There are 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute; so of course a second is only 1/(24x60x60) or 1/86400 of a day, right? Turns out the timing wasn’t so easy.

We are used to thinking of the second as a fixed increment of time, but this small unit has changed many times over the centuries.

“Originally the second measurement was based on the length of the day,” Peter Wibberley, a senior scientist at Britain’s National Physical Laboratory, told LiveScience. “People watched the sun pass overhead and began measuring its movement with sundials. ‘Such devices give time directly based on the position of the sun in the sky, this is called apparent solar time.’

However, sundials have some disadvantages. Besides the obvious problem of not being able to read a sundial when the sun is not visible, relying on the Earth’s daily rotation (also known as astronomical time) is surprisingly inaccurate.

“The rotation is not exactly constant,” Wibberley said. “The Earth speeds up and slows down over time. There are seasonal fluctuations, large unpredictable fluctuations from decade to decade due to changes in the molten core, and long-term slowdowns caused by the back-and-forth movements of the tides.”

So how can we measure time accurately if using the length of the day is so unreliable?

In the 16th century, people turned to technological solutions to this problem, and the first recognizable mechanical watches began to appear.

“The essence of creating a clock ranged from tracking time relative to the position of the sun to creating an oscillator and determining a fixed number of oscillations equivalent to one second,” said Sumit Sarkar, a physicist at the University of Amsterdam. he told LiveScience.

The earliest mechanical examples were pendulum clocks designed to tick at a specific frequency equivalent to an average astronomical second over the course of a year. Over the next several hundred years, scientists worked to create better, more accurate oscillators and developed a variety of other timing systems, including springs and gears.

By about the 1940s, quartz watches had become the new gold standard. “If you apply voltage to a carefully shaped piece of quartz, it vibrates, and you can tune the frequency of that oscillation very precisely,” Sarkar said. “But while this accuracy is good for general use, it is not good enough for truly technical applications such as the Internet, GPS systems, or basic research studies.”

The problems arise because each piece of quartz is unique and resonates slightly differently depending on physical conditions such as temperature and pressure. To be truly accurate, clocks must be set to an independent, unchanging standard. Atomic clocks come to our rescue here.

“Atoms naturally have a fixed resonance. They exist only in certain energy states and can only move from one state to another by absorbing or emitting a certain amount of energy,” Wibberley explained. “This energy corresponds to a precise frequency, so you can use this frequency as a reference to tell time.”

The first practical atomic clock, introduced in 1955, measured the number of microwave-induced energy transitions of cesium atoms during one astronomical second. In 1967, the world scientific community agreed to redefine the second according to this number, and the International System of Units and Measurements defines the leap second as the duration of 9,192,631,770 energy releases in a cesium atom.

Since then, the astronomical second has continued to change, while the atomic second has remained at 9,192,631,770 oscillations. These differences in astronomical time actually mean that scientists have to add a leap second every few years to ensure that Earth’s slow rotation keeps pace with atomic time. Wibberley said the leap second will disappear by 2035, but scientists and government agencies haven’t yet figured out how to deal with this minor discrepancy.

However, scientists are not satisfied with this definition, which is accurate to 10^-15 seconds, or one quadrillionth of a second. Research groups around the world are working on more accurate optical atomic clocks that use visible-light-induced, higher-energy atomic transitions in elements such as strontium and ytterbium to increase this accuracy by more than 100-fold. Scientists are even debating whether it is time to once again define seconds by the oscillations of the optical clock, using ultraviolet and visible light sources instead of microwaves.

But before that happens, a few important questions still need to be answered, but it’s clear that the exact definition of a second can change.