How did 1972 become the longest year in modern history?

By the mid-20th century, scientists already knew with precision that the Earth’s rotation was gradually slowing. In 1972, this knowledge translated into an extraordinary fact: it became the longest year of the past two millennia, thanks to the first-ever use of leap seconds to keep atomic clocks in sync with the planet’s rhythm.

While we might be inclined to think that, aside from leap years every four years, a year always lasts the same amount of time, the reality is far less orderly. The Earth’s rotation is far from uniform, and our clocks merely attempt to keep pace. There was, however, one year when this discrepancy became particularly pronounced, ultimately earning 1972 the distinction of the longest year in modern history, according to IFLScience.

A year of momentous events

1972 was hardly lacking in historic events. In the United States, President Richard Nixon’s administration appeared to be collapsing under the weight of the Watergate scandal; Northern Ireland was shaken by the infamous “Bloody Sunday” events; and in cinemas, The Godfather premiered, ushering a new era in filmmaking.

That same year also marked the last time humans walked on the Moon: Eugene Cernan, commander of Apollo 17, took the final human steps on the lunar surface in December 1972.

However, it was not the density of these events that made 1972 the longest year of the past two millennia. The Earth’s rotation had slowed slightly, creating a small but significant discrepancy between human-measured time—atomic time—and the planet’s rotation, which needed to be corrected.

The birth of the leap second

By the mid-20th century, scientists had long recognised that the Earth’s rotation was not perfectly uniform. Atomic clocks, which measure time based on the vibrations of caesium atoms, could remain accurate for millions of years—but the same could not be said for the Earth. Oceans, the atmosphere, ice sheets, and tectonic activity all influence its spin.

Initially, this difference seemed negligible, but by the 1960s it could no longer be ignored. The gap between atomic time and astronomical time, while still small, threatened systems that relied on precise timekeeping—navigation, international communication, and even financial transactions.