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Measuring time
Beginner
Clocks, stopwatches and atomic oscillations define the second.
Time is measured by counting a regular repeating process: a pendulum swing, a quartz crystal vibration, or an atomic transition. The second is defined by 9,192,631,770 oscillations of caesium-133 radiation. Human reaction time of roughly 0.2 s limits stopwatch accuracy.
Timing 20 swings of a pendulum and dividing by 20 gives the period with 20 times less relative error than timing one swing.
Start counting at zero, not one, when you start the watch on a swing, otherwise you count one extra period.
In more detail
Time is measured with clocks, stopwatches and, for the most precise work, atomic oscillations. The second is defined by a fixed number of oscillations of the caesium atom, which is why atomic clocks are so accurate.
Real-world uses
- GPS satellites carry atomic clocks so that their signals can be used to compute position.
- Stopwatch timing in sports and lab experiments is limited by human reaction time, around a fifth of a second.
- GPS satellites carry atomic clocks, and their timing must be accurate enough that position errors stay small on the ground.
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Frequently asked questions
Why do we need such precise clocks?
Small timing errors in navigation or communications cause large position or data errors, so precision matters.
Is a pendulum clock accurate?
It can be very good, but its period depends on temperature and length, so it drifts compared with an atomic clock.
Why does an atomic clock beat a quartz watch?
Atomic transitions have a very stable, reproducible frequency, while a quartz crystal drifts with temperature and age.
Related topics in Measurement & Math Tools
Length measurement · Vernier calliper · Screw gauge · Mass vs weight · Significant figures · Rounding rules