Aug 10, 2023 · By Erik Olsen

Mt. Wilson Observatory, Where We Learned the Universe Is Expanding

High above Los Angeles, Mt. Wilson Observatory is where Edwin Hubble discovered other galaxies and the expanding universe. It nearly burned in 2020, and you can still look through its telescopes.

Updated Oct 12, 2026

---

Drive up the Angeles Crest Highway from La Cañada Flintridge, take the turnoff and keep climbing, and you’ll end up at a cluster of white domes on top of the San Gabriel Mountains. That’s the Mt. Wilson Observatory, and it’s where we found out how big the universe really is.

It started with George Ellery Hale, an astronomer with a talent for raising money and building enormous telescopes. Hale picked Mt. Wilson because it’s 5,710 feet up, and the air coming in off the Pacific is unusually calm and steady, which makes for sharp views of the sky. In 1904, he set up the observatory’s first instrument there, the Snow Solar Telescope, built to study the sun.

George Ellery Hale

Plenty of great astronomers worked on that mountain, but the most famous was Edwin Hubble. In the 1920s, using the 100-inch Hooker Telescope, then the largest in the world, he made two discoveries that changed everything.

First, some context. A hundred years ago, most astronomers thought the Milky Way was the whole universe. Those fuzzy spirals they saw through their telescopes, called “nebulae,” were assumed to be clouds of gas inside our own galaxy.

In 1923 and 1924, Hubble found stars in the Andromeda Nebula whose brightness let him work out how far away they were, and the answer was way too far to be inside the Milky Way. Andromeda was its own galaxy. Suddenly the universe was full of galaxies, maybe billions of them.

Expanding Universe

Then he went further. Measuring distant galaxies with the same telescope, Hubble found that nearly all of them were moving away from us, and the farther away a galaxy was, the faster it was going. That meant the universe is expanding. It wasn’t a fixed, unchanging place, and if it’s expanding now, it must have been smaller in the past. That idea led to the Big Bang.

Edwin Hubble

Hubble wasn’t the only one. Harlow Shapley used Mt. Wilson to map globular clusters, tight balls of old stars, and showed that the sun is nowhere near the center of the Milky Way. We’re off in the suburbs.

During World War II, Los Angeles was blacked out at night, and the German-born astronomer Walter Baade took advantage of the unusually dark skies. He found that galaxies contain two different populations of stars, younger and older. That distinction later let him correct a key yardstick astronomers used to measure distances, and when he did, the known universe roughly doubled in size.

The telescopes

Over the years, Mt. Wilson has collected some famous instruments:

• The Hooker Telescope: At 100 inches, it was the world’s largest when it opened in 1917, and it’s the one Hubble used.
• The Snow Solar Telescope: The observatory’s first instrument. Paired with a spectrograph, it was used to study the sun’s magnetic fields and helped scientists understand the solar cycle.
• The CHARA Array: Six telescopes linked together so they work like one giant telescope about 1,000 feet across. Light from each is piped to a central room and combined, producing sharper images of stars than even the Hubble Space Telescope.

Mt. Wilson Observatory

In 2020, all of it nearly burned. The Bobcat Fire, one of the largest fires in Los Angeles County history, burned more than 115,000 acres over three months, and the flames came within 20 feet of the observatory. It survived because the staff had prepared. Every year they clear out flammable invasive plants and keep their big water tanks full, and just months before the fire they had installed new high-capacity hydrants. A dozen fire crews, each with 40 to 50 firefighters, worked day and night to hold the line.

Scene at Mt. Wilson after the 2020 Bobcat Fire (Erik Olsen)

You can go. The observatory runs guided tours, and on public Telescope Nights you can look through the 60-inch or 100-inch telescopes yourself. The 100-inch is the largest telescope in the world open to the public. Book early, because the nights sell out. Standing in that dome, looking through the same telescope Hubble used to discover the universe, is hard to beat.