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.

The solution was clear: from time to time, an extra second must be added to Coordinated Universal Time (UTC) to keep the two systems aligned. Thus, the concept of the “leap second” was born.

The longest year ever?

In June 1972, the global scientific community applied this solution in practice for the first time. One second was added to world time on 30 June, and another on 31 December. This made 1972 the only year in history to gain two separate leap seconds.

Over the past 50 years, around thirty leap seconds have been added to UTC, but never two in a single year again—meaning 1972 still retains its unique status.

However, the title of the longest year ever goes further back: 46 BCE, when Julius Caesar, following astronomical advice, extended the year to 445 days to realign the Roman calendar with the solar year.

What happens if the Earth’s rotation speeds up?

Recent observations suggest that over the past five years, the Earth’s rotation has experienced a noticeable acceleration. This could mean that in the future, a “negative leap second” may be required—effectively removing a second from time rather than adding one.

Researchers caution, however, that the consequences of such a step are unpredictable. Even adding leap seconds is often complex and cumbersome, and this would represent an unprecedented intervention.

Judah Levine of the U.S. National Institute of Standards and Technology (NIST) estimates that over the next decade, there is only a 30% chance that it will be necessary to remove a second from world time.

He primarily points out that, in the long term, global warming—with melting polar ice caps and rising sea levels—will slow the Earth’s rotation, eventually counteracting the current acceleration. Thus, the present increase in rotation speed is likely only a temporary phenomenon.

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