Science corner

Sophie Duncan looks at objects which have been measuring time for more than 3,500 years

This beautiful object is a sundial. The dragon's tail casts a shadow, the position of which indicates the time. The first sundials were constructed more than 3,500 years ago comprising a gnomon (Greek for "the one who knows"), which cast a shadow on a dial that was marked with divisions to show the time.

Once the motion of the Earth was understood, sundials were designed that were very accurate. When clocks were introduced, sundials were used to set the clock to the correct time.

Sundial designs were often elaborate and several different types were made. One design made use of the fact that if the gnomon was inclined at an angle equal to the latitude of the user, and pointed north, the sundial could be used all year round. The largest is in Jaipur, India and was constructed in the early 1800s. The gnomon is angled at 27 degrees and is 27 metres high.

The easiest way to make a sundial involves placing a stick vertically in the ground and then marking in the soil the positions of the shadow every half hour. This sundial can be used successfully for the next couple of weeks, after which it becomes inaccurate. This can lead to interesting discussions about why the position of the shadows has changed. To make a more accurate dial try the British Sundial Association website at www.sun-dialsoc.org.uk Sophie Duncan is a physicist and programme manager for Planet Science (formerly Science Year). www.planet-science.com

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