The Earthan Exodus: A series of setbacks in Earth's environmental remediation lead to the mass resettlement of humanity elsewhere in the galaxy. Tens of millions of scientists, engineers, historians and artisans remain on Earth to pursue the planet's renewal, hundreds of millions remain scattered across the solar system, mostly on the Moon and Mars. January 1st of the year following the beginning of the Earthan Exodus is declared to be Year 1 of the Star Era, with the year the exodus began being Year 0. The previous year is the final year of the Anno Domini era. Though the AD calendar continues to be used well into the first century of the Star Era, today it's considered as obsolete as the Roman AUC calendar and is only used for historical purposes (and most people don't know the AD year without looking it up).