A mechanical watch movement is a tiny-but-complex system made up of components in equilibrium, operating flawlessly. But because the those components are delicate – especially the parts that make up the escapement – its operation can be influenced by external factors, including shock, moisture, and more commonly, the position of the watch, whether on the wrist or off.
The position of a watch determines how gravity affects the moving parts of the movement. In other words, the timekeeping of a watch can vary according to whether it is laid on its back or on its side. That resulting variation is known as positional error, and it is largely because of gravity’s effect on the balance wheel – the oscillator in the regulating organ of the movement.
Beyond the position of a watch, positional error is also shaped by factors like the amplitude of the balance wheel, the type of hairspring, and the poise of the balance. All are inextricably linked and must be adjusted just right – properly regulated in watchmaking parlance – in order for a watch to have minimal positional error and thus keep good time.