A floor that feels still can still turn
The absence of a felt daily spin is often treated as evidence that Earth does not rotate. The pendulum asks a different question: does its swing direction drift against the floor by the amount predicted at that latitude?
Rate and direction are the test
Foucault pendulums exhibit slow changes of swing direction relative to their surroundings. At a pole the ideal precession makes one turn per Earth rotation; at the equator the rotational contribution is zero. Intermediate latitudes give intermediate rates, with opposite directions in the two hemispheres.
A real suspension can introduce its own motion. An elliptical swing, an uneven support or a disturbed release can add drift. A useful comparison records latitude, elapsed time and swing-plane bearings and distinguishes those disturbances from the predicted rotational signal.
Project the rotation onto the local vertical
Using the rounded 23.93-hour sidereal rotation period, the signed precession rate is 360 sin(φ) / 23.93 degrees per hour. Here positive means clockwise when viewed from above; southern latitudes are negative and give counterclockwise drift.
The full-turn period is 360 divided by the magnitude of that rate. At zero latitude there is no finite full-turn period: the tool reports that explicitly rather than dividing by zero. Elapsed turn is rate multiplied by hours; it is not reduced modulo 360, so long runs keep their completed revolutions.
This is the slow precession of the swing direction, not the seconds-long back-and-forth oscillation of the bob. Changing the pendulum length changes that oscillation period, not the leading latitude factor used here.
Predict the swing-plane drift
Latitude is signed: north positive, south negative. Positive output means clockwise viewed from above.
Positive is clockwise; negative is counterclockwise, viewed from above.
The slow change in swing direction, not the back-and-forth swing angle.
360 ÷ |7.521939| = 47.86 h
Working tape
- Latitude × π ÷ 180
30 × 0.017453 = 0.523599 - Sine of latitude
sin(0.523599) = 0.5 - Signed turn per Earth rotation
360 × 0.5 = 180 - Signed turn per hour
180 ÷ 23.93 = 7.521939 - Rate × elapsed hours
7.521939 × 1 = 7.521939
One instrument supplies a geographic prediction
The 30° north scenario produces about 7.52° of clockwise drift per hour and a full precession turn in about 47.86 hours. The same southern latitude reverses the sign without changing the magnitude.
Try the equator, then either pole. A latitude-independent mechanical push would not reproduce this entire pattern merely by matching one demonstration.