The second was originally defined as 1/86,400 of a mean solar day, since there are 24 hours × 60 minutes × 60 seconds in a day. That fraction was essentially arbitrary, inherited from the ancient Babylonian base-60 number system that gave us 60-minute hours and 60-second minutes. Over centuries, as clocks got more precise, scientists discovered that Earth’s rotation isn’t steady enough to anchor a reliable unit of time. Today, the second is defined by counting exactly 9,192,631,770 oscillations of radiation from a cesium-133 atom.
Dividing the Solar Day
For most of human history, the second was simply a subdivision of the day. Ancient civilizations split daylight and nighttime into 12 hours each, and by the medieval period those hours were further divided into 60 minutes of 60 seconds. The entire system traces back to Babylonian mathematics, which favored base-60 arithmetic. A mean solar day, the average time it takes for the Sun to return to the same position in the sky, contains exactly 86,400 of these seconds.
This definition worked well enough when sundials and church bells were the main timekeeping tools. But it carried a hidden problem: it assumed every day is exactly the same length. They aren’t.
Clocks Revealed Earth’s Imperfections
The cracks in the solar definition started showing once mechanical clocks became precise enough to expose them. In 1656, Christiaan Huygens built the first pendulum clock, which improved timekeeping accuracy from about 15 minutes of drift per day (the best that earlier spring-driven clocks could manage) down to roughly 10 to 15 seconds per day. For the first time, people had a tool that could measure time more consistently than Earth itself rotates.
By the early 20th century, quartz crystal clocks were even more revealing. Researchers at Germany’s national physics institute demonstrated that Earth’s rotation fluctuates with the seasons. Beyond those seasonal wobbles, tidal friction from the Moon is gradually slowing the planet’s spin. Four hundred million years ago, a year contained about 400 days, each one shorter than today’s. There are also unpredictable rotational hiccups, likely caused by shifts of mass deep inside Earth, that change the day’s length by tiny but measurable amounts. A unit of time pegged to something this unreliable was becoming a scientific liability.
The Ephemeris Second: A Brief Fix
In 1956, the international weights and measures community tried a workaround. Instead of tying the second to Earth’s daily rotation, they tied it to its yearly orbit around the Sun. The “ephemeris second” was defined as 1/31,556,925.9747 of the tropical year as calculated for the start of 1900. A tropical year is the time between two spring equinoxes, and basing the definition on a specific historical year locked in a fixed value rather than a moving target.
This was more stable than the solar day, but it had a practical drawback: you couldn’t directly observe the ephemeris second in a laboratory. Verifying it required painstaking astronomical measurements over long periods. Scientists needed something they could reproduce on a benchtop, anywhere in the world, at any time.
How Atoms Replaced Astronomy
The solution came from quantum physics. Atoms of a given element absorb and emit electromagnetic radiation at extremely precise, unchanging frequencies. Unlike a spinning planet, these frequencies don’t drift over centuries or wobble with the seasons. They are, as far as physics can tell, identical for every atom of that element in the universe.
In 1955, physicist Louis Essen and his colleague Jack Parry at the UK’s National Physical Laboratory built an atomic clock based on cesium-133. Cesium was ideal: it has a single stable isotope, one outer electron, and a sharp, narrow resonance at a microwave frequency that existing electronics could generate and count. Essen and Parry measured that resonance frequency against the existing ephemeris second and arrived at a value of 9,192,631,770 cycles per second.
In 1967, the General Conference on Weights and Measures made it official. The second was redefined as the duration of exactly 9,192,631,770 periods of radiation from the cesium-133 atom’s ground-state hyperfine transition. In plain terms, if you watch a cesium atom flip between two specific energy states and count the microwave radiation it produces, one second elapses every time you reach that count. This definition remains the global standard today.
Keeping the World’s Clocks in Sync
Defining the second is one thing. Maintaining it globally is another. The Bureau International des Poids et Mesures (BIPM) in Paris calculates International Atomic Time (TAI) by combining data from roughly 450 atomic clocks operated by more than 80 timing laboratories worldwide. Every five days, these labs report how their clocks compare, and an algorithm produces a weighted average optimized for long-term stability. No single clock defines the world’s time. The collective average is far steadier than any individual device.
Coordinated Universal Time (UTC), the time standard your phone and computer use, is derived from TAI but periodically adjusted with leap seconds to stay within 0.9 seconds of mean solar time. Because Earth’s rotation is slightly and unpredictably slowing, atomic time gradually drifts ahead of solar time. The International Earth Rotation and Reference Systems Service monitors this gap and announces a leap second, a one-second insertion into UTC, several months before it’s needed. Since 1972, when the current system was adopted, these leap seconds have kept clock time aligned with the Sun’s position in the sky. Today’s mean solar day is about 3 milliseconds longer than exactly 86,400 atomic seconds, so the corrections add up over months and years.
Why Cesium’s Number Looks So Odd
The value 9,192,631,770 isn’t a round number because it wasn’t chosen for neatness. It was chosen for continuity. Essen and Parry measured the cesium frequency against the ephemeris second, which itself had been calibrated to match the older solar second as closely as possible. The goal was to make the new atomic second the same length people had always used, just defined by something far more stable. The awkward digit string is simply what nature gave back when scientists asked, “How many cesium oscillations fit inside the second we already have?”
That chain of inheritance means today’s second is still, in a meaningful sense, 1/86,400 of the day as it existed in roughly 1900. It’s just no longer tied to what Earth happens to be doing right now. The planet keeps slowing down. The second stays the same.