Excerpt:

At its heart, a mechanical watch is a fancy spring. A metal coil stores power when the crown is wound tight. A series of gears harnesses that energy in even increments. It spins a central wheel, whose oscillations are geared to turn the watch’s hands.

Once gears spin, it’s possible to add more complications, as watchmakers call them. A date display, for example, can be accomplished by adding a reduction gear mechanism to cause a calendar disc to rotate every two full revolutions of the hour hand. A similar mechanism can track the phases of the moon. A more complex one, called a perpetual calendar, can account for months less than 31 days and even leap years. The more complications, the more complexity, cost, labor, value, and mechanical drama.

For the full article, read here.