Old Slavic (Russian Ярослав) name, which was derived from the words яр [yar] - "ardent; fierce; Yarilo (pagan God of the Sun)" + слав [slav] - "glory, fame"="ardent; famous by Yarilo (sun)".
Yaroslav the Wise
Yaroslav I, known as Yaroslav the Wise (c. 978 - 1054) was thrice Grand Prince of Novgorod and Kiev, uniting the two principalities for a time under his rule.
A son of the Varangian Grand Prince Vladimir the Great, he was vice-regent of Novgorod at the time of his father’s death in 1015. Subsequently, his eldest surviving brother, Svyatopolk the Accursed, killed three of his other brothers and seized power in Kiev. Yaroslav, with the active support of the Novgorodians and the help of Varangian mercenaries, defeated Svyatopolk and became the grand prince of Kiev in 1019. Under Yaroslav the codification of legal customs and princely enactments was begun, and this work served as the basis for a law code called the Russkaya Pravda ("Russian Justice"). During his lengthy reign, Rus' reached the zenith of its cultural flowering and military power.