Mount Wilson Observatory: Where Astronomy History Was Made

Mountain Wilson Observatory
Mount Wilson Observatory and the CHARA array.

 Gerard T. Van Belle, Public domain.

High in the San Gabriel mountains, north of the busy Los Angeles basin, the telescopes at the Mount Wilson Observatory have been watching the skies for more than a century. Through its venerable instruments, astronomers have made discoveries about the universe that have changed humanity's understanding of the universe.

Fast Facts: Mount Wilson Observatory

  • Mount Wilson Observatory has four telescopes, three solar towers, and four interferometer arrays. The largest telescope is the 100-inch Hooker Telescope.
  • One of the most important discoveries made at Mount Wilson in its early years was by Edwin P. Hubble. He found that the Andromeda "Nebula" is actually a separate galaxy.
  • The CHARA Array on Mount Wilson was used in 2013 to detect starspots on the star Zeta Andromedae, and in 2007, it made the first measurement of the angular diameter of a planet around another star.

Today, Mount Wilson remains one of the premier observatories in the world, despite the incursions of light pollution that threaten its clear views of the sky. It is run by the Mount Wilson Institute, which took over the administration of the observatory after Carnegie Institution for Science planned to shut it down in 1984. The site has been kept open and running again since the mid-1990s.

Mount Wilson and Observatory ridge aerial photo.
Mount Wilson and Observatory ridge aerial photo. Doc Searls, CC BY 2.0 

History of Mount Wilson Observatory

Mount Wilson Observatory was built on the 1,740-meter tall Mount Wilson (named for the early settler Benjamin Wilson). It was founded by George Ellery Hale, a solar astronomer devoted to studying and understanding sunspots, and was also one of the key people involved in building telescopes in the early 20th century. He brought the 60-inch Hale reflecting telescope to Mount Wilson, followed by the 100-inch Hooker telescope. He also built a 200-inch telescope at nearby Palomar Mountain south of Los Angeles. It was Hale's work that eventually inspired Griffith J. Griffith to give money for the Griffith Observatory in Los Angeles.

The observatory at Mount Wilson was originally built with funding by the Carnegie Institution of Washington. In more recent times, it has received funding from universities. It also solicits support from the public in the form of donations for the continued operation of the facilities. 

The 100-inch Hooker telescope, once the largest in the world. It is still in use today.
The 100-inch Hooker telescope, once the largest in the world. It is still in use today. Ken Spencer, CC BY-SA 3.0 

Challenges and Telescopes

Building world-class telescopes atop the mountain posed a number of challenges for the observatory's founders. Access to the mountain was limited by the rough roads and even rougher terrain. Still, a consortium of people from Harvard, University of Southern California, and Carnegie Institutions began to work on building the observatory. Two telescopes, a 40-inch Alvan Clark instrument, and a 13-inch refractor were ordered for the new site. Harvard astronomers began using the observatory in the late 1880s. Encroaching tourists and the owners of the land made things difficult, and for a time the observatory site shut down. The planned 40-inch telescope was diverted for use at the Yerkes Observatory in Illinois. 

Eventually, Hale and others decided to return to Mount Wilson to build new telescopes there. Hale wanted to do stellar spectroscopy as part of new advances in astronomy. After much back-and-forth and negotiations, Hale signed a contract to lease 40 acres at the top of Mount Wilson to build an observatory. In particular, he wanted to create a solar observatory there. It took several years, but eventually, four great telescopes, including the world’s largest solar and stellar instruments, would get built on the mountain. Using those facilities, astronomers such as Edwin Hubble made significant discoveries about stars and galaxies. 

The Original Mount Wilson Telescopes

The Mount Wilson telescopes were behemoths to build and transport up the mountain. Since few vehicles could make the drive, Hale had to rely on horse-drawn carriages to bring up the mirrors and equipment needed. The result of all the hard work was the building of the Snow Solar Telescope, which was the first one to be installed on the mountain. Joining it was the 60-foot solar tower, and then a 150-foot solar tower. For non-solar viewing, the observatory built the 60-inch Hale Telescope, and then finally the 100-inch Hooker Telescope. The Hooker held the record for many years as the world's largest telescope until the 200-inch was built at Palomar. 

Transporting a telescope up to Mount Wilson
The Hale telescope being transported up to the summit of Mount Wilson. Public domain.  

Current Instruments

Mount Wilson Observatory eventually gained several solar telescopes over the years. It has also added instruments such as the Infrared Spatial Interferometer. This array gives astronomers another way to study infrared radiation from celestial objects. In addition, there are two stellar interferometers, a 61-cm telescope, and the Caltech Infrared Telescope are also in use on the mountain. In 2004, Georgia State University built an optical interferometer called the CHARA Array (named for the Center for Angular Resolution Astronomy). It's one of the most powerful instruments of its kind. 

The top of the solar tower on Mount Wilson.
The top of the solar tower on Mount Wilson.  Dave Foc, CC BY-SA 3.0. 

Each piece of the Mount Wilson Observatory collection is equipped with state-of-the-art CCD cameras, detector arrays, and spectrometers and spectrographs. All these instruments help astronomers record the observations, create images, and dissect the light that streams from distant objects in the cosmos. In addition, to help correct for atmospheric conditions, the 60-inch telescope has been outfitted with adaptive optics that allow it to get sharper images.

Notable Observations at Mount Wilson

Not long after the largest telescopes were built, astronomers began flocking to use them. In particular, astronomer Edwin P. Hubble used the Hooker to peer out at distant objects that were (at the time) called "spiral nebulae." It was at Mount Wilson that he made his famous observations of Cepheid variable stars in the Andromeda "nebula", and concluded that this object was really a distant and distinct galaxy. That discovery in the Andromeda Galaxy shook the foundations of astronomy. Then, a few years later, Hubble and his assistant, Milton Humason, made further observations that proved the universe is expanding. These observations formed the basis of the modern study of cosmology: the origin and evolution of the universe. Its views of the expanding universe have informed cosmology's constant search for an understanding of such events as the Big Bang

Edwin P. Hubble, the astronomer who used the Mount Wilson 100-inch telescope to observe distant galaxies. His work led to the discovery of the expanding universe.
Edwin P. Hubble, the astronomer who used the Mount Wilson 100-inch telescope to observe distant galaxies. His work led to the discovery of the expanding universe. Public domain 

Mount Wilson Observatory has also been used to look for evidence of such things as dark matter, by astronomer Fritz Zwicky, and further work on the different types of stellar populations by Walter Baade. The question of dark matter has been studied by other astronomers as well, including the late Vera Rubin. Some of astronomy's most prominent names have used this facility over the years, including Margaret Harwood, Alan Sandage, and many others. It's still heavily used today and allows remote access to observers from around the world. 

vera rubin
Dr. Vera Cooper Rubin in 1970, working on measuring galaxy rotation rates. Vera Rubin

Mount Wilson in the Public Eye

The administration of Mount Wilson Observatory is also dedicated to public outreach and education. To that end, the 60-inch telescope is used for educational observing. The grounds of the observatory are open to visitors, and there are weekend observing sessions and tours available as the weather permits. Hollywood has used Mount Wilson for a filming location, and the world has watched several times via Webcam as the observatory was threatened by wildfires.


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