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Do you know what the time is? Chances are you could make a good guess without even looking at your watch. |
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We're constantly checking the time, to keep us punctual or to make sure we don't miss our favourite TV shows.
But what is time – and how do we measure it? |
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Some of the first things we invented were timekeepers.
Over 2,500 years ago, the Egyptians made the sundial. |
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But instead of 24 hours in the day, they divided daylight up into twenty sections, each one lasting just over half an hour.
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Trouble was, the sundial didn't work at night, of course!
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The Greeks used candles.
They would light a candle at sundown, and then make a mark to see where the candle burned down to by sunrise.
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Then they'd make some more evenly spaced marks to show roughly how long to go until dawn.
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The nail made a good alarm clock – because when it got to dawn, the nail would drop out and clatter onto the metal tray beneath.
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This is one of the first chiming clocks – small rocks tied to an incense burner with a thin piece of cotton. |
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As the stick burns, the cotton breaks and the rock drops onto the metal plate.
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The problem with all these clocks was that they didn't last very long.
Incense sticks and candles burn very quickly, and even the sun disappears at the end of the day.
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What they needed was something that could be measured and which never ran out. Something that flowed continuously like time itself. Water. |
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Water clocks were used by the Greeks and Egyptians and particularly the Chinese.
As the water level rose, it pushed up a float which moved a hand to tell the time.
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But water clocks required a never-ending and constant flow of water.
The trick is to get a bucket with a hole in the bottom.
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When the bucket is full, we know with scientific accuracy that the flow of water out of the hole will always remain the same. |
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That's because the flow of water out of the hole depends on the water pressure. And the water pressure comes from all the water piled up inside above the hole.
So we know that the flow of water won't stop, and it won't get faster or slower. |
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