Why does a clock only have 12 numbers?


Human beings have been thinking about and counting time for thousands of years. The first time-keeping machines used sand, water or the position of the Sun in the sky to mark out the passage of time. These days, we use clocks and watches for telling the time. But however we do it, our measurement of time is based on the movement of our planet Earth in relation to the Sun.

The Earth rotates or spins around an imaginary line that runs between the North Pole and the South Pole. When the place where you live faces towards the Sun and you can see the Sun’s light, it is daytime; when the place where you live faces away from the Sun and is dark, it is nighttime. We call the amount of time it takes the Earth to do one full spin, one ‘day’.

But days are not useful enough on their own to help us count time because they are a bit too long. So we divide them up into smaller units called hours, minutes and seconds.

The Ancient Egyptians liked to count in twelves and they divided the day into two lots of twelve hours – 12 for daytime and 12 for nighttime. So one whole day lasts for 24 hours. Each hour lasts the same amount of time and can be divided up into 60 minutes; and each of those minutes can be divided up into 60 seconds.

On mechanical or analogue clocks (like the one in my photo, above) the 12 hours are marked out in a circle. The number 12 is always at the top, 6 is always at the bottom and the rest of the numbers in between are spaced out evenly in order around the edge of the circle. Two pointers, called hands, show the hour and the minutes.

To tell the time on a 12-hour clock, we count the hours from 1 o’clock in the morning until midday; then from 1 o’clock in the afternoon until midnight. To make it a bit less confusing we say that the morning hours are ante meridiem (a.m.) or post meridiem (p.m.), which comes from Latin words that mean ‘before midday’ and ‘after midday’.

But on digital clocks, we can count all 24 hours in one day. If we do this, then the morning (or a.m.) hours are shown with the numbers 1 to 12 and the afternoon (or p.m.) hours are shown with the numbers 13-24. So, 2 o’clock in the afternoon or 2p.m. would be 14:00 hours.

This video shows you how to tell the time and what it looks like on a 12-hour clock, a digital clock and written out in words.

You’ll find clocks all over the place – in your home, at school or work, on computers and in public spaces like train stations, bus stops and shops and libraries. Have a look around and see how many different clocks you can find!

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