The Japanese emperor picks a regnal name when he takes the throne, just like the Pope.
The name is actually the name of the emperor, not the name of the era. In Japan, nobody uses the emperor's birth name; they just use the regnal name. For example, nobody in Japan says "Hirohito"; they refer to him as "Emperor Showa" instead (though nobody calls Akihito "Emperor Heisei" now, because it's considered rude to refer to the living emperor by name... he's just "His Majesty, the Emperor").
Note that this wasn't the case until the 19th century. Until Emperor Meiji (Mutsuhito) took the throne in 1867, the name of the emperor and the name of the era were completely decoupled, and the emperor would just arbitrarily start eras whenever he felt like it. Meiji changed all this and standardized Japan on a one-emperor, one-era system. And in 1979 (!), it actually became law.
The name is actually the name of the emperor, not the name of the era. In Japan, nobody uses the emperor's birth name; they just use the regnal name. For example, nobody in Japan says "Hirohito"; they refer to him as "Emperor Showa" instead (though nobody calls Akihito "Emperor Heisei" now, because it's considered rude to refer to the living emperor by name... he's just "His Majesty, the Emperor").
Note that this wasn't the case until the 19th century. Until Emperor Meiji (Mutsuhito) took the throne in 1867, the name of the emperor and the name of the era were completely decoupled, and the emperor would just arbitrarily start eras whenever he felt like it. Meiji changed all this and standardized Japan on a one-emperor, one-era system. And in 1979 (!), it actually became law.