On the 9th of August AD 117, Trajan’s letter of adoption (litteras adoptionis) was made public when it reached Hadrian in Antioch. Hadrian was now Caesar.
As previously mentioned (see here), there was some uncertainty about whether or not Trajan had adopted Hadrian before his death or whether the adoption was staged by Plotina. The ancient sources give us two different versions of this event. The Historia Augusta, writing long after the events, confirms that Hadrian was adopted. However Cassius Dio, also writing after the events, rejects the adoption.
On the fifth day before the Ides of August, while he was governor of Syria, he learned of his adoption by Trajan, and he later gave orders to celebrate this day as the anniversary of his adoption. HA Hadr. 4.6-7
Hadrian had not been adopted by Trajan; he was merely a compatriot and former ward of his, was of near kin to him and had married his niece, — in short, he was a companion of his, sharing his daily life and had been assigned to Syria for the Parthian War. Dio 69.1
If the HA statement is correct, this indicates that Trajan adopted Hadrian some days before his death since one must allow a minimum of two days for the letter to be delivered from Selinus to Antioch, located 400 kilometres away. Therefore the adoption was probably not posthumous and the rumours were unfounded. Most modern scholars have reached the same conclusion and do not doubt the authenticity of Hadrian’s adoption by Trajan.
Soon afterwards, arrangements were made for the imperial mint in Rome to issue coinage for dissemination throughout the Empire. ADOPTIO became the main theme of the earliest coins of Hadrian’s reign.
Another coin may have helped emphasize Hadrian’s legitimacy to the succession; a gold aureus with the legend ADRIANO TRAIANO CAESARI (Hadrian son of Trajan Caesar). It is mentioned by Anthony R. Birley in his biography of Hadrian as being dated to late summer AD 117. The coin shows a partly draped bust of Hadrian wearing a laurel wreath on one side and a bust of Trajan, also laureled, on the other side.
Hadrian later gave orders to celebrate this day (August 9) as the anniversary of his adoption.
- Historia Augusta, The Life of Hadrian (link)
- Cassius Dio 69.1 (link)
- Birley, Anthony R. (1997). Hadrian. The restless emperor (p. 77-80)
Burnett, Andrew. “The Early Coinage of Hadrian and the Deified Trajan at Rome and Alexandria.” American Journal of Numismatics (1989-), vol. 20, 2008, pp. 459–477. JSTOR, http://www.jstor.org/stable/43580323.