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I have always heard that the first and most prominent definition of the metre was to use the length of the seconds pendulum - pendulum with the period of exactly 2 seconds. However, in the end it was abolished because of the fact that gravity is different on every point on Earth, affecting the length of the pendulum.

However, I am confused about one other thing: the period of any pendulum depends not only on its length, but also noticeably on the initial angle (amplitude) the pendulum makes with the vertical.

Why were these scientists so eager to adopt the seconds pendulum definition of the metre in light of the fact that the length would be different for different amplitudes?

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First of all, the amplitude of the pendulum swing has a very small effect until you get to rather large swings. That's the beauty of the sine function. :-) .

Next, it took rather a while for science to develop the concept of repeatability, let alone repeating this precise pendulum measurement in several locations -- and observing a repeatable difference.

In the meantime, it was only 10 years after Wren's original pendulum proposal that Gabriel Mouton suggested setting the meter relative to the Earth's meridian arc. At that time (late 17th century) this method was undoubtedly more difficult.

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