Foucault pendulum

By kind permission of the Dean of Guernsey, the Very Reverend Canon Paul Mellor, the Astronomy Section of La Société Guernesiaise will be installing a Foucault Pendulum in the Town Church from Monday 12 July to Saturday 17 July 2010.  There will be explanatory posters and Astronomy Section members will sometimes be present to explain it.


At 1.00 pm on Friday, 16th July there will be a 30-minute presentation by David Le Conte, Fellow of the Royal Astronomical Society, entitled The Earth Moves: the Story of the Foucault Pendulum.  Part of the Church’s Friday Meeting Point series, the talk will be preceded at 12.30 pm by a sandwich lunch in the Church.  All are welcome.



What is a Foucault pendulum?


The 9-metre pendulum swings freely in space, while the Earth turns beneath it.  It thus appears that the angle of swing turns slowly, thereby demonstrating the daily rotation of the Earth on its axis.  To illustrate this effect, the pendulum will knock over pegs every few minutes.


Jean Bernard Léon Foucault (1819-1868) was a French physicist.  He first set up a pendulum at the Paris Observatory in 1851, and then at the Panthéon.  His pendulum was 67 metres high and had a bob weighing 28 kg.  Our pendulum is just 9 metres long, with a bob weighing just 2 kg, but it demonstrates the same principles.


Although the Earth rotates on its axis once every 24 hours, the apparent rate of rotation of the pendulum depends upon the latitude.  At the north and south poles it would take exactly 24 hours to make a complete rotation, but at the latitude of Guernsey (49.5°) the pendulum takes 31h 35m.   Here, in the northern hemisphere the pendulum rotates clockwise, but in the southern hemisphere it would rotate anti-clockwise.  At the equator it would not rotate at all




For further information about Foucault pendulums go to:



This page was last updated on 2010 July 06.

Astronomy Section Home Page

About the Astronomy Section

About the Observatory

Diary of events

Astronomical events this year


International Space Station  Visibility predictions 

Image gallery


La Société Guernesiaise


myspace hit counters

visitors since 25 May 2009.