Here’s a deceptive question: Is Long Island an island? It’s surrounded on all sides by water, yet for legal purposes, it isn’t an island. In 1985, all nine justices on the Supreme Court agreed. Long Island, that mass of land completely surrounded by water, is not an island. It’s a peninsula. One that just happens to have a little water between it and the mainland.
The case in question, United States v. Maine, arose over disputes between states and the federal government over who legally controls the water between the eastern tip of Long Island and Rhode Island (Block Island Sound).
If Long Island were legally an island, that water would be considered the open sea, and therefore be regulated by the federal government. As a peninsula, the water around it (and the soil at the bottom of the sound) comes under the authority of the states surrounding it.
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Thanks to the Supreme Court’s geographically unsound ruling, the island is an extension of the New York mainland, and ships passing through the bay to its north need a state-licensed captain, per New York and Rhode Island law. Here’s the Supreme Court’s logic on why the island is not an island, according to the Long Island-based newspaper Newsday:
The court was led to its conclusion as a result of Long Island's shape and relation to the corresponding coast. According to the ruling, Long Island's north shore follows the south shore of the opposite mainland. But the shapes of the two lands almost completely surround the Long Island Sound. The court also determined that Long Island and the adjacent shore share a common geological history, which contributes to its lack of island-ness. Deposits of sediment and rocks from the mainland formed the shores by ice sheets that retreated thousands of years ago, according to the ruling.
Technically, the East River, the body of water that separates Long Island from Manhattan and the Bronx (on the New York mainland), is a tidal strait, rather than a river. Since the East River is relatively shallow, difficult for ships to navigate, and not an outlet to the sea, it doesn’t count, the Court essentially argued. Newsday points out that scientific experts don't support this argument—geologically, the two islands are made of very different kinds of rock that formed at millions of years apart. But, as a matter of political expediency, it’s more convenient for Long Island to be a peninsula so New York can exercise jurisdiction over it (and reap whatever natural resources it can from that).
While it may have simplified interstate squabbles, the demotion to peninsula status did cause Long Island to lose out on its designation as the longest island in the lower 48 states.