Pendulum clocks - the usual electronic movement type (battery feedback pulses solenoid when magnet on pendulum passes by). My experience with these movements is that they are somewhat delicate: you have to carefully adjust the clock movement vertically and horizontally until the pendulum swings reliably. However after running for several hours, sometimes the pendulum stops. I think that I've ruled out the battery as the guilty party. That set me to wondering: does gravity vary? If so, what are its characteristics? Could gravity changes account for the pendulum stopping?