The decade of the 1920s on Earth is set between the first two World Wars. During that time, the Charleston is a popular dance. Some women are referred to as flappers. The United States economy starts to deteriorate, an event which causes the Great Depression and has consequences even in an alternate timeline (see 1929). During the Prohibition era, organized crime rises to never-seen heights. The book Chicago Mobs of the Twenties chronicles this era, and serves as a guide for the civilization on Sigma Iotia II.
Scientific breakthroughs on Earth include the launch of the first liquid propellant rocket. Radio becomes a popular form of entertainment during this decade. In art, the film Metropolis is released and Irving Berlin writes the song "Blue Skies". The song "Someone to Watch over Me" is also released.