<text><span class="style10">ime (3 of 3)</span><span class="style7"></span><span class="style10">Measuring time</span><span class="style7">The earliest device for measuring time was the </span><span class="style26">sundial</span><span class="style7">, which can be traced back to the Middle East c. 3500 BC. A sundial comprises a rod or plate called a </span><span class="style26">gnomon</span><span class="style7"> that casts a shadow on a disc; where the shadow points indicates the position of the Sun and hence the time of day. </span><span class="style26">Mechanical clocks</span><span class="style7">, driven by falling weights, appeared in the 14th century, and the first </span><span class="style26">mechanical watches</span><span class="style7">, driven by a coiled mainspring, in the 16th century. The first </span><span class="style26">pendulum clock</span><span class="style7"> was invented by Christiaan Huygens (1629- 95), a Dutch physicist, in the middle of the 17th century.Pendulum clocks could not be used on board ships owing to the vessel's motion. In 1714 the British Longitude Board offered a prize for the development of a </span><span class="style26">marine chronometer</span><span class="style7">, as being able to tell the time accurately is vital to navigation. Fourteen years later the English clockmaker John Harrison (1693-1776) set to work on the problem, producing his first marine chronometer in 1735.The first </span><span class="style26">quartz clock</span><span class="style7">, operated by the vibrations of a quartz crystal when an electrical voltage is applied, appeared in 1929. The quartz clock is accurate to within one second in ten years. This was followed in 1948 by the </span><span class="style26">atomic clock</span><span class="style7">, which depends on the natural vibrations of atoms. The most accurate modern atomic clocks are accurate to one second in 1.7 million years.GE </span><span class="style37">THE DAYS OF THE WEEK</span><span class="style10"></span><span class="style7"> </span><span class="style10">English name Named after</span><span class="style7"> Sunday The Sun Monday The Moon Tuesday Tiw, the Anglo-Saxon counterpart of the Nordic god Tyr, son of Odin Wednesday Woden, the Anglo-Saxon counterpart of Odin, the Nordic god of war Thursday Thor, the Nordic god of thunder, eldest son of Odin Friday Frigg, the Nordic goddess of love, wife of Odin Saturday Saturn, Roman god of agriculture and vegetation</span></text>
</content>
<content>
<layer>background</layer>
<id>26</id>
<text><span class="style10">he equipment responsible</span><span class="style7"> for creating the shortest `slice' of time ever - a pulse of light from a laser lasting only 30 femto seconds. A femto second is 10 to the power of -15 seconds (0.000000000000001 or a millionth billionth of a second). Scientists use such pulses of light as `stopwatches' to study subtle physical and chemical changes, such as how electrons move in semiconductor materials. In one second, a pulse of light can travel almost to the Moon, but in 30 femto seconds it travels no further than one third of the thickness of a human hair.</span></text>
</content>
<content>
<layer>background</layer>
<id>23</id>
<text>ΓÇó THE UNIVERSE AND COSMOLOGYΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE INNER PLANETSΓÇó QUANTUM THEORY AND RELATIVITYΓÇó THE REVOLUTIONARY CALENDAR</text>
