Question

What is a second's pendulum ? What is its length ?

Answer 1

A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz

If time period is kept constant, length of a seconds pendulum is directly proportional to acceleration due to gravity. As its length on Earth is approximately 1 m, it will be about 1/6th of a metre on the moon.