In 1931, a Foucault pendulum, the invention of a French astronomer of 1851 that demonstrates the rotation of the Earth, was installed beneath the dome of Saint Isaac's Cathedral

And if the pendulum that Jean Foucault hung beneath the dome of the Paris Panthéon on a 67-metre cable weighed 28 kilograms, the one installed in "Isakiy" was almost twice as heavy and, with a cable 98 metres long, was recognised as the largest in the world. The pendulum was set in motion on the Easter night of 11 to 12 April.

The pendulum operated in one of Russia's principal churches until 1986, when its suspension mechanism was judged faulty. Since then it has been kept motionless in the museum "Saint Isaac's Cathedral", while the hook from which it hung has been given back its dove, a metre and a half across, which had hovered beneath the dome long before this scientific instrument appeared.