400

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Year 400 (CD) was a leap year starting on Sunday of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Stilicho and Aurelianus (or, less frequently, year 1153 Ab urbe condita). The denomination 400 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Events

January 9 – Emperor Arcadius gives his wife Aelia Eudoxia the official title of Augusta. She is able to wear the purple paludamentum and is depicted in Roman currency.
Anthemius, praetorian prefect of the East, is sent on an embassy to the Persian capital, Ctesiphon, to congratulate King Yazdegerd I on his accession the year before.{{cite book|first=Christopher|last=Kelly|title=The End of Empire|publisher=W.W. Norton & Company|location=New York|year=2010|isbn=978-0-39333-849-2|page=76}}
A riot breaks out in Constantinople; the Great Palace is burned to the ground. Gainas, a Gothic leader, attempts to evacuate his soldiers out of the city, but 7,000 armed Goths are trapped and killed by order of Arcadius. After the massacre, Gainas tries to escape across the Hellespont, but his rag-tag ad hoc fleet is destroyed by Fravitta, a Gothic chieftain in imperial service.
Winter – Gainas leads the remaining Goths back to their homeland across the Danube. They meet the Huns and are defeated; the Hunnic chieftain Uldin sends the head of Gainas to Constantinople, where Arcadius receives it as a diplomatic gift.

Richū, the eldest son of Nintoku, becomes the 17th Emperor of Japan.{{cite book|first=Jonathan Edward|last=Kidder|title=Ancient Japan|publisher=Elsevier-Phaidon|location=Oxford|year=1977|isbn=978-0-72900-047-5|page=146}}

Resurrection and "Two Marys with Angel near the Empty Tomb", panel of a diptych, found in Rome, is made. It is now kept at Castello Sforzesco, Milan (approximate date).

The Vergilius Vaticanus, an illuminated manuscript containing fragments of Virgil's Aeneid and Georgics, is made in Rome.
The Yoga Sutras of Patanjali are composed.{{cite book|last1=Maas|first1=Philipp André|title=Samādhipāda das erste Kapitel des Pātañjalayogaśāstra zum ersten Mal kritisch ediert|trans-title=Samādhipāda critically edited the first chapter of the Pātañjalayogaśāstra for the first time|language=German|date=2004|publisher=Shaker|location=Aachen|isbn=3832249877}}

Caelius Aurelianus, Roman physician, is practising his work "De morbis acutis et chronicis" (Concerning Acute and Chronic Illness), a guide to acute and chronic diseases.

Hypatia, Greek philosopher, distinguishes herself as one of the first female scientists and becomes head of the Neo-Platonist school at Alexandria.

The mausoleum of Galerius in Salonica (Greece) is converted into a church.
Bishops from Gaza (Palaestina Prima) arrive at Constantinople to ask Arcadius that he close the pagan temple of Marnas.