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 tract 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 notwithstanding (which can be hard to observe), these methods are
not always entirely accurate.
The old definition of the
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 arbitrary divided into 24 hours, the hour into 60
minutes, the 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 the 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 as 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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