PASADENA, Calif. (AP) — Astronomers are celebrating the centennial of the 100-inch (2,540-millimeter) Hooker Telescope at Southern California‘s Mount Wilson Observatory .

The reflector telescope’s first observations occurred in 1917 on the night of Nov. 1 and into the morning of Nov. 2.

At the time it was the world’s most advanced telescope and was used by astronomer Edwin Hubble to prove that the universe is expanding.

The telescope is named for businessman John D. Hooker, who funded the instrument when it was proposed by astronomer George Ellery Hale.

Creating the massive, 100-inch-diameter glass disk for the telescope’s mirror took years before it went into use on Mount Wilson, a peak at an elevation of more than 5,700 feet (1,737 meters) in the San Gabriel Mountains above Pasadena.