Perched atop a lofty peak in the San Gabriel Mountains of California sits a historic treasure, one of the great astronomical tools of the 20th century: the Mt. Wilson Observatory. For more than 100 years, it has been a centerpiece for major astronomical discoveries, playing a pivotal role in our understanding of the universe. The observatory, with its rich history shaped by devoted scientists, advanced technology, and revolutionary discoveries, stands as a testament to humanity’s relentless pursuit of knowledge.

The story of the Mt. Wilson Observatory begins with the visionary astronomer George Ellery Hale. Recognizing the importance of location, Hale selected Mt. Wilson for its elevation of 5,710 feet and its proximity to the Pacific Ocean, which provided consistently stable, clear skies—perfect conditions for astronomical observations. In 1904, he installed the first telescope at the site, the Snow Solar Telescope, specifically designed for studying the Sun. This telescope marked the beginning of groundbreaking solar research at the observatory and set the stage for future advancements in astronomy.

Many brilliant minds walked the halls and explored domes of Mt. Wilson, but few shone as brightly as Edwin Hubble. In the 1920s, using the Hooker Telescope – then the largest in the world – Hubble made two groundbreaking discoveries:

Before Hubble’s observations at Mt. Wilson, the prevailing belief was that our galaxy, the Milky Way, constituted the entirety of the universe. The existence of other galaxies was not yet confirmed, and what we now know as galaxies were often referred to as “nebulae” and thought to be part of the Milky Way.

Hubble’s groundbreaking discovery in 1923-1924, using the 100-inch Hooker telescope at Mt. Wilson, revealed that the Andromeda Nebula (now known as the Andromeda Galaxy) was far beyond the Milky Way, providing the first concrete evidence that the universe extended far beyond our own galaxy. This discovery fundamentally altered our understanding of the cosmos, leading to the realization that the universe is vast and filled with countless galaxies.

Using the powerful Hooker telescope once again, Hubble carefully observed distant galaxies and made a groundbreaking discovery: these galaxies were moving away from us. Even more astonishing was that the farther a galaxy was, the faster it was receding. This finding provided clear evidence that the universe itself was expanding. Hubble’s revelation shattered the long-held belief in a static universe and laid the groundwork for the Big Bang theory, suggesting that the universe had a specific beginning and has been expanding ever since. Through Hubble’s meticulous observations, humanity gained a new understanding of a dynamic, ever-evolving cosmos, far more vast and mysterious than anyone had previously imagined.

Many other scientists have also made major discoveries at Mt. Wilson. One luminary, Harlow Shapley, used the observatory to gauge more specifically our place in the Milky Way. Before Shapley, Earth was believed to be at the galaxy’s center. However, through his observations of globular clusters, he pinpointed our more humble location on a distant spiral arm.

Another notable scientists who made significant contributions at Mt. Wilson Observatory was Walter Baade. Baade, a German-American astronomer, played a key role in refining our understanding of the universe by studying stars in different populations. During World War II, when Los Angeles experienced blackout conditions, Baade took advantage of the clearer skies at Mt. Wilson to observe celestial objects with unprecedented clarity. He discovered that there were two distinct types of stars in the Milky Way, which led to the realization that galaxies had different stellar populations. This breakthrough allowed Baade to correct the scale of the universe, doubling the previously estimated size of galaxies and distances to them. His work helped refine Hubble’s expanding universe theory and provided a deeper understanding of the evolutionary stages of stars. Baade’s observations were critical in the advancement of modern cosmology and our comprehension of the vastness of space.

Instruments of Enlightenment

Over the years, Mt. Wilson has housed a suite of powerful telescopes:

• The Hooker Telescope: At 100-inches, it was the world’s largest when it was installed in 1917. It’s the very instrument Hubble used for his revolutionary work.
• The Snow Solar Telescope: The observatory’s inaugural instrument remains crucial for solar studies.
• The CHARA Array: The Center for High Angular Resolution Astronomy array is an impressive configuration of six telescopes that function as an interferometer. It allows for sharper images of stars than even the Hubble Space Telescope. The CHARA Array has a spatial resolution equivalent to a single telescope 331 meters (over 1000 ft) in diameter. Light from each of the six telescopes is transported through fiber optics to a special beam-combining room. 

Not only has Mt. Wilson been instrumental in observing distant stars, but it also has a unique device: the Snow Horizontal Solar Telescope. This apparatus, combined with the spectrograph, was used to study the sun’s magnetic fields. It has since been fundamental in understanding solar cycles and the impact of solar phenomena on Earth’s climate.

In 2020, the Bobcat Fire, the second largest fire on record in Los Angeles County to date, burned over 115,000 acres and was active for more than three months. Annually, the team at Mount Wilson Observatory takes measures against potential forest fires, removing fire-hazardous invasive plants and ensuring their extensive water tanks are full for the fire suppression system. Just a few months prior to the blaze, they had fitted new high-capacity hydrants. These proactive steps played a pivotal role in safeguarding the Observatory when the Bobcat Fire flames approached within a mere 20 feet of its perimeter. A dozen fire squads, each consisting of 40 to 50 firefighters from various units, tirelessly worked day and night to protect this cherished landmark.

Visitors to the Mt. Wilson Observatory have a rare chance to not only tour the grounds but also look through the same telescopes that revolutionized astronomy. Public “Telescope Nights” offer the exciting opportunity to observe the night sky through the famous 60-inch or 100-inch telescopes, the latter being the largest in the world open to the public. These sessions allow people to view celestial objects like planets, star clusters, and nebulae in stunning detail. Reservations are necessary for these events, as spots fill up quickly due to high demand. Additionally, private group sessions and special events can be arranged, providing an unforgettable, up-close experience with the universe. Guided tours are also available for those who want to dive into the observatory’s rich history, tracing the steps of astronomers who made some of the greatest discoveries of the 20th century.