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|AD 40 by topic|
|Gregorian calendar||AD 40
|Ab urbe condita||793|
|Balinese saka calendar||N/A|
|Chinese calendar||己亥年 (Earth Pig)
2736 or 2676
— to —
庚子年 (Metal Rat)
2737 or 2677
|Coptic calendar||−244 – −243|
|- Vikram Samvat||96–97|
|- Shaka Samvat||N/A|
|- Kali Yuga||3140–3141|
|Iranian calendar||582 BP – 581 BP|
|Islamic calendar||600 BH – 599 BH|
|Julian calendar||AD 40
|Minguo calendar||1872 before ROC
|Seleucid era||351/352 AG|
|Thai solar calendar||582–583|
166 or −215 or −987
— to —
167 or −214 or −986
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AD 40 (XL) was a leap year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus without colleague (or, less frequently, year 793 Ab urbe condita). The denomination AD 40 for this year has been used since the Early Middle Ages, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- The emperor Caligula is consul without colleague.
- Caligula starts on a campaign to conquer Britain, which fails miserably. He declares himself victorious regardless.
- Noricum and Mauretania are incorporated 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. He also appoints his horse, Incitatus, a senator.
- Approximate date of start of construction on the Pont du Gard aqueduct in Gallia Narbonensis.
- Vardanes I becomes king of Parthia, opposed by his brother Gotarzes II.
- The Vietnamese Trưng Sisters rebel against the rule of the Chinese Emperor Guangwu of Han.
Arts and sciences
- Philo teaches that all men are born free.
- Christianity comes to Egypt as a church is founded in Alexandria. Mark the Evangelist founds the Coptic Church as the first Patriarch.
- An early Christian church is erected at Corinth (most probable date).
- June 13 – Gnaeus Julius Agricola, Roman governor of Britain (d. AD 93)
- Pedanius Dioscorides, Greek physician (approximate date) (d. AD 90)
- 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. AD 62)
- Empress Ma of the Han dynasty (d. AD 79)
- January – Gnaeus Domitius Ahenobarbus, husband of Agrippina the Younger, brother-in-law of Caligula (dropsy) (b. 17 BC)
- Ptolemy of Mauretania (executed by Caligula) (b. 1 BC)
- Fabre, Guilhem; Fiches, Jean-Luc; Paillet, Jean-Louis (1991). "Interdisciplinary Research on the Aqueduct of Nimes and the Pont du Gard". Journal of Roman Archaeology. 4: 63–88.