Cornerstones of time measurement, astronomical complications illustrate what the ancient civilizations observed and they continue to punctuate our lives today.
Seasons, equinoxes, solstices and signs of the Zodiac indicated by the hand on the sun
The multiple seasonal calendar with astronomic scale is both ingenious and precise. Placed at the edge of one of the two main dials on the Reference 57260 watch, it carries three separate pieces of information that may be read simultaneously via the central gold hand identifiable by its sun-shaped counterweight.
Completely to the exterior, we find the display of the perpetual calendar of the months of the year with their respective numbers of days.
The next concentric circle displays the division of the year into the constellations of the Zodiac, with a display of the dates of the spring and autumn equinoxes —the two points at which the duration of the day and night are almost equal— and the dates of the summer and winter solstices, when the Sun is at its zenith or its lowest point in relation to the celestial equator.
The four connected seasons — spring, summer, autumn and winter — are displayed more towards the interior on an additional concentric circle.
The two annual equinoxes occur in March and September. In the Northern hemisphere the March equinox, called the Spring or Vernal Equinox, falls on the 19th, 20th or 21st of the month. In September the Autumn Equinox falls on the 22nd, 23rd or 24th of the month. The Earth's axis is always inclined at approximately 23.5° compared to the ecliptic, an imaginary chart of the Earth's journey around the Sun. On non-equinox days the Earth's axis leans slightly towards the Sun in both the Southern and Northern hemispheres. At the equinoxes, as the Earth's axis is perpendicular to the Sun's rays the lengths of the day and night are more or less equal, hence the term equinox. Like equinoxes, solstices are directly connected to the seasons. In many cultures the solstices mark either the beginning or the middle of winter and summer. In the Northern hemisphere, on the day of the Summer Solstice, around June 20th-21st, the Sun is at its highest in the sky. This is the longest day of the year. By contrast, on the day of the Winter Solstice, around December 21st-22nd, the Sun is at its lowest point. This is the shortest day of the year.
Star chart (for the owner's city)
The map of the night sky is a very rare complication found only on a select few of the most sophisticated and important watches in the world. The rotating blue disc precisely indicates the constellations visible at night from the owner's place of residence, with the months of the year inscribed on the edge. The rotation of the map depends on the sidereal or stellar time which may be read on an exterior 24-hour scale: choose a star and follow its development on the 24-hour scale in order to know the current sidereal time at any moment.
Sidereal time hoursSidereal time minutes
Sidereal time is generally used by astronomers and navigators. The unit is the sidereal day, or the interval of time between two consecutive passings of the vernal point on the meridian. As it may be measured from the Earth's rotation by calculating the distance of the stars, this is known as sidereal or stellar time (from the Latin sider meaning star). The sidereal day lasts 23 hours, 56 minutes and 41 seconds and the sidereal year has an additional day to the solar year. By using the stars as a point of reference, sidereal time gives a more regular timescale than that which takes the Sun as a reference. Of course, the star chart is mainly used at night when the constellations are visible. That is why the most prominent indications on this watch are the times between 8 p.m. and 4 a.m. (indicated in red).
Equation of time
The equation of time sector indicates the difference in minutes between the true solar time and the conventional mean time. It varies throughout the year. Mean time can be ahead by a maximum of 16 minutes 33 seconds (around November 3rd) and behind by a maximum of 14 minutes 6 seconds (around February 12th). Solar and mean time only coincide four times a year. The hand's movement is driven by an irregular cam that depends on the sidereal time mechanism. The equation of time also corresponds to the season indicated by the sun-shaped hand, which is itself controlled by the sidereal time mechanism. Historically, knowledge of the equation of time was useful to navigators who observed the height of the Sun at a given moment, then comparing it to the original time given on a conventional dial.
The equation of time exists for two reasons. Firstly, the Earth's equator is at a tilt compared to the Earth's orbit. Secondly, the Earth orbits the Sun in an ellipse, not a circle.
Sunrise times (for the owner's city)Sunset times (for the owner's city)Length of day (for the owner's city)Length of night (for the owner's city)
Rare is the watch that can indicate the times of the sunrise and sunset as well as the lengths of the day and night in their owner's city of residence throughout the year. On the dial of the Reference 57260 watch, the information is displayed on two specific double sectors that depend on the perpetual calendar mechanism. The latter corrects the displays automatically for the entire duration of the year, so that the sunrise and sunset times, as well as the day and night lengths, are suited to each of the 24-hour periods. Each sector has two gold hands mounted on the same axis. The sunrise hand indicates the corresponding time throughout the year, between 4 a.m. and a little before 8 a.m. depending on the time of year. The sunset hand opposite displays the corresponding time which varies between 4 p.m. and approximately 7.30 p.m., also depending on the time of year. The lower sectors are reserved for the lengths of the day and night: hands display the appropriate numbers, the number of hours in the day counted from sunrise and the number of hours in the night counted from sunset. For example, if the sun rises at 4.30 a.m. and sets at 7.30 p.m., the displayed day length is 15 hours and the displayed night length is nine hours (from 7.30 p.m. to 4.30 a.m.). The total always corresponds to a 24-hour period.
The display of sunset and sunrise time is controlled by cams connected to the perpetual calendar mechanism and the correction is made automatically, every day at midnight. On the owner's request, the cams may be adapted to any location in the world and made to measure at the time of manufacture of the watch. The times indicated are to the nearest five minutes of the actual sunset and sunrise times coinciding with the perception of the upper part of the Sun on the actual horizon, at sea level. The actual sunrise and sunset times are conventionally corrected to take into account the refraction of the Earth's atmosphere.
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