On this page
You cannot feel Earth spin. In 1851 Léon Foucault found a way to let anyone see it: hang a heavy ball on a long wire, set it swinging, and watch the line of the swing slowly turn across the floor. This post covers who Foucault was, how the experiment works, how to calculate the turning time for your own city, and what his way of working can teach a physics student.
Who was Léon Foucault?
Foucault was born in Paris on 18 September 1819 and died there on 11 February 1868, according to his MacTutor biography from the University of St Andrews. He was a frail child who studied poorly at school, started medicine in 1839 and gave it up after fainting at the sight of blood. He then worked as an assistant to the microscopist Alfred Donné and became science editor of a Paris daily.
He had loved building machines since his teens, and that skill shaped his science. With his school friend Hippolyte Fizeau he took the first photograph of the Sun in 1845, and in April 1850 he showed, with a spinning mirror, that light travels more slowly in water than in air. His friend's story is in our post on Fizeau and the first Earth-based speed of light.
How does a pendulum show that Earth rotates?
Foucault's note to the Academy of Sciences, Démonstration physique du mouvement de rotation de la terre au moyen du pendule, was printed in the Comptes rendus for 3 February 1851. He started with an ideal case. Put a pendulum at the North Pole and release it without any sideways push. Inertia keeps its plane of swing fixed in space. Earth turns underneath, so to someone standing on the ice the swing seems to rotate, making a full turn in a day.
Away from the pole the effect is weaker. Foucault stated that the angle the plane turns through equals Earth's rotation in that time multiplied by the sine of the latitude. At the equator the plane does not turn at all, and in the southern hemisphere it turns the other way.
His first pendulum hung in his cellar: a steel wire 2 metres long carrying a 5 kg polished brass sphere, swung through 15 to 20 degrees. To avoid giving it a sideways push, he tied the ball back with a thread and burned the thread. After half an hour the shift was obvious. He then used an 11-metre wire in the Meridian room of the Paris Observatory.
The famous version came later that year. The Panthéon describes it on its own site: a 67-metre steel wire hung from the dome, a 28 kg sphere of brass and lead, and a stylus that cut a groove in a bed of sand at each pass. The invitations read "Vous êtes invités à venir voir tourner la Terre": you are invited to come and see the Earth turn. A pendulum has hung there permanently since 1995.
How long would a Foucault pendulum take to turn in India?
Earth's sidereal rotation period, one turn relative to the stars, is 23.93 hours according to NASA's Earth fact sheet. Use that in the sine law.
Paris, latitude about 48.8° N. sin 48.8° = 0.753, so T = 23.93 / 0.753 = 31.8 hours. The plane turns 360 / 31.8 = 11.3 degrees per hour.
New Delhi, about 28.6° N. sin 28.6° = 0.479, so T = 23.93 / 0.479 = 50.0 hours, or 7.2 degrees per hour.
Chennai, about 13.1° N. sin 13.1° = 0.227, so T = 23.93 / 0.227 = 105 hours, a little over four days.
The swing itself is a simple pendulum, which is in the JEE (Main) 2026 syllabus under oscillations. For the Panthéon wire, T = 2π√(L/g) = 2π√(67 / 9.8) = 16.4 seconds for one full swing. The turning of the plane is a rotating-frame effect, which that syllabus does not list, so treat it as enrichment rather than exam content.
Three mistakes to avoid:
Assuming one turn per day everywhere. That is true only at the poles.
Mixing up the two periods. The 16.4-second swing period depends on length; the turning time depends on latitude, not on length or mass.
Using cosine instead of sine. Check with the limits: the answer must be one day at 90° and never at 0°.
Practise the period formula with our simple harmonic motion problems.
What can a student learn from Foucault's way of working?
Start with the simplest version of the problem. Foucault reasoned about a pendulum at the pole before dealing with Paris. Do the same with a hard question: solve the special case first.
Remove hidden causes. Burning the thread meant no hand could push the ball sideways. In a lab, list what else could produce your result and design it out.
Test the objection yourself. He checked that twisting the wire did not turn the plane of swing, using a vibrating steel rod clamped in a lathe, which he described in a footnote.
Scale up until the effect is obvious. Two metres, then eleven, then sixty-seven. A small, real effect becomes convincing when it is big enough to see.
Combine tools you already have. By his own account, quoted in his MacTutor biography, he did not invent the spinning mirror or the achromatic lens; his skill was putting existing instruments together well.
Some ideas, like a swing that holds its direction while the room turns, are easier to see than to read. The Fermi Simulation Lab has interactive simulations for ideas like that.
Your next step: look up your city's latitude, calculate the turning time with the sine law, and check that your answer sits between one day and never.
