The Parable of the Two Rabbis

{% capture manuscript %} # The Parable of the Two Rabbis Long ago, two rabbis lived in the Holy Land. Let us call them Rabbi Ezra and Rabbi Nathan. Both were learned men. Both had studied the movements of the heavens, and neither doubted that the Earth was a sphere. They also shared the habit common to learned men of many ages: each was quite certain that the other would benefit from further instruction. One day their travels took them away from home in opposite directions. Rabbi Ezra traveled east. Rabbi Nathan traveled west. Their journeys were long. They traveled by land where roads permitted it and by sea where oceans required it. They passed through kingdoms whose languages they did not understand, ate unfamiliar foods, and slept beneath the same stars far from home. But wherever they went, they never lost track of the days. They were rabbis. For them, knowing whether it was Monday or Thursday was not merely a convenience. They needed to know when the Sabbath would begin. Each therefore counted his days faithfully. Six days of travel and work. Then Shabbat. Another six days. Then Shabbat again. Years passed, and the two men continued around the spherical Earth, one eastward and the other westward. Eventually, on the far side of the world from the Holy Land, their paths crossed. Their reunion was joyful. They embraced, exchanged stories of the places they had visited, and marveled that after traveling in opposite directions for so long, Providence had brought them together again. As evening approached, Rabbi Ezra looked toward the lowering Sun. “We must prepare,” he said. “Shabbat is coming.” Rabbi Nathan stared at him. “Shabbat?” “Yes.” Rabbi Nathan shook his head. “My friend, tomorrow is Friday.” Rabbi Ezra laughed. Then he saw that Rabbi Nathan was not joking. “No,” said Ezra. “Today is Friday. Shabbat begins this evening.” “No,” Nathan replied. “Today is Thursday.” Each rabbi regarded the other with the particular expression reserved for a respected colleague who has just said something obviously absurd. And so they debated, as good rabbis do. Rabbi Ezra produced his journal. Every day of his journey had been recorded. He showed where one Sabbath ended and the next week began. Rabbi Nathan produced his own journal. His record was equally careful. They counted Ezra’s days. They counted Nathan’s days. They counted them again. They examined departures, arrivals, storms at sea, nights spent in ports, and days spent crossing deserts. Neither man had skipped a day. Neither had counted a day twice. Neither had forgotten the Sabbath. Yet their calendars differed by exactly one day. “This is impossible,” said Rabbi Ezra. Rabbi Nathan considered this. “No,” he said. “It is merely inconvenient.” That began a second debate. They knew the Earth was round. They also knew what they had done differently. Ezra had traveled around it toward the east. Nathan had traveled around it toward the west. Each had kept an entirely reasonable calendar as he moved from place to place. But after circling the Earth in opposite directions and meeting again, those reasonable calendars could no longer agree. Rabbi Ezra drew a circle in the dirt. “This is the Earth.” “I know what a circle is,” said Rabbi Nathan. “I am establishing the argument.” “Then establish it accurately.” Ezra marked the Holy Land on the circle. He traced his journey eastward with one finger and Nathan’s westward with the other. The two paths eventually met on the opposite side. They stared at the drawing. At last Nathan said, “The difficulty is not in our counting.” “No.” “And it is not in the week.” “No.” “And we cannot have Shabbat beginning on two different days in the same village.” “Certainly not.” “Then somewhere,” Nathan said, “one of us must change his date.” Ezra frowned. “Why me?” “I did not say you.” “You were thinking it.” They debated that for a while as well. Eventually the larger principle became clear. If people were going to carry calendars all the way around a spherical Earth, there had to be some agreed place where one calendar day ended and the next began. Otherwise travelers approaching one another from opposite directions could faithfully keep time and still arrive with different dates. The place could not sensibly run through Jerusalem. Nor should it divide a kingdom, a city, or, if it could be avoided, someone’s house. So the rabbis looked at their maps. On the opposite side of the world from the Holy Land lay an enormous ocean. “There,” said Rabbi Ezra. Rabbi Nathan considered it. “For once,” he said, “you may be right.” Thus, according to a history that never happened but perhaps ought to have, two rabbis invented the International Date Line. There is, of course, no line painted across the Pacific Ocean. A sailor does not cross a seam in the water. The stars do not jump. The Earth does not pause while one calendar page is torn away and another takes its place. The Earth simply turns. The Sun rises and sets. Human beings supply the dates. That does not make the dates false. It makes them conventions: agreements we construct so that people living on a spherical, rotating world can describe time to one another consistently. The same is true of time zones. It is true of the International Date Line. And, for astronomical work, it is true of our adoption of a common universal time reference. Nature gives us motion. We draw the lines. And sometimes it takes two rabbis, standing on the far side of the world and disagreeing about Shabbat, to discover why the lines are necessary. When I was learning to fly, I was taught to use Zulu time—the common aviation name for the universal time reference associated with Greenwich. The conversion seemed simple: take today’s date and convert the local hour to Zulu time. But there was a subtle error in that instruction. Sometimes the conversion crosses midnight. When that happens, the hour changes and the date must change with it. A correct time attached to the wrong date is still the wrong time. Years later, this struck me as a real-world version of the problem faced by the two rabbis. We do not merely need a common clock. We need a common way of saying which day that clock belongs to. {% endcapture %}