How Long Is One Second, Really?

It's the sort of question that can really take you down the rabbit hole: How long is a second, anyway?

Happily for everyone concerned, we specialize in rabbit hole navigation here at Seeker. As Julian Huguet details in today's DNews dispatch, the unit of time known as the second has actually gone through some changes over the years. More on the topic of years in a bit.

But first, the news: An international team of extremely punctual researchers recently announced an improved method for timekeeping involving optical clocks. The new technique measures the precise length of a second -- or any unit of time -- by counting the oscillations of strontium atoms. It gets rather technical, but the upshot is that the new optical clock method is more accurate than the traditional atomic clock standard. The improved timekeeping could have significant real-world effects, like GPS systems that are accurate down to the centimeter.

As to the history of the second, as it were, that's where things get interesting. It turns out that the very definition of that term has gone through several changes over the course of history.

For millennia, our primary unit of measure was the solar day -- the amount of time it takes for the sun to return to its same spot in the sky. The length of the second was essentially determined off the solar day, using basic math: 24 hours in a day, 60 minutes to an hour and 60 seconds to a minute means a there are 86,400 seconds in a solar day.

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That was the official definition of the second until the 1950s, when scientists realized the Earth's rotation wasn't precise enough for the matter at hand. It was actually more accurate to measure seconds as a subdivision of a full year, the amount of time it takes the planet to make a full circuit of the sun.

But the length of different years can vary slightly as well. So in 1960 it was decreed by the Eleventh General Conference on Weights and Measures that seconds would be based off one particular year -- the year 1900. Since 1900 was calculated to be 365.242198 days long, the second was officially defined by the conference as 1/31,556,925.9747 of the year.

Clearly, this solution lacked something in the way of elegance. That's when the world's official timekeepers decided to go micro instead of macro. The atomic standard was adopted in 1967, based on the cesium atom, which oscillates 9,192,631,770 times every second, give or take 20 oscillations. More accurately stated, the second is defined by the amount of time it takes for the cesium atom to oscillate 9,192,631,770 times.

The new optical clock technique, published in the journal Optica, hopes to improve matters still further. But optical clocks aren't likely to be implemented for 10 years or so, which in seconds calculates out to ... oh, never mind.

-- Glenn McDonald

Learn More:

Independent: Scientists want to change the way a second is defined

Scientific American: How does one arrive at the exact number of cycles of radiation a cesium-133 atom makes in order to define one second?

How Stuff Works: How Atomic Clocks Work