Historical City Eruvin and Modern Population Growth

A historical city eruv is valuable halachic precedent tied to a particular city as it existed at a particular time. Cities grow. Their boundaries expand, populations rise, roads change, walls disappear, bridges and railways are built, and patterns of travel can be transformed. The name of the city may remain the same while several facts relevant to reshus harabim no longer do.

This is especially important when an older city eruv is cited in a discussion of a modern metropolis. To understand the precedent, one must know what the Rabbonim of that city were enclosing, which physical features they relied upon, how they understood shishim ribo, and what the population and public traffic were at the time.

Population is not the same thing as passage through the streets

Warsaw is a useful example because Rav Moshe Feinstein discusses it directly in Igros Moshe, Orach Chaim V:28.

In section 5, while addressing the practice of making eruvin in large cities, Rav Moshe acknowledges explicitly that eruvin had been made in Warsaw. He calls Warsaw a well-known major city of Russia and Poland. He then answers the population argument by distinguishing the number of people who lived in a city from the number present in its streets:

Although eruvin were made in Warsaw, and it was a well-known large city among the important cities of Russia and Poland, it did not have six hundred thousand in its streets. That occurs only in a city with close to three million residents. Warsaw did not have that many people, nor even half that number, and its passersby were not numerous enough to complete the figure.

The wording matters. Rav Moshe does not say that a municipal census above 600,000 automatically makes 600,000 people "present in the streets." In this passage he is working with a street-use calculation, and his figure of close to three million residents is an approximate factual estimate for reaching 600,000 in the streets, not a universal mathematical ratio stated as a separate halachic rule.

Official Warsaw statistics illustrate why the distinction cannot be skipped. The 1921 census counted 936,713 residents in Warsaw. By 1938, the city's population was about 1.295 million, and its municipal area had also expanded from the 1921 boundaries. Thus Warsaw could plainly have a resident population above 600,000 while Rav Moshe still maintained, when discussing the Warsaw precedent, that it did not have 600,000 people in its streets.

That does not settle the broader dispute over how shishim ribo is defined. It shows something more basic: before a historical population figure is used in a halachic comparison, one must know what is being counted. A city census, a borough population, daily traffic on one road, aggregate traffic across many roads, visitors, and commuters are different numbers. The full discussion of shishim ribo addresses the different halachic approaches to those numbers.

A long road cannot be counted as one crowded point

Rav Moshe returns to Warsaw later in the same responsum from another angle. In section 16 he discusses an argument based on the long road from Warsaw to Petersburg. The argument cited the millions of people found along or using that route as evidence that a tzuras hapesach could permit even a road used by shishim ribo.

Rav Moshe rejects the calculation. People using different stretches of a very long road cannot simply be added together as though all of them passed through one place. He describes one stretch near Warsaw as having roughly a thousand travelers, another stretch as having hundreds of different travelers, and other portions as having still other people. He says that there were places on the route between villages and small towns where scarcely anyone passed on a given day, and that the claim of millions traveling the entire Warsaw to Petersburg road in one day was itself an exaggeration.

The precise historical traffic estimates in that passage are Rav Moshe's own estimates. The lasting analytical point is clear. A large total attached to a road network says little until its geography is specified. If different people use different sections, their numbers cannot automatically be treated as the number passing through one halachically relevant point.

That principle is just as important in a modern city. Traffic counts collected at different bridges, avenues, subway entrances, or road segments answer different questions. Adding them together without first defining the area or street under discussion can produce a very large number that does not correspond to the halachic calculation being made.

Growth can change the facts without changing the halacha

The same Igros Moshe responsum gives an unusually clear example of a city whose relevant facts changed over time.

Rav Moshe had earlier suggested that Yerushalayim was intentionally left without an eruv because Yidden came there from everywhere. Visitors who saw carrying in a properly enclosed Yerushalayim might return to places that lacked the same enclosure and carry there too. In Orach Chaim V:28 he expressly says that this concern did not ordinarily apply to other cities, because visitors generally came only from nearby places.

New York, he says, had become different. In his time people came there מכולי עלמא, from all over the world. Airplanes allowed travelers from distant places to arrive within hours, and Rav Moshe says that before such circumstances the Yerushalayim-based concern would not have applied even to New York. He suggests that the rail era may already have begun to create a similar American reality.

This is a direct example of historical change affecting the application of an unchanged halachic analysis. The rule did not change because airplanes were invented. The facts to which the rule was applied changed.

Population growth can work the same way. So can the removal of city walls, the opening of new roads, the absorption of adjoining neighborhoods, the construction of bridges and tunnels, or a change in the way most people move through a city. A psak concerning an earlier form of a city cannot be transferred to its later form without asking whether the facts on which it rested are still present.

What a historical eruv can and cannot establish

A historical eruv can establish that the Rabbonim responsible for it considered carrying possible within the arrangement and circumstances they addressed. That precedent deserves to be understood in its own setting, even when the city later became much larger.

Its force remains tied to those circumstances. Later expansion may have moved neighborhoods beyond the earlier enclosure, traffic patterns may have changed, and the historical record may not tell us without further evidence whether the Rabbonim relied on walls, tzuras hapesach, omed merubeh, the absence of a mefulash street, a particular understanding of shishim ribo, or several considerations together.

Warsaw itself shows why these distinctions matter. Its population and municipal footprint changed markedly. Rav Moshe discusses its historical eruv in terms of the number present in the streets. Elsewhere in the same responsum he rejects aggregating travelers from widely separated portions of the Warsaw to Petersburg road. His New York discussion also shows that patterns of visitors can change sufficiently over time to alter the practical comparison with Yerushalayim.

Historical practice therefore belongs in a modern eruv discussion once the old and new facts have been placed side by side. For Crown Heights, that means that precedents from Warsaw, Yerushalayim, Manhattan, or any other city can illuminate a halachic principle. None substitutes for determining the actual present-day boundaries, streets, public use, population calculations, and ruling of the Rabbonim responsible for Crown Heights itself.