The descending weight provides the motive force for the movement as well as the indication of time. The method of timekeeping in Japan at this period was based on a day divided into two parts ? from sunrise to sunset and from dusk to dawn. Each part was divided into six hours. As the duration of daylight and night hours vary in length in accordance with the seasons, it was necessary to provide for these variances with the different time scales. This clock is unusual for the fact that it has interchangeable scales to account for these differences. The majority of this type of clock has adjustable hour markers, which need to be manually adjusted for the seasons.