What was the name of the telescope Hubble used to discover Cepheids in Andromeda?

Edwin Hubble used the Hooker Telescope to discover Cepheid variables in Andromeda. The 100-inch Hooker reflector stood at Mount Wilson Observatory in California and was among the world’s most powerful telescopes when Hubble began using it.

On October 5–6, 1923, Hubble photographed a region of the Andromeda Nebula, now called the Andromeda Galaxy or Messier 31. One object initially marked as a nova turned out to change brightness periodically. He crossed out the “N” on his photographic plate and wrote “VAR!”, identifying it as a variable star later known as M31-V1.

The star was a Cepheid variable, whose pulsation period reveals its intrinsic brightness. By comparing that known brightness with how faint the star appeared, Hubble estimated Andromeda’s distance. The result placed it far beyond the Milky Way, helping settle the Great Debate over whether spiral nebulae were objects within our galaxy or separate galaxies.

The Hooker Telescope is often confused with the Hubble Space Telescope, which launched in 1990 and was named for Edwin Hubble. Hubble’s landmark Andromeda observations were made decades earlier with a ground-based telescope.