He had been an officer of the guard under Julian and Jovian, and had risen high in the imperial service.
His election caused considerable surprise, and it is suggested by Ammianus Marcellinus that he was wrongly identified with another Jovian, chief notary, whose name also had been put forward, or that, during the acclamations, the soldiers mistook the name Jovianus for Julianus, and imagined that the latter had recovered from his illness.
Julian pressed forward to Ctesiphon but succumbed to a wound; and his successor Jovian soon found himself in such straits, that he could only extricate himself and his army by a disgraceful peace at the close of 363, which ceded the possessions on the Tigris and the great fortress of Nisibis, and pledged Rome to abandon Armenia and her Arsacid protg, Arsaces III., to the Persian.
He accompanied this emperor, for whom he expresses enthusiastic admiration, in his campaigns against the Alamanni and the Persians; after his death he took part in the retreat of Jovian as far as Antioch, where he was residing when the conspiracy of Theodorus (371) was discovered and cruelly put down.
The long-sought cause of the "great inequality" of Jupiter and Saturn was found in the near approach to commensurability of their mean motions; it was demonstrated in two elegant theorems, independently of any except the most general considerations as to mass, that the mutual action of the planets could never largely affect the eccentricities and inclinations of their orbits; and the singular peculiarities detected by him in the Jovian system were expressed in the so-called "laws of Laplace."