||This article may contain original research. (May 2013)|
Hillel II, (Hebrew: הלל נשיאה, Hillel the Nasi) also known simply as Hillel held the office of Nasi of the ancient Jewish Sanhedrin between 320 and 385 CE. He was the son and successor of Judah III. He was a Jewish communal and religious authority, circa 330 - 365 CE. He is sometimes confused with Hillel the Elder, as the Talmud sometimes simply uses the name "Hillel".
In two instances his name is quoted in connection with important decisions in Jewish law: in one, Jose ben Abin expounds to him a law; in the other, Hillel cites a mishnah to establish a law (Yer. Ber. ii. 5a; Yer. Ter. i. 41a).
He is traditionally regarded as the creator of the modern fixed Hebrew calendar. However this attribution is tenuous. It first appears in a responsum of R. Hai Gaon (early eleventh century) cited by R. Avraham b. Hiyya in his Sefer Ha'ibbur, written in 1123. The topic of that responsum is the 19-year cycle for leap-year intercalations, so the most that can be inferred from that attribution is that Hillel was responsible for the adoption of that cycle for the regulation of the distribution of leap-years. Scholars who have studied the history of the Hebrew calendar are in general agreement (and there is much evidence for this in the Talmud itself and in other rabbinic sources) that in practice, the evolution of the calendar into its present form was a gradual process spanning several centuries from the first to about the eighth or ninth century CE. The champion of the view that the calendar was developed in the eighth or ninth century CE is Sacha Stern. This quote is from page 184-5 of his book Calendar and Community
“Of far greater importance, however, is a much later document from the Cairo Geniza: a letter of a Babylonian exilarch - one of the main leaders of the Rabbanite community - with detailed calendrical instructions for the year 835/6 CE. The letter reveals that Passover (15 Nisan) in that year was due to occur on a Tuesday; whilst according to the present-day rabbinic calendar, it should have occurred on Thursday. According to the exilarch, the setting of Passover on Tuesday was dictated by a concern to avoid visibility of the new moon before the first day of the month. This concern does not exist in the present-day rabbinic calendar. Once discovered and published in 1922, the exilarch's letter proved beyond doubt that almost five hundred years after R.Yose and 'Hillel the Patriarch', then fixed calendar in its present-day form had still not been instituted."
This proof by Sacha Stern is flawed. The exilarch was using the very same calculation that has been attributed to Hillel II.
Presume neither the Dehiyya Molad Zaqen nor the Dehiyya Lo Adu Rosh (at least for yom Shlishi) were being implemented by the Calendrical Court in Israel as certainly was the case at the time of Hillel II. The Molad of Tishrei for 835 CE was Friday, August 27, at 22 hours and 660 parts (4:36 PM). The year 835 CE was the seventeenth year of the cycle and thus a leap year. Rosh HaShanah 836 Ce would be Thursday, September 14, 836. Basic apportionment of days to months would be Tishrei 30, Cheshvan 29, Kislev 30, Tevet 29, Shvat 30, Adar I 30, Adar II 29, for a total of 207 days. Thus Rosh Chodesh Nisan would fall on Tuesday, March 21, 836, and Passover would also fall on Tuesday. Nisan 30, Iyar 29, Sivan 30, Tamuz 29, Av 30, Elul 29 would complete this year to a total of 384 days.
In Sacha Stern’s translation of the exilarch’s letter the exilarch is concerned about seeing a Moon before Tuesday, i.e. Monday Night after sunset. Yet from Nasa’s Tables the actual lunar conjunction will be Tuesday, March 21, 836 at 2:19 AM so the New Moon would actually appear after sunset on Wednesday, nothing to fear it all. It was of the Old Moon that the exilarch had reason to be concerned.
The exilarch’s fears were based on what had happened Rosh HaShanah 835/6. On Thursday morning, August 26, 0835 the [Old] Moon would have risen in Bavel at 3:48 AM with the Sun rising at 5:33 AM. This moon would have been 44 hours from the conjunction so of considerable brightness. This is the problematic old moon by the light of which R’Hiyya walked four miles and would cause difficulties for the exilarch in his conflict with the Karaitim as it proved that the new moon was not really witnessed.
Rabbinic tradition ascribes to him an enactment which proved of incalculable benefit to his coreligionists of his own and of subsequent generations. The Jewish calendar is lunisolar. That is, its months are synchronized with the phases of the moon, but its average year length approximates the mean length of a solar year. The purpose of the latter is to ensure that the festivals, all of which occur on fixed dates of the lunar months, are also observed each year in the seasons designated for them in the Bible. To ensure the former, occasional intercalations of a day in a month were required; to ensure the latter, occasional intercalations of an extra month in a year were required.
These intercalations were determined at meetings of a special committee of the Sanhedrin. But Constantius II, following the precedents of Hadrian, prohibited the holding of such meetings as well as the vending of articles for distinctly Jewish purposes.
The entire Jewish community outside the land of Israel depended on the calendar sanctioned by the Judean Sanhedrin; this was necessary for the unified observance of the Jewish holidays. However, danger threatened the participants in that sanction and the messengers who communicated their decisions to distant congregations. Temporarily, to relieve the foreign congregations, Huna ben Abin once advised Rava not to wait for the official intercalation: When you are convinced that the winter quarter will extend beyond the sixteenth day of Nisan declare the year a leap year, and do not hesitate (R. H. 21a). But as the religious persecutions continued, Hillel decided to provide an authorized calendar for all time to come, though by doing so he severed the ties which united the Jews of the diaspora to their mother country and to the patriarchate.
The emperor Julian the Apostate was gracious to Hillel, whom he honored on a number of occasions. In an autograph letter to him, Julian assured him of his friendship and promised to ameliorate further the condition of the Jews. Before setting out for the war with Persia, Julian addressed to the Jewish congregations a circular letter in which he informed them that he had "committed the Jewish tax-rolls to the flames," and that, "desiring to show them still greater favors, he has advised his brother, the venerable patriarch "Julos", to abolish what was called the 'send-tax'".
In Christian tradition 
- Letters of Julian, Letter 51. To the community of the Jews
See also 
320 CE - 365 CE