The moon-phase complication provides an excellent allegory for the appeal of luxury watches as a whole. It has been a very long time – several hundred years, at least – since mankind needed to refer to the Moon’s progress through its monthly wax and wane for practical purposes, yet a great many fine watches (themselves not a strict necessity since the 1970s) display just that information.

In fact, not only do they display it, their makers devote great time and ingenuity – both mechanical and mathematical – to improving its accuracy. And having done so, they ensure that it is given pride of place on the dial and picked out with beautiful hand-crafted decoration. This is not because they are under any illusions that the wearer of, say, a Jaeger-LeCoultre Master Calendar will be troubled to notice that the golden disc representing the moon is a day out of sync with Earth’s satellite. They do it because it respects tradition, namely the Moon’s ancient role in establishing our system of months and seasons, and because having committed to doing anything, it should be done as well as humanly possible.

In short, if you understand the presence of the moon-phase complication, you understand a lot about the watch industry. A Jaeger-LeCoultre moon phase is accurate to one day in 122 years, and its appearance at six o’clock on this model’s dial epitomises the fine balance between functionality and beauty that all watches strive for. In this video, we explain exactly how the moon phase works, touch on the maths behind its accuracy, and highlight its aesthetic impact on two elegant classics from the Jaeger-LeCoultre Master collection. And – never say we are not all about the practical, useful knowledge – we recap exactly how you’re supposed to read it.

If you want to know more about the moon phase and its history, we recommend Ms Laura McCreddie-Doak’s in-depth piece, covering the very earliest origins of mechanical timekeeping and the latest genius watchmakers creating moon phases so accurate they might just outlast all of human civilisation.

You can watch other episodes of our About Time video series here, or on our YouTube channel.