Mount Wilson Observatory: Building the 100 inch Telescope

The 100-inch (2.5 m) Hooker telescope located at Mount Wilson Observatory, California, was completed in 1917, and was the world's largest telescope from 1917 to 1949. It is one of the most famous telescopes in observational astronomy of the 20th century. It was used by Edwin Hubble to make observations with which he produced two fundamental results which changed the scientific view of the Universe. Using observations he made in 1922–1923, Hubble was able to prove that the Universe extends beyond the Milky Way galaxy, and that several nebulae were millions of light-years away. He then showed that the universe was expanding.

Some Important Accomplishments:
1923: Edwin Hubble conclusively proves the Andromeda nebula to be external to the Milky Way galaxy
1929: Hubble and Milton Humason confirmed that the Universe is expanding, measure its expansion rate, and measure the size of the known Universe
1930s: Fritz Zwicky finds evidence for dark matter
1938: Dr. Seth Nickolson finds two satellites of Jupiter, referred to as #10 and #11.
1940s: Walter Baade's observations lead to the distinction of stellar populations and to the discovery of two different types of Cepheid variable stars, which double the size of the known universe previously calculated by Hubble