In the video above, I talk about how pendulums actually work. If you haven’t watched it, the principle is simple: an object is suspended from a fixed point and allowed to swing back and forth – the mass of the object and the time it takes to swing back and forth are independent of each other, relying only on the length of the string and the strength of gravity.
Normally, you’d think of gravity on Earth’s surface as being constant, but the Earth isn’t a perfect sphere, meaning that the force of gravity near the equator is slightly weaker than at higher or lower latitudes. And how did we discover this fact? Pendulums!

In the year 1671, a French scientist named Jean Richer travelled to French Guiana. Among several experiments and astronomical observations during his two-year trip was to take measurements with a clock pendulum.
He set up the pendulum in the same way I did in my video, but he adjusted the length of the pendulum so that one half-swing took exactly one second, a common technique at the time. What he found was that the pendulum length needed to be slightly shorter than it did back in Paris, by about 3 millimeters. Though a small difference, it was significant enough to begin a discussion about the varying gravitational field of Earth.

This was later proved by Isaac Newton by determining that due to the Earth’s rotation, it was thicker at the equator, meaning the surface was further away from Earth’s center of mass. This was further supported by Newton’s idea that gravitational force decreases as the distance between two objects increases.
Scientists started to use pendulums to take measurements of the gravitational field in other locations and began to create a model of the Earth’s true oblong shape. Since then, we’ve developed more accurate methods to measure the same thing, but they were pioneered by those first efforts.
Written By: Scott Yarbrough

Video Music: Funky Chunk Kevin MacLeod (incompetech.com)