How did we “calibrate” the second?

Question

It seems like everything with our calendar is based around 24hr days and the number of 24hr days to revolve around the sun. But a 24hr day can be broken down to 1,440 minutes and in turn 86,400 seconds. How did we (humans) calibrate the second so that exactly 86,400 would be 1 rotation of the earth to the point where we never need something like a “leap second” like we have with leap years?

In: Planetary Science

Answer 1

I read in the book that if a weight is tied to a string of a certain length, the time for the pendulum to swing back again will always be a second.