18 April

1955: On 18 April 1955, Albert Einstein died.

Born in Germany, Einstein developed the special and general theories of relativity and won the Nobel Prize for Physics in 1921 for his explanation of the photoelectric effect.

Recognised as one of the most creative intellects in human history, in the first 15 years of the twentieth century Einstein advanced a series of theories that proposed entirely new ways of thinking about space, time, and gravitation. His theories of relativity and gravitation were a profound advance over the old Newtonian physics and revolutionised scientific and philosophic inquiry.

Many ESA missions are either testing Einstein's theories or observing phenomena predicted by Einstein.

1756: On 18 April 1756, Jacques Cassini died.

Cassini was a French astronomer who compiled the first tables of the orbital motions of Saturn's satellites. Cassini was the son of Jean Dominique (Giovanni Domenico) Cassini. His father was a greatly esteemed astronomer, head of the Paris observatory, an academicien, and active in the cartographical projects of France.