273-236 bc
In 231 BC
The reign of Hadrian began when the previous Emperor's reign ended. His cousin was the previous Emperor and it was said by those close to Trajan, the previous Emperor, that he named Hadrian as his next in line. His reign began in 117 AD and ended in 138 AD at his death.
ashok
1900
Domitian was the second son of the emperor Vespasian and the last of the Flavian dynasty. His reign followed the short reign of his brother Titus.
Emperor Hirohito.
Qin Shi Huang ~ see related link below .
During the reign of the emperor Contantine.
The period beginning with Augustus and ending with Aurelius can loosely be called the Pax Romana. I say loosely because trouble was brewing on the borders for years. Marcus Aurelius had to spend most of his reign fighting wars.
The emperor who wanted the Terracotta Warriors was Qin Emperor Shi Huangdi.
400 years
The reign of Augustus was known as the golden age.