Mount Wilson’s 100-inch telescope turns 100

By | November 1, 2017

One hundred years ago today, astronomers at Mount Wilson Observatory got their first look through the observatory’s newest instrument, a 100-inch telescope that for a while was the largest in the world. At the time, this location in the San Gabriel Mountains near Pasadena was a great dark-sky site, and the telescope would prove to be tremendously useful.

Although most public activities at the observatory are suspended because of a recent fire, there will be an anniversary celebration this Saturday, November 4, from 2pm to 10pm. The observatory’s historic telescopes (the 100-inch and a slightly older 60-inch telescope) will be open to the public, and guides will be available to describe their contributions to astronomy. After dark, weather permitting, visitors will be able to take a look through the 100-inch telescope (first come, first served). If you’re in the Los Angeles area, check it out. Note that the observatory is at 5715 feet, so dress warmly. Because the observatory is otherwise closed, the Cosmic Cafe will not be open.

View of twilight sky from inside the telescope dome

100-Inch Hooker telescope at Mount Wilson Observatory. Photograph by Tracie Hall (who also took the photograph appearing as a thumbnail); shared under a Creative Commons license.

The telescope was the brain child of the observatory’s founder, George Ellery Hale, who contributed greatly to astronomical observation over the course of his life. It’s named for his John D. Hooker, a businessman who put up the money for the 100-inch glass blank and a shop in which to shape it into a mirror for the telescope. On the MWO’s web site, you can read about the construction of the telescope and the arrival of the huge mirror on the mountain. (I love the idea of this tremendous piece of glass being moved up to that elevation by 1917 technology, and the excitement everyone must have felt.)

The telescope has a wonderful history. Observations by the 100-inch provided the data that Harlow Shapley used to figure out the size of the Milky Way galaxy. Then Edwin Hubble found that the Andromeda nebula, as it was then known, was so far away that it must be a separate galaxy (an “island universe,” in the lingo of the time), and not part of our own galaxy. This discovery resolved an intense debate among astronomers. Hubble and Milton Humason later found that the entire universe was expanding.

The telescope’s valuable work didn’t stop there, but light pollution eventually overcame the site, and the optical telescopes at the observatory are mostly used for public viewing these days. However, one solar telescope is still in operation, and there are two infrared interferometers, which use multiple telescopes to gather data at infrared wavelengths.