40

Year 40 (XL) was a leap year starting on Friday of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus without colleague.

• The emperor Caligula is consul without colleague.
• Caligula starts a campaign to conquer Britain. It fails miserably. He names himself the winner.
• Noricum and Mauretania are added into the Roman Empire.
• Caligula reforms the principatus into a Hellenistic Autocracy. He distributes honors carelessly, declares himself a god and orders that all the heads of the Greek deity statues be replaced by his. According to legend, this is when he appointed his horse, Incitatus, a senator.
• Approximate date of start of construction on the Pont du Gard aqueduct in Gallia Narbonensis.
• Philo teaches that all men are born free.
• An early Christian church is erected at Corinth (most probable date).

• The Germanic Quadi tribe begin settling in present-day Moravia and Slovakia.

• Vardanes I becomes king of Parthia. He was opposed by his brother Gotarzes II.
• The Vietnamese Trung sisters rebel against the rule of the Chinese Emperor Guangwu of Han.

• Christianity comes to Egypt. Mark the Evangelist founds the Coptic Orthodox Church as the first pope.

• July 13 – Gnaeus Julius Agricola, Roman governor of Britain (d. 93 AD)
• Pedanius Dioscorides, Greek physician (approximate date) (d. 90 AD)
• Frontinus, Roman general and military author (approximate date) (d. 103)
• Dio Chrysostom, Greek philosopher and historian (approximate date d. c. 120)
• Claudia Octavia, daughter of Claudius and Messalina (d. 62)
• Empress Ma of the Han Dynasty (d. 79)

• January – Gnaeus Domitius Ahenobarbus, husband of Agrippina the younger, brother-in-law of Caligula (b. 17 BC)
• Ptolemy of Mauretania (b. 1 BC)