Ernst Waldfried Josef Wenzel Mach, probably best known today for the "Mach number," a measure of velocity that obviously takes his name, is slightly less well known for the "Mach" lunar crater, and the optical illusion "Mach bands." It is even more often forgotten that Einstein credited Mach as "the precursor of the general theory of relativity." Oddly, the "Mach principle," to which Einstein was referring was something Mach himself never formalized in writing, preferring instead to relate the concept verbally to students and colleagues, though Einstein was probably also referring to this passage written by Mach:

The natural investigator must feel the need of further insight - of knowledge of the immediate connections, say, of the masses of the universe. There will hover before him as an ideal an insight into the principles of the whole matter, from which accelerated and inertial motions result in the same way.

Mach served in the upper house of the Austrian parliament after leaving the University of Vienna and in 1916 died in Munich at the age of 78 on February 19th. That makes today the day of his death, the day after his birthday, and the fourth anniversary of finem respice.