A pendulum is a weight suspended from a fixed pivot that can swing. When you swing a pendulum, you can assume it is a simple harmonic oscillation.
oscillating period = 2π x √pendulum length / gravity acceleration
This equation tells us that the pendulum weight and the oscillating period are irrelevant.
The gravity acceleration on the earth is approximately 9.8[m/s2].
Length → Osillating Period
Osillating Period → Length
Osillating Period & Length → Gravity Acceleration
When you are outside of the earth, the oscillating period changes due to the gravity acceleration change. Therefore, measuring the oscillating period will tell you the gravity acceleration where you are.
In the science fiction "Project Hail Mary," Ryland Grace used this technique to identify the acceleration of the spaceship he was boarding.