Hillel II

The Calculated Hebrew Calendar has been attributed to a Nasi named Hillel II. One would have thought that the calculation established by Hillel II would be widely published and known in his day. Yet neither Hillel II nor even a discussion of the idea of supplanting witnesses with a calculated calendar is found in the Talmud Yerushalmi or the Talmud Bavli. 

The first mention of Hillel II is over seven hundred years after the fact.

עד ימי הלל בר' יהודה בשנת תר'ע לשטרות, שמאותה שנה לא הקדימו ולא אחרו, אלא אחזו הסדר הזה אשר היה בידם.

. . . until the days of Hillel b. R. Yehuda in the year 670 of the Seleucid era (358/9 CE), from when they did not bring forth or postpone, but kept to this cycle which was at hand . . . This from a responsum of R. Hai Gaon (early eleventh century) cited by R. Avraham b. Hiyya. 

Sacha Stern because of a lack of any reference to a Hillel II as a patriarch or establisher of the Hebrew Calendar in the Yerushami or in the Bavli and because of convincing evidence that Bavil was still waiting for Jerusalem to sanctify the moon as late as 835/6 AD and for other reasons on pages 175 though 179 of his book Calendar and Community concludes “The very existence of a ‘Hillel the Patriarch’ in the mid fourth century may thus treated as uncertain".

However Sacha Stern missed the ramifications of the date 358/9 CE. It is not often that such a precise date is provided by our sources. The Molad for Hillel's Rosh HaShanah occurred September 19 358 CE at 17:12 PM (23 hours and 233 Parts). From the chart provided by R H van Gent the actual lunar conjunction was September 19 358 CE at 19 hours 44.1 minutes (UTC). According to the basic calendar rules (pre-dehiyyot) Rosh HaShanah would start on the day the molad occurs. This means it would start September 18 358 CE at approximately 6:00PM.  On September 18 at 5:00 AM the "Old Moon" would be clearly visible, in Israel, being some 40 hours before the conjunction, rising at 3:50 AM. The sun will rise that morning at 5:25 AM. As can be seen from the chart this is one of the dates of a 3 day discrepancy. A 3 day discrepancy is always indicative of a visible old moon on the morning when Rosh HaShanah will begin in the evening. You can see a full size visibility chart here.

Hillel b. R. Yehuda like R' Yehoshua before him realized that something was wrong when the Nasi sanctified the month of Tishrei on the very evening that an old moon was seen. But unlike Rabbi Joshua ben Hananiah, Hillel realized or was told by the Nasi that Rosh Chodesh was not a function of witnesses but rather that witnesses were selected because they conformed with the selection of a calculated calendar.

Hillel b. R. Yehuda documented the fact not of the initiation of the Hebrew Calculated Calendar but of the fact of the continued use of this system. This documentation somehow made it to the library of Pumpedita and ultimately to the attention of Rav Hai Gaon.

Sacha Stern was correct that Hillel II was not the Nasi and that he did not originate the Calendar. But he very much existed.