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101: A Love Letter to The Moonphase


A classic complication stays timeless. The Celestia Series brings to mind the transfixing power of a shooting star with every glance. 

As a matter of opinion, the moonphase may be the loveliest of all complications, those horological features that extend your watch's functionality beyond the basic timekeeping of hours and minutes. 

Developed to aid astronomers' calculations and sailors' navigation, moonphase tracking is still the tool of practitioners of primal technologies. Farmers seeking the best days to plant. Astrologers looking for luck or love.

You don’t have to be a stargazer or seafarer to understand why humankind wants to know its place in the cosmos—or to appreciate the ingenuity and history of the complication.



The hands on your watch track the sun’s movement across 24 hours in two turns around a 12-hour dial.

The moonphase tracks the lunar cycle, which is roughly a month in duration—or 29 days, 12 hours, 44 minutes, and 2.8016 seconds.

To set the moonphase, use the crown, which functions as an easy key for your watch's mechanics.

Twisting the crown rotates the watch hands along with the moonphase disc.

When the moon’s current phase appears in the aperture, push the crown back in to set the time and moonphase and to restart the watch.



Moonphase watches, like Celestia, have a curved aperture, or opening, in the dial that reveals a partial view of an illustrated disc.

The rotating graphics include a full moon, typically shown as a solid white circle, and a representation of the new moon, which is often depicted as a starry sky.

The starry-sky background reflects the appearance of the natural sky when the moon is new.

When it is exactly new—which can be calculated to fractions of a second—the sky appears dark to us. That’s because the moon is new when it is closest to passing between the Earth and the sun. Until the moon no longer keeps pace with the sun, we can't see it without special lenses or filters.

As the moon in our sky advances through the two major phases of new and full, with the cycles of waxing and waning between them, twin scallops along the inner edge of your watch's aperture carve out the silhouette of a crescent or gibbous moon.

Astronomical clocks have appeared and disappeared from horology in their own phases.

Royal horologists from China and the region now split between Turkey, Iraq, and Syria created water-driven astronomical clocks from the 11th through 14th centuries. These attempts, most of which were successful, extend the technology of history's great Antikythera mechanism. The mechanism is an analog computer-calendar hybrid, found aboard a ship off the coast of Antikythera, a Greek island, by an archeologist in the early 20th century. Later analysis revealed the artifact tracked the sun across the zodiac, but was primarily a moonphase mechanism. It could predict eclipses and model the moon's irregular orbit, including increases in its velocity at perigee, when the moon is nearest to Earth, compared to apogee, when it is farthest. Two millennia later, we've got tracking it down to an exact science, plus a beautiful horological complication.


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