True or false? Long Island is an island.

False. Well, sort of.

In 1985, the U.S. Supreme Court ruled by a vote of 9-0 that, legally, Long Island is not an island. It is part of mainland New York State.

The ruling came as part of a decision in a case in which the federal government and the states of New York and Rhode Island had been fighting for control of Long Island Sound and Block Island Sound. While the states won the case, Long Island lost its island-ness, according to an April 30, 1985, Newsday article.

“Both the proximity of Long Island to the mainland, the shallowness and inutility of the intervening waters as they were constituted originally, and the fact that the East River is not an opening to the sea, suggest that Long Island be treated as an extension of mainland,” Justice Harry Blackmun wrote in the court decision. Blackmun died in 1999.

The issue became the focal point of the case because if Long Island was part of the mainland, as New York and Rhode Island argued, then under international law, the sounds would be inland bays controlled by the states instead of open waters under federal control.

Long Island, which is deemed "unusual" in the ruling, is an island that should be considered an extension of the mainland, according to the case.

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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.

But scientists begged to differ.

“Of course, Long Island is an island,” R. Lawrence Swanson, a trained oceanographer who ran the Waste Management Institute at the State University at Stony Brook, said in an Oct. 5, 1987, article published in Newsday's series, “Long Island Our Story.” Swanson was an independent expert witness in the case.

Swanson argued that Manhattan and Long Island are different geologically. Manhattan is made up of exposed bedrock hundreds of millions of years old. Long Island is mostly loose sand and came into existence during the last 150,000 years, according to the 1987 article.

Blackmun also wrote that humans widened and deepended the East River, which was previously too narrow, shallow and navigationally dangerous to be a barrier between Long Island and New York City.

However, Swanson said in response that the two lands are separated by “a series of very complex tidal straits.”

Other experts agreed. “I never understood that Supreme Court decision,” Ralph Lewis, a geologist and an expert on the history of Long Island Sound, said in 1987.

An article published in the academic journal Annals of the Association of American Geographers in 1959 was referenced in the case. It stated that an island is identified as that of a mainland when the two lands are separated by little water.

According to the U.S. Board on Geographic Names, a federal body within the U.S. Geological Survey, an island is simply an “area of dry or relatively dry land surrounded by water or low wetland.” But apparently that was not enough to convince the court.

“Well, at least people will no longer talk about our insular habits,” Suffolk County Executive Peter Cohalan said at the time of the court decision. “Now they will talk about our peninsular habits.”

But don’t expect a name change to Long Peninsula.

While the Supreme Court thinks otherwise, the geographic names board still classifies Long Island as an island since it’s surrounded by water. For them it really is that simple.