Stationary pendulum experiment during the August 11 1999 solar eclipse            my home page

This page last revised on March 27, 2011

This pendulum was meant to be at rest (stationary) in the lab frame, i.e. without initial motion.

Location of observation: Marigliano,  4055N,  1428E. Local data: eclipse starts (C1) at 09:21:08 UT, maximum eclipse at 10:47:28 UT, altitude of the sun 64 deg, 78.3% solar coverage, eclipse ends (C4) at 12:13:43 UT.

The pendulum was 5.5m long. The suspension was made of stainless steel, 7x7 counter-twisted thin wires with an overall diameter of 0.9mm. The bob was a brass cylinder about 7cm in diameter and 14cm in height, weighing about 4.6Kg. The isotropic suspension was carefully realized, with particular attention to the hinge point and to the connection to the bob. All mounting accessories were made of brass.

The pendulum was installed in the staircase, and the bob was about 1m under the ground level. During the experiments the doors were sealed with tape.

Figure 1. Snapshots of the video showing the bottom of pendulum No. 1. This is what the display looked like. The actual size is much smaller, see description. The speed of motion is largely exaggerated in this  four-frames animation. This is also a moment of quite large motion, about 60 microrad wide). This is the display at 10:17 UT (the timer shows UT+2).

The recording means consisted of a video camera with macro lens installed under the bob and facing upwards toward the bottom face of the cylinder. Concentric circles were drawn on that face, and a transparent reference template, solid to the environment, was placed between the bob and the camera lens. The camera was connected to a remote VCR and monitor. Figure 1 is a snapshot of the video. The circles and the radial line (torsion indicator) are on the bottom face of the bob, the central dot is 1mm in diameter, and the inner circles are spaced 0.5mm. The reference cross is on the transparent template solid to the environment. The marks on the cross are spaced 1mm. Spacing of circles and marks do not coincide in the images due to parallax effect. For a 5.5m length, 1mm is subtended by an angle of 1.8^10−4 rad, which rounds to 2^10−4 rad or 200 microrad. Angles of 1/20th the above were easily observable in the magnified image on the monitor, corresponding to a resolution of about 10 microrad. Note that this is a bottom view, hence North and South are inverted.

Around the eclipse the pendulum always showed micro-oscillations with periods of about 4.7s, but the periods were often  disturbed, the oscillations being abruptly reduced or widened. The amplitude ranged from almost stationary to nearly 80 microrad, with several changes. The oscillation was characterized by rapid changes of the plane of major axis. The direction of the almost always elliptical motion changed a number of times, and the reversals showed a certain symmetry about the time of maximum eclipse.

It was also observed that, a few times over the observing period, the bob oscillated like a torsion pendulum about its vertical axis with a period of about 1 minute and an angle of about 3 degrees. Here  is an example. This is a fast movie covering more than 10 minutes. Look at the radial line. No torsion was observed over the long period.

Regarding tilts of the vertical, they were superficially analysed with this pendulum, because the procedure was rather complex since the pendulum was in continuous motion. Anyway, it has been possible to notice a discontinuity in the progression of the tidal tilt about 30 minutes before max eclipse. The tilting vertical is seen here.

Abnormal tilts will be better seen with pendulum No. 2. See here.

The most outstanding effect was a kind of wobble, or tremor,  or vibration of the bob, due to an unexpected response of the instrument. This wobbling happened nine times over the period of observation. At each of these occurrences, the outermost reference circles got off focus, then again on focus, alternately a few times within a few seconds. The reference cross stayed on focus. This suggests that the bob was moving up and down. Note that, in front of a macro lens at very short distance, a small motion puts the image off focus, more easily near the edges of the frame. One of these wobbles is seen here.

Control tests were made on August 24-25-26 and September 8-9-10. In brief, the always present motion was much quieter and less erratic, and the Foucault rotation was often observable over periods of hours, despite the small oscillations. Tidal tilts were observed too. In substance, the  motion was not as chaotic as it appeared on the eclipse day.


        The duration of recordings indicated in the page below is wrong, it was 12 hours