How It Works

How long is a second?

There are two main ways of measuring time: dynamic and atomic time. The former relies on the motion of celestial bodies (including the Earth) to keep track of time, whether it’s the rotation time of a distant spinning star such as a pulsar, the motion of a star across our night sky or the rotation of the Earth. However, a spinning star not withstanding (which can be hard to observe), these methods are not always entirely accurate.

The old definition of a second was based on the rotation of the Earth. As it takes the Sun one day to rise in the east, set in the west and rise again, a day was almost arbitrarily divided into 24 hours, the hour into 60 minutes, and the minute into 60 seconds. However, the Earth doesn’t rotate uniformly. In fact, it’s rotation decreases at a rate of about 20 millionths of a second every calendar year due to tidal friction caused by the Moon.

Atomic time relies on the energy transition with an atom of a certain element, commonly caesium. By defining a second as the number of these transitions (where an electron ‘quantum jumps’ between shells in an atom after gaining or losing a specific amount of energy), time can be so accurately measured that only a tiny portion of a second is lost every million years. The definition of a second is currently defined as 9,192,631,770 transitions within a caesium atom.




  • CarverUpton

    I feel like we should have better standards for time than the traditional ones and rather than arbitrarily try to convert the old standards we should have a system based on the atomic transitions.

    • Bailey Wrathmann

      I thought that at one point as well, but when it comes to units, we try to make things as utilitarian as possible. When it comes to length, the metric system is best because its so easy to remember and we dont necessarily need any specific measurements. But with time, the most useful things is to base it off the Earth’s rotation and revolutions around the sun. The Mayans did an excellent job creating the calendar we use today that bases time off Earth, and because its already built into society, theres really no good reason to change it. So by saying that a second is 919… cesium transitions isn’t really an attempt to convert time to a new “metric” type of system, its more just a way of creating a record of how long we’ve decided a second is. Not to mention, all units are arbitrary, even metric. It’s just based off water, so you get some nice relationships with regard to water, but not much else. One thing that would be good, would be to make an hour be 100 minutes, and a minute be 100 seconds. But theres not much of a point in doing that because it doesn’t really improve anything for how much infrastructure would be required to change.

      • CarverUpton

        Very nice. Very valid points. I think you’ve won me over.