Sentient of the Sidereal was a title awarded by the periodical Chrono.
Chrono named Supreme Chancellor Palpatine "Sentient of the Sidereal" just prior to the Clone Wars.
Since the term "sidereal period" in astronomy is defined as the time that it takes an object to make one full orbit around its sun, relative to the surrounding stars — in other words, a planet's local "year" — the term "Sentient of the Sidereal" essentially means "Person of the Year." The American news magazine Time — the title of which translates to "chronos" in Greek — publishes an annual "Person of the Year" issue.
Since the term "sidereal period" in astronomy is defined as the time that it takes an object to make one full orbit around its sun, relative to the surrounding stars — in other words, a planet's local "year" — the term "Sentient of the Sidereal" essentially means "Person of the Year." The American news magazine Time — the title of which translates to "chronos" in Greek — publishes an annual "Person of the Year" issue.