Why 1972 Was The Longest Year in History

Jul 8, 2021 4 comments

Some years seem longer than others, especially when you are passing through a bad phase in life such as being stuck at home because a pandemic is playing havoc around, but officially the longest year on record was 1972. It was longer than the average year by a whole two seconds. The two extra seconds were leap seconds added on June 30 and again on December 31 the same year.

Photo: Pertusinas | Dreamstime.com

What are leap seconds?

From time to time, leap seconds are added to clocks on Earth to accurately reflect the observed solar time. This is due to the fact that the Earth’s rotation around the axis and its revolution around the sun is imprecise and varies from year to year.

The Earth’s rotation is slowing down, which throws the clocks on Earth out of sync. On average, an Earth day is about 0.002 seconds longer than 24 hours. These differences, however small, add up and grows to one second in about 1.5 years. To keep the clocks synchronized with the actual solar day, we have to add an extra second every few years.

“By adding an extra second to the time count, we effectively stop our clocks for that second to give Earth the opportunity to catch up,” explains Timeanddate.com. 

On average, a leap second is inserted about every 21 months, but because the Earth's rotation speed varies in response to climatic and geological events, leap seconds are irregularly spaced and unpredictable. Every year, the International Earth Rotation and Reference Systems Service decides whether to add them. If the international body responsible for maintaining global time and reference frame standards determines that a leap second is due, one is added to either on June 30 or on December 31. 1972, the year this system was introduced, was the only year when two leap seconds were added. As of today, there have been 27 leap seconds applied, the most recent on December 31, 2016.

Why is the Earth slowing down?

The main reason for the slowing down of the Earth's rotation is the tidal tug-o-war the Earth has been playing with the moon. The mass of the moon is sufficiently large to change the shape of the Earth, by creating a slight bulge towards the direction of the moon’s pull. However, the bulge doesn’t lie directly underneath the moon, because in the solid Earth there is a delayed response due to the dissipation of tidal energy. This causes the Earth to drag the bulge along as it rotates, creating a torque that affects both the moon and the Earth. In the case of the moon, it boosts the natural satellite into a higher orbit, away from the Earth, while in the case of the Earth, it is loses rotational energy and slows down.

Image by AndrewBuck/Wikimedia Commons

Scientists have attempted to look back in time and measure how long the day was in ancient time. One group of researchers estimates that 1.4 billion years ago a day was just 18.7 hours. At that time, the moon was also some 27,000 miles closer to Earth than it is now. By 3.5 billion years ago, the day had lengthened to about 21 hours. The Earth maintained this length throughout much of the Precambrian period, until about 700 million years ago, when the Earth began to slow down again.

A simulated history of Earth's day length. Graph by Bencbartlett/Wikimedia Commons

Tidal acceleration alone lengthens the day by 2.3 milliseconds per century. But there are other contributing factors such as the relative motion between the Earth’s crust and the molten core that creates a turbulence within the Earth, which changes the Earth's moment of inertia, affecting the rate of rotation due to the conservation of angular momentum. Large-scale geographic events such as earthquakes and post-glacial rebound, that has been going on since the last ice, also changes the distribution of Earth's mass, thus affecting the moment of inertia of Earth and hence its rotation. Certain manmade structures can also influence the earth’s rotation. For example, NASA scientists calculated that the water stored in the Three Gorges Dam has increased the length of Earth's day by 0.06 microseconds due to the shift in mass.

The future of leap seconds

Many international bodies propose elimination of leap seconds because of the problems it creates. Many computer systems fail because software are unable to handle the extra second. The Intercontinental Exchange, parent body to 7 clearing houses and 11 stock exchanges including the New York Stock Exchange, broke down for one hour when a leap second was added to June 30, 2015. The same year, several network failures occurred in routers across the world causing outage across major services such as Twitter, Instagram, Pinterest, Netflix, and Amazon.

Those who oppose leap seconds suggest keeping International Atomic Time (TAI) and Global Positioning System (GPS) time, used by computer systems, separate from Universal Coordinated Time (UTC). Computers, for example, could always take TAI and convert it to human-readable UTC or local civil time whenever necessary without causing UTC to alter atomic time.

Others object that adding leap seconds isn’t necessary because the drift between solar time and atomic time is too small to be noticeable.

"By my predictions, which would only be correct if the Earth's rotation is remarkably predictable, it will take over 700 years before the clock says 11:30 when it would have said 12:00," says Demetrios Matsakis, the chief scientist for time services at the US Naval Observatory.

This gives us plenty of time to prepare for some other way to put the world's clocks and computers back in sync with the earth's rotation.

# Wikipedia
# What Is a Leap Second?, Timeanddate.com
# Cade Metz, We Should Drop the Leap Second Before It Causes Real Damage, Wired


  1. Beg to differ. In 49BC, Julius Caesar added 2 "leap months" (for lack of a better term) in order to let his new "Julian Calendar" match-up with the seasons.
    -It was 445 days long, and was nicknamed the "Year of Confusion".

  2. The shortest year in recorded history was probably 1923 - but only in Greece. It was only 352 days long, the missing 13 days being deducted from the end of February. This is because that's when the country switched from the Julian calendar to a variant of the Gregorian calendar, and had to skip ahead 13 days to line up with the rest of the world.

  3. This is a contradiction to that other article, about the 445 day year.
    Maybe this article title should be changed from "longest year in history" to "longest year since the Gregorian calendar"?

    1. 46 BC was the longest year only for the Roman empire (and parts of Europe perhaps that followed the Roman calendar). But 1972 was a global event that affected all regions.


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