840

840 in various calendars
Gregorian calendar	840; DCCCXL
Ab urbe condita	1593
Armenian calendar	289; ԹՎ ՄՁԹ
Assyrian calendar	5590
Balinese saka calendar	761–762
Bengali calendar	247
Berber calendar	1790
Buddhist calendar	1384
Burmese calendar	202
Byzantine calendar	6348–6349
Chinese calendar	己未年 (Earth Goat); 3537 or 3330; — to —; 庚申年 (Metal Monkey); 3538 or 3331
Coptic calendar	556–557
Discordian calendar	2006
Ethiopian calendar	832–833
Hebrew calendar	4600–4601
Hindu calendars
- Vikram Samvat	896–897
- Shaka Samvat	761–762
- Kali Yuga	3940–3941
Holocene calendar	10840
Iranian calendar	218–219
Islamic calendar	225–226
Japanese calendar	Jōwa 7; (承和７年)
Javanese calendar	737–738
Julian calendar	840; DCCCXL
Korean calendar	3173
Minguo calendar	1072 before ROC; 民前1072年
Nanakshahi calendar	−628
Seleucid era	1151/1152 AG
Thai solar calendar	1382–1383
Tibetan calendar	阴土羊年; (female Earth-Goat); 966 or 585 or −187; — to —; 阳金猴年; (male Iron-Monkey); 967 or 586 or −186

Millennium:	1st millennium
Centuries:	8th century; 9th century ; 10th century;
Decades:	820s; 830s; 840s ; 850s; 860s;
Years:	837; 838; 839; 840 ; 841; 842; 843;

Year 840 (DCCCXL) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar.

Events

June 20 – Emperor Louis the Pious falls ill and dies at his hunting lodge, on an island in the Rhine, near his imperial palace at Ingelheim, while suppressing a revolt. His eldest son Lothair I succeeds him as Holy Roman Emperor, and tries to seize all the territories of the late Charlemagne. Charles the Bald, 17, becomes king of the Franks, and joins with his half-brother Louis the German, in resisting Lothair.

King Wigstan of Mercia, grandson of former ruler Wiglaf (see 839), declines his kingship in preference of the religious life. He asks his widowed mother, Princess Ælfflæd, to act as regent. A nobleman of the line of the late king Beornred, named Berhtric, wishes to marry her but he is a relative. Wigstan refuses the match, and is murdered by followers of Berhtric at Wistow. He is buried at Repton Abbey, and later revered as a saint. The Mercian throne is seized by Berhtric's father, Beorhtwulf.^[1]
Vikings make permanent settlements with their first 'wintering over', located at Lough Neagh in Northern Ireland (approximate date).

Emperor Wen Zong (Li Ang) dies after a 13-year reign, in which he has failed to break the power of his palace eunuchs. He is succeeded by his brother Wu Zong, as Chinese ruler of the Tang Dynasty.
The Yenisei Kirghiz settle along the Yenisei River, and sack with a force of around 80,000 horsemen the Uyghur capital, Ordu-Baliq (driving the Uyghurs out of Mongolia). This ends the Uyghur Khaganate.^[2]
840 Erzurum earthquake. It took place in the city of Qaliqala (modern Erzurum).^[3]

Nobis becomes bishop of St. David's, in the Welsh Kingdom of Dyfed (approximate date).

January – Michael III, Byzantine emperor (d. 867) This uncertain, though January 840 is the most probable, however, 839 is also possible.
October 25 – Ya'qub ibn al-Layth al-Saffar, founder of the Saffarid dynasty (d. 879)
Abu al-Hassan al-Nuri, Muslim Sufi (approximate date)
Adalhard II, Frankish nobleman (approximate date)
Berengaudus, Benedictine monk (d. 892)
Clement of Ohrid, Bulgarian scholar (approximate date)
Eudokia Ingerina, Byzantine empress (approximate date)
Hucbald, Frankish music theorist (or 850)
Lothar I, Frankish nobleman (d. 880)
Notker the Stammerer, Benedictine monk (approximate date)
Richardis, Frankish empress (approximate date)
Sunyer II, Frankish nobleman (approximate date)
Theodard, archbishop of Narbonne (approximate date)
Theodore II, pope of the Catholic Church (d. 897)
Unruoch III, margrave of Friuli (approximate date)