WEEK DAY
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Displaying the seven days of the week is one of the simplest complications to create. This indication is rarely seen as the sole addition to the time display; it is generally combined with the day in simpler cases and is also found in complete calendars, and most frequently in annual, perpetual, or secular calendars.
The seven-day weekly cycle is constant and regular. Thus, it simply involves advancing a disc or a hand each time the hands indicate midnight. A derivation from the motion work allows the driving wheel of the week days to rotate at a rate of one turn per 24 hours. A finger drives the day disc or the 7-pointed star (the wheel that carries the day hand). The disc and star are positioned by a jumper.
Different driving systems exist and can be classified into two main categories: semi-instantaneous mechanisms and instantaneous mechanisms.
In the first system, the disc or the star is gradually advanced (in a trailing manner) by the finger of the driving wheel, and then the jumper snaps over the next tooth of the disc or star, positioning the disc or hand instantly in its next position.
The instantaneous mechanism involves gradually arming a spring during the final minutes before midnight. At midnight, the energy stored in the spring is released instantaneously. The spring then drives the finger of the driving wheel in a rapid movement that causes the disc or hand of the week day to jump instantly.
The instantaneous mechanism avoids reading confusion in the minutes following midnight; however, this system requires more energy than the semi-instantaneous system.
In most cases, the days of the week are indicated by a disc displaying the current day in an aperture or by a hand on a subsidiary dial. In such cases, the day of the week display can also be of the retrograde type.