On this day in 1917 - July 1st - the 100" mirror for the Hooker telescope arrived at Mt Wilson Observatory.
This telescope was the biggest in the world for three decades. Edwin Hubble calculated the distance to Andromeda from a Cepheid variable he discovered there with the telescope. It showed Andromeda was a separate galaxy. Hubble and Milton Humason also observed the redshifts of a number of galaxies and found that they were moving away from us.
Philosophiæ Naturalis Principia Mathematica (Latin for 'Mathematical Principles of Natural Philosophy') states Newton's laws of motion and of universal gravitation. It's regarded as one of the most important works in the history of science.