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860P
Explanation of the functions
The moon-phase indicator:
Watches with moon phase indicators depict the cycle of the moon (new moon, first quarter, full moon, last
quarter) on their dials.
As the moon orbits the Earth, the relative positions of the sun, Earth and moon change constantly.The phases of
the moon correspond to the illuminated half of the moon being visible from different angles; they are therefore
not due to the shadow cast by the Earth on the moon.The moon merely reflects the light of the sun on the Earth.
The phases of the moon depend on the relative positions of the sun, moon and Earth.The moon is full when the
sun and moon are on either side of the Earth. However they become invisible (new moon) when they are both
aligned on the same side of the Earth.
The first lunar phase is the newmoon. The main feature of this phase is the invisibility of the moon in the sky.
At this precise moment, the moon is located between the sun and the Earth.
Little by little, the illuminated side of the moon becomes visible to the human eye as it moves around the Earth.At
first, a slim crescent can be observed in the evening sky and, as the days pass, the crescent becomes thicker and
remains visible for increasingly longer periods of time.After the full moon, the visible part of the moon diminishes.
A complete cycle is called a lunation, or lunar month, and corresponds to the duration between two full moons
(synodic period). It lasts 29 days, 12 hours, 44 minutes and 2.9 seconds, or approximately 29.530588 days.
The Piaget moon phase is said to be “astronomical”.The wheel that indicates the moon has 135 teeth. With this
system, the precision of the lunar month is 29 days, 12 hours and 45 minutes (29.53125 days as opposed to
29.5 days in general). The gap between this mechanism and the true lunar cycle thus only amounts to one day
every 122 years (instead of one day every 3 years in general). The complication is modelled on the Northern
hemisphere.
860P
Explanation of the functions
The complication is symbolised by a gold plate fixed atop the movement, on top of which a disc in the
shape of the number 8 rotates according to the lunar calendar. Thus, the mechanism illustrates the
interplay between the sun and the Earth perfectly. The moon is made from a gold plate, which is heated
and brought to the boil, before being polished. This is how the bumps reminiscent of lunar craters are
obtained. This process ensures that each “moon” is “unique”.
ENGLISH
15th night 11th night 7th night 4th night night 0
29th night26th night22nd night18th night
Sun
Earth
Gibbous moon
First quarter
(quadrature) First crescent
New moon
(conjunction)
Last crescent
Last quarter
(quadrature)
Gibbous moon
Full moon
(opposition)
