Conventional moon-phase displays must be corrected by one day every two and a half years because they round the lunar month down to 29.5 days. The display of the Lange 1 Moon Phase reproduces the so-called synodic month, which lasts 29.531 days, to an accuracy of 99.998 per cent. Once it has been correctly set, the moon-phase display will deviate from the true position of the moon by only one day every 122.6 years.
On the celestial disc, the different times of day are represented by varying blue hues caused by interference effects. During the day, it shows a bright-blue sky without stars, while at night it depicts a dark-blue starry sky. Thus, the moon always orbits against a realistic background.
Caliber L121.3 is based on the 2015 caliber L121.