This
really simple experiment, which you could carry out easily if you school has a stair well,
proved that the Earth was spinning on its axis. If you lived at the North Pole you could also
show that it takes one day to make one revolution.
It was first performed on a large
scale by the French scientist Jan Bernard Foucault in 1851. He hung a 28 kg brass sphere
with a spike on the base from the roof of the Pantheon in Paris by a 67m long wire and on
31st March set it swinging in one plane. At each end of the swing the spike grazed through a
heap of sand.
As time went by it looked as if the plane of swing of the pendulum
was changing but this is not what was happening. The pendulum with its heavy bob stays
swinging in the same plane because of its large inertia while the Earth rotates beneath it.
The apparent movement
of the pendulum is in a clockwise direction in the northern hemisphere but it would move in
the opposite direction in the southern hemisphere. The rate of rotation measured using this
experiment depends on where it is done. If the experiment had been done at the pole the
rotation period would have been 24 hours, while at the equator it would have looked as it the
Earth was not rotating at all. At latitudes between the pole and the equator the rotation period
would have been more than 24 hours (32 hours in Paris and 31 hours in
London.
The time rotation in seconds (T) for a latitude A is:
T = 86164/sin
A therefore for latitude 51o the time period is: T = 86164/0.7771 = 110872s = 30 hrs
48 mins.
However in Nairobi (Kenya) (1.3o S) the period would be an enormous 91
hrs 26 mins
It is very easy to see this from the following diagrams.
