Long Island — not really an island?

Sure, it’s long, but it’s no island. The U.S. Supreme Court says so.

In a 12-year-old ruling that still prompts eye-rolling from scientists who study Long Island’s geography, the high court declared that for purposes of international law Long Island is merely “an extension of the mainland’’ that is “integrally related’’ to Manhattan and the Bronx. The decision was unanimous, although many Long Islanders doubtless would have dissented if given the chance.

“Of course, Long Island is an island. But what I think is important as a scientist doesn’t mean a hill of beans to the lawyers,’’ said R. Lawrence Swanson. A trained oceanographer who runs the Waste Management Institute at the State University at Stony Brook, Swanson was an independent expert witness in the case.

For the last 11,000 years or so, ever since the rising sea filled in Long Island Sound, Long Island has inarguably met the geographic definition of an island: a piece of land surrounded by water at low tide. But the legal definition, naturally, is much more abstruse, and it took a long and costly court fight to determine that Long Island didn’t meet it.

The issue wound up in court because the federal government and the states of New York and Rhode Island couldn’t agree on who controls Long Island Sound and Block Island Sound. The states wanted to retain the power to regulate fishing, and to require state-licensed pilots to be on board ships passing through the waterway.

The is-the-Island-an-island controversy quickly became the key issue in the case. If Long Island was merely an extension of the mainland, as the two states argued, then under international law Long Island Sound and Block Island Sound would be inland bays controlled by the states instead of open waters under federal control.

Supreme Court Justice Harry Blackmun settled the issue — for the lawyers, at least — in a 1985 majority opinion that sided with the states and included a flawed geology lesson.

“Long Island and the adjacent shore also share a common geological history, formed by deposits of sediment and rocks brought from the mainland by ice sheets that retreated approximately 25,000 years ago,’’ Blackmun wrote.

Not really, according to Swanson and several other experts. Glaciers did cover both places, but Manhattan and Long Island are different geologically.

Much of Manhattan consists of exposed bedrock hundreds of millions of years old, while Long Island is mostly loose sand and didn’t even exist as a distinct entity until it was built up by bulldozing ice sheets over the last 150,000 years.

Blackmun also opined that before humans widened it, the East River was too narrow, shallow, and navigationally dangerous to be a significant barrier between Long Island and the boroughs of Manhattan and the Bronx.

Wrong again, according to Swanson. “It’s just very clear from a geophysical and geological point of view that Long Island is an island separated by the mainland by a series of very complex tidal straits,’’ he said.

Even off-Island experts agree the Island is an island.

“I never understood that Supreme Court decision,’’ said Ralph Lewis, an associate state geologist at the Connecticut Geological and Natural History Survey and an expert on the history of Long Island Sound. “We didn’t build those channels. It’s a natural island.’’