Clyde Tombaugh discovers Pluto at Lowell Observatory; in 2006 it is reclassified as a dwarf planet.
Edwin Hubble publishes the relation between galaxies' distances and recession velocities, evidence that the universe is expanding.
Martin Luther King Jr., leader of the American civil rights movement, is born in Atlanta, Georgia.
The New York stock market collapses in late October, heralding the Great Depression that spreads around the world.
Charles Lindbergh flies solo and nonstop from New York to Paris in the Spirit of St. Louis.
The Jazz Singer, the first feature-length film with synchronised dialogue, premieres in New York, launching the era of talking pictures.
Franklin D. Roosevelt becomes president and launches the New Deal programs in response to the Great Depression.