The sages were furious. True, Hananiah was a great scholar - among the greatest of his generation. But after he left for Babylon, Hananiah continued to arrange the Jewish calendar - deciding whether to add a leap month and determining the first day of the month.
Two scholars were dispatched to Hananiah with the following warning: if you insist on setting the calendar outside the Land of Israel, then go build your own altar and publicly declare that you and your Babylonian community have left the Jewish people and 'share no portion in the God of Israel.' When the Babylonian Jews heard this shocking message, they began to wail. 'Heaven forbid! We still belong to the Jewish people!'
The Talmud [Brachot 63b] explains that the sages in Israel based their action on the verse, "For Torah will go forth out of Zion, and the word of God from Jerusalem" . The source of Torah instruction - including determining the Jewish calendar - is Jerusalem and the Land of Israel.
Why were the sages so upset with Hananiah setting the calendar in Babylon? Two simple words: Zion and Jerusalem.
Zion vs. Jerusalem
We must first understand the source text, "For Torah will go forth out of Zion, and the word of God from Jerusalem." What is the difference between Zion and Jerusalem?
Zion and Jerusalem refer to the same city, but they indicate different aspects of the holy city. The word Zion literally means 'marked' or 'distinctive.' It refers to those inner qualities that distinguish the Jewish people - a 'people who dwells alone' with their own unique spiritual traits. Jerusalem, on the other hand, indicates the holy city's function as a spiritual center, influencing the nations of the world. Jerusalem is the means by which the Godly spirit found in Israel penetrates the inner life of distant peoples.
In short: 'Zion' looks inwards, at the city's inner significance for the Jewish people, while 'Jerusalem' looks outwards, at the city's external role as a spiritual focal point for the entire world.
It is axiomatic that the Torah's overall spirit and ideals can only flourish - both in Israel and among the nations - when the Jewish people observe the detailed mitzvot of the Torah. For this reason, the verse first stipulates that "Torah will go forth out of Zion." First the Torah and its practical mitzvot must be fulfilled in Israel; only then "the word of God" can disseminate from Jerusalem to the rest of the world. The two parts of the verse thus correspond to the two aspects of Jerusalem. There must first be Torah in Zion, focusing inwards; and then the word of God, the universal prophetic message, can spread to the rest of the world - emanating from Jerusalem, the external side of the holy city.
Sun and the Moon
What does all this have to do with setting the calendar?
There are two aspects to setting the Jewish calendar. The first is to determine when the new moon occurs. The second is to calculate whether it is necessary to introduce an extra month so that the lunar cycle will remain in synch with the seasons and the solar year.
To the Sages, the sun represents the nations of the world, while the moon is a metaphor for the Jewish people. The two aspects of the calendar correspond to the two sides of Jerusalem. Using the new moon to declare the beginning of a Jewish month is needed for the special lunar calendar of Israel - this is the inner Torah of Zion. And declaring a leap year is needed to maintain the proper balance between the solar year of the nations and the lunar year of Israel - this corresponds to the universal message emanating from Jerusalem.
National Torah
But why can the Jewish calendar only be set in the Land of Israel?
The requirement to set the Jewish calendar in Israel reflects a fundamental understanding of Torah. For the Torah to influence and enlighten the world, it must be established as a complete Torah, a Torah that governs all spheres of life. By setting the calendar outside the Land, Hananiah disconnected the Torah from the Jewish people as a nation living a full life in their own land. He reduced the Torah to a personal religion, one that relates only to the morality of the individual and one's private feelings towards God. Such an approach impoverishes the vibrant richness of Torah.
As the sages warned Hananiah, this was akin to setting up a private altar to serve God - a break from the true goal of Torah and the people of Israel.
To refute such views, the prophet Isaiah promised, "For the sake of Zion I will not be silent." Only when Jerusalem is secured as the center of Torah will the continuation of this prophecy be fulfilled: "Then the nations will see your righteousness, and all kingdoms your honor" .
[Adapted from Ein Ayah vol. II p. 385]