Åk 6–9
 
English/فارسی
2.5 The Clock
There are many ways to measure time.  The most common is that we use some type of clock. There are many types of clocks which have been developed during a long period of time.




Sun Dial

The sun dial consists of an arrow and a bow with time marked out.  Then the sun shines on the arrow, a shadow is produced on the bow, and the time can be read.

 This is probably the oldest time measuring device.  The first sun dials were simple and had a base of wood, stone, or metal and were read using a horizontal shadow. 


Foto: Fredrik Enander

During the Middle Ages, the sun dial was developed so that the shadow could be cast parallel with the Earth’s axis and which was pointed at the North Star.



Water Clock
The simplest form consists of a marked holder with a hole in the bottom where water can slowly run out.  Simple water clocks were already used from about 2,000 b.c.

In more advanced water clocks, called clepsydra, water from the container (where the water level was held constant) could be dropped down onto a water wheel, which in turn drove a cog with a hand on it.



Hourglass

Even called a sandglass.

Consists of two containers which are connected in the middle so that sand can run from the one container to the next.

Is commonly from the Middle Ages until the 1800’s.  They were used in navigation for example where the work and rest periods were 30 minutes, the time it took for the sand to run down.


Foto: Fredrik Enander




Analog Clock

The mechanical clock was developed during the 900’s and exacted afterwards with pendulums and springs.

The first pocket watch was introduced in the beginning of the 1500’s, but only first during the 1700’s did it become small enough to comfortably carry.

An analog clock normally has an hour, minute and second hand.

Foto: Fredrik Enander




Digital Clocks

Digital clocks began being used during the later part of the 1900’s.  The time is shown with digits.  Either using 12 or 24 hour time.
 
The watch in the picture shows the time one minute over two in the afternoon.

 


Foto: Fredrik Enander




Atomic Clocks

A clock which is based on how electrons move in an atom or molecule.  These clocks are the most accurate clocks in use today.  Since 1967, a cesium clock is used to give our time standards.  The atomic clock keeps time to 1 second every million years  (+-1 s/million years).  Newer atomic clocks are even being developed that are more accurate.