Equatorial Pendulum
At the equator the rotation of the Earth is not observable in relation to the pendulum swing because the plane of the pendulum swing is moving along with the rotation of the Earth and no net change in relationship occurs. It is shown by the diagrams that the surface velocity vectors of the Earth underneath the swing of the pendulum are either balanced in the same direction or included within the same plane as the pendulum swing.
For the equatorial pendulum with a swing in the longitudinal (north-south) direction the surface velocity vectors on either side of the swing are balanced in the same direction and the center-point of the pendulum swing is moving along with the direction of rotation (see Figure 2A).
For the equatorial pendulum with a swing in the latitudinal direction (along the equator) the surface velocity vectors on either side of the swing are not balanced in the same direction but are acting within the same x-y plane as the pendulum swing (see Figure 2B). These vectors are all of the same magnitude, that of the equatorial velocity. On one side of the pendulum swing the surface velocity vectors are angled upwards but on the opposite side the vectors are angled downwards. Since these vectors are all in the same plane as the pendulum swing there is no change of the surface in relationship to the plane of the pendulum swing.
Read more about this topic: Foucault Pendulum Vector Diagrams
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