end the plumb bob so that it is hanging straight down. Then give it a push so that it starts swinging straight ahead. Watch this “Foucault Pendulum” for a long time. The direction of motion of the pendulum will change. In the Northern Hemisphere, the pendulum will rotate in the clockwise direction. In the Southern Hemisphere, the pendulum will rotate in a counter-clockwise direction. At the Equator the pendulum does not rotate.
After a certain amount of time, the direction of the pendulum will return to its initial orientation (but not at the Equator). A Foucault pendulum set in motion at the Equator will remain in the same orientation. At the North and South Poles, the pendulum will rotate by 360º in one day (but will rotate in opposite directions). At any other latitude, the pendulum will rotate with angular speed ω = 360º sin φ/day, where φ is the magnitude of the latitude. Thus, when φ = 30º (North or South latitude), it will take 2 days for the Foucault pendulum to make one full revolution, while for φ = 45º it will take 1.41 days to rotate.
The rotation of the Foucault pendulum is a direct demonstration of the rotation of the Earth. Flat-Earthers claim that the “rotation” is not reproducible, that the rotation is random, and that the pendulum will eventually come to rest and have to be re-started. The last of these statements is correct; the combined effects of friction and air resistance will cause the pendulum to slow down. The pendulum needs to be “re-started” at regular intervals. It is also true that great care must be taken to insure that the pendulum begins its trajectory with no initial sideways motion. Unless the initial conditions are very carefully set, the circular motion from the Earth’s rotation will mix with another normal mode to produce elliptical motion.
However, the pendulum does not have to be observed for an exceptionally long time to estimate its period. And the fact that the period increases as one goes from the North Pole to the Equator (where it does not rotate), and that the direction of rotation changes from the Northern to the Southern Hemisphere, is sufficient to demonstrate the qualitative effects of the Earth’s rotation.
However, most of these difficulties can be avoided if one substitutes a gyroscope for the pendulum. With a high-quality gyroscope one can minimize several sources of error. The gyroscope will rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. One does not need to observe a complete rotation of the gyroscope, one simply needs to observe the amount of rotation and determine that the angular speed ω is as determined by our formula.
Flat Earther Bob Knodel has achieved notoriety for his “Globebusters” YouTube videos. A scientific debunker who runs the Web site “Flat Earth Lunacy” challenged Knodel to purchase a high-quality ring laser gyroscope, to carry out measurements that would show the Earth’s rotation. A paper by Beverini et al. discussed how one could measure the Earth’s rotation with this instrument.
Apparently Knodel purchased a ring laser gyroscope, with the aim of demonstrating that the Earth does not rotate. According to the Debunking Flat Earth Misconceptions Web site, Knodel’s measurements indeed demonstrated the Earth was rotating at an angular rate of 15o/hour (one full rotation per day). Furthermore, the gyroscope showed rotation in the “correct” direction (a specific direction in either Hemisphere). It is claimed that Knodel refuses to publicly announce the results of his gyroscope test until he can produce the “correct result” (i.e., a stationary Earth). This shows the lengths to which Flat-Earthers are willing to go to maintain their predetermined conclusions. They have no interest in actually obtaining the correct result, or in minimizing errors in their measurements. Any result, no matter how crappy, that is consistent with their Flat-Earth biases is accepted, while they go to incredible lengths to “explain away” anything that demonstrates the motion of the Earth.
II.6: Size and Distance of the Sun
Flat-Eart