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 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 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 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, an hour into 60 minutes and a minute into 60 seconds. However, the Earth doesn’t rotate uniformly. In fact, its rotation decreases at a rate of about 30 seconds every 10,000 years due to factors such as tidal friction. Scientists have devised ways to account for the changing speed of Earth’s rotation, introducing ‘leap seconds’, but for the most accurate time you have to go even smaller. Atomic time relies on the energy transition within an atom of a certain element, commonly caesium. By defining a second using the number of these transitions, time can be measured with an accuracy of losing a tiny portion of a second in a million years. The definition of a second is now defined as 9,192,631,770 transitions within a caesium atom.

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