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On September 23, 1846, astronomers Johann Galle and Heinrich d'Arrest at the Berlin Observatory confirmed the existence of Neptune, the eighth planet of the solar system. Its position had been mathematically predicted by Le Verrier and Adams from perturbations in Uranus's orbit. It was the first planet discovered through calculation rather than observation.

On September 23, 1932, King Abdulaziz ibn Saud proclaimed the unification of the regions of Hejaz and Nejd under the name of the Kingdom of Saudi Arabia, founding the state that bears his family's name. The country controls the Two Holy Cities of Islam — Mecca and Medina — and holds the world's largest oil reserves.

On May 14, 1811, Paraguay proclaimed its independence from Spain — and also from Buenos Aires — making it one of the first countries in South America to do so. Under the leadership of Doctor Francia, who governed for decades as dictator, Paraguay isolated itself from the outside world in an unprecedented political experiment in Latin American history.