After an earlier false start with a leaking companion ship, the Speedwell, the Mayflower finally departed Plymouth harbor alone on September 6 by the old calendar (September 16 new style), though passengers had boarded and prepared over preceding days including September 24. The 66-day crossing brought Puritan separatists and other settlers to Cape Cod, where they would draft the Mayflower Compact and found Plymouth Colony, a founding episode in American colonial history.

Urbain Le Verrier had calculated the position of an unseen planet from irregularities in Uranus's orbit, then mailed his predictions to Berlin Observatory. On the night of September 23-24, Johann Galle and student Heinrich d'Arrest located Neptune within about one degree of Le Verrier's forecast. The triumph of Newtonian mechanics sparked a bitter international priority dispute with England's John Couch Adams, who had made similar calculations but published later.

On September 28, 1864, a large meeting at St. Martin's Hall in London brought together British trade unionists, French workers, and continental exiles to form the International Workingmen's Association, commonly called the First International. Karl Marx, present at the founding, was elected to its General Council and soon drafted its inaugural address, steering the organization toward his revolutionary socialist vision and setting the stage for decades of ideological struggle within the international labor movement.