Foucault pendulum
No need to look at the sky to prove that the Earth rotates.
In an inertial frame the pendulum keeps its plane of swing fixed, but the ground turns with the Earth. Seen from the ground, the plane of swing slowly rotates with angular velocity Ω·sinφ — clockwise when viewed from above in the Northern Hemisphere. At the North Pole it makes a full turn in one day; at the equator it does not turn at all. This is a direct manifestation of the Coriolis effect.
Time for a full turn vs. latitude
The lower the latitude, the slower the rotation; near the equator the time tends to infinity, so the curve breaks off there.
Think about it
At latitude 30° N, how long does the swing plane of a Foucault pendulum take to make a full turn?
T = 23.93 h / sin30° ≈ 47.9 h, nearly two days. Set the latitude to 30° and the reading will show about 47.9 h.
Model assumptionsAir resistance and ellipticity ignored · sidereal day 23.93 h · swing and rotation sped up on screen
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.