The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,710-foot (1,740 m) peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.

The 100 inches (2.5 m)Hooker telescope saw "first light" on November 2, 1917. The 100-inch telescope’s most important discovery was American astronomer Edwin Hubble’s determination of the distance to the Andromeda Nebula in 1924. He showed that the nebula lay beyond the bounds of the Milky Way Galaxy and hence was a galaxy in its own right. Then in 1929, building on the work of American astronomer Vesto Slipher, Hubble and his assistant Milton Humason demonstrated that galaxies were moving away from one another. This movement is the expansion of the universe. Throughout the 1930s and ’40s, Hubble and his associates used the 100-inch reflector to refine the extragalactic distance scale and to probe the large-scale structure of the universe.

The Hooker's reign of three decades as the largest telescope came to an end when the Caltech-Carnegie consortium completed its 200-inch (5.1 m) telescope in 1948 at Palomar Observatory, 90 miles (140 km) south, in San Diego County, California.