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Counting something reliable

How Do Clocks Tell Time?

A clock is anything that repeats at a perfectly steady rate, plus a way to count the repeats.

1 Somethingrepeatssteadily2 Gears count therepeats3 The count isshown as time
Every clock ever built works this way.

  1. Step 1 of 5

    Time needs something that repeats.

    1 Sun's shadow2 A swing3 A quartzvibration

    You cannot count time directly. You count something dependable that happens over and over — sunrises, swings, vibrations.

  2. Step 2 of 5

    The repeat must be perfectly regular.

    Irregular repeat Useless for timingPerfectly steady repeat A good clock

    A dripping tap repeats, but unevenly, so it makes a terrible clock. Accuracy comes entirely from how steady the repetition is.

  3. Step 3 of 5

    Gears turn many small counts into hours.

    Second hand60 steps perminuteMinute hand60 steps perhourHour hand12 steps perhalf day

    A pendulum swings once a second. Gears divide that down: 60 swings move the minute hand one step, 60 minutes move the hour hand.

  4. Step 4 of 5

    Better repeats made better clocks.

    Atomic clock1 second in 100million yearsQuartz watch1 second a monthPendulum clock1 second a day

    Pendulums beat sundials. Quartz crystals, vibrating 32,768 times a second, beat pendulums. Each jump in steadiness was a jump in accuracy.

  5. Step 5 of 5

    Atomic clocks count atoms vibrating.

    An atomic clock Counting 9 billionatomic flips per second

    A caesium atom flips 9,192,631,770 times a second, always. We now define the second by that count — the clock defines the unit.

    Satellite navigation only works because atomic clocks in orbit agree to within billionths of a second.

The short version

A clock counts a perfectly steady repeat — better clocks simply found steadier things to count.

Try it yourself

Time a minute using your pulse, then check against a real clock. Your heart is a clock, just not a very steady one.

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