Long Count of the Mayan calendar will cycle.