Did you know? Sundials

This Month . . . . Sundials. The earliest type of timekeeping device when people relied on the passage of the Sun through the sky to tell the time.




A sundial is an instrument that tells the time like a clock. It has a pole, or gnomon, at its centre and markings that show hours. It works on the concept of the Sun changing its location in the sky throughout the day as it rotates from east to west. Image above: Sundial in Royal Botanic Gardens, Sydney, 2009. Photo J.Bar


The gnomon, Greek for “pole”, at the centre of the sundial needs to have two requirements: It must point toward the celestial pole, and it must be inclined at an angle to the horizon exactly equal to the observer's latitude.


The largest sundial in the Southern Hemisphere is situated in Singleton, NSW. It is 14.6 metres long, 7.92 metres high and has a gnomon 12 metres long. It weighs 30 tonnes. Image below of the giant sundial courtesy Lianne Griffiths.


Sundials are still being used scientifically by NASA and the furthest sundial from Earth is currently on Mars: it is part of the Mars Rover. The sundial is always in view of the cameras and is used as a backup navigation aid if the other instruments fail.


It is still not known exactly who invented the sundial, it is one of the oldest scientific instruments in the world, but they have been invented independently in every major culture and become more accurate and sophisticated as the culture developed.