What does the equation of time mean?

　　Do you know what the equation of time in watches means? Let us find out together below.

　　Definition of the equation of time

　　Also known as the time difference, or equation of time. It refers to the difference between clock time (i.e. mean solar time, also known as civil time) and true solar time (also known as actual solar time).

　　Equation of time watch: Based on an ordinary watch (mean solar time), it adds the display of (true solar time) to become an "equation of time" watch.

　　Development of the equation of time in watches

　　The earliest pocket watches with an equation of time were the Marie-Antoinette by master watchmaker Abraham-Louis Breguet and the Calibre 89 by Patek Philippe (which later inspired the movement of the Star Calibre 2000). In contemporary wristwatches, Breguet took the lead in 1991 by launching the first equation of time perpetual calendar moon phase watch, which indicates the solar time difference with a hand on a sector-shaped display at 1 and 2 o'clock. This not only won major awards but also lived up to the innovative craftsmanship passed down by master Breguet. In 2000, inspired by an 1925 sunrise and sunset pocket watch in the Audemars Piguet Museum, the Swiss manufacture Audemars Piguet successfully developed and released the world's first mechanical wristwatch featuring sunrise and sunset times, the equation of time, and a perpetual calendar.

　　In the early years, most equation of time watches added a plus/minus 15 minutes display device on the dial to show the time difference between actual solar time and civil time. However, if it was only displayed this way, one could only know the difference between the 2, without showing which one was the reference point for the deviation. The first equation of time watch to display civil time and solar time simultaneously with 2 hands was Blancpain's Le Brassus Equation du Temps Marchante watch. This intuitive display method allows people to clearly and explicitly know the time difference between the 2 and the specific fast or slow status of the time difference, making the display method clear, intuitive, and simple.

　　True solar time and mean solar time

　　True solar time: Refers to the daily duration of sunlight generated by the Earth's orbit around the Sun. Within a year, true solar time varies every day all over the world. This is because the Earth's distance from the Sun constantly changes along its orbital path. The Earth's orbit is elliptical rather than circular, and there is a 23° angle between the plane of the Earth's rotation and the plane of its orbit. This means that the actual length of a day is not the universally assumed 24 hours. The "24 hours" we use to indicate the time of a day, when precisely measured, is actually not exactly 24 hours, but increases or decreases by about 24 minutes on the basis of 15 hours. Days that are exactly 24 hours long occur only 4 times a year: 4 15, 6 14, 9 1, and 12 24.

　　However, because true solar time is different every day, it poses a very difficult problem for horological timekeeping. Therefore, a relatively simple timekeeping method—mean solar time—came into being.

　　Mean solar time: The timekeeping system of exactly 24 hours a day used on the vast majority of watches is "mean solar time", or more commonly referred to as "civil time". It is a simple timekeeping system that ignores the variations of the equation of time.

　　In astronomy, it is assumed that a sun (the mean sun) moves at a constant speed along the celestial equator (rather than the ecliptic), and its speed is equal to the average speed of the true sun moving along the ecliptic. The time interval between two consecutive transits of this imaginary sun across the meridian is called a mean solar day. This is also equivalent to calling the average of the true solar days in a year a mean solar day, and 1/24 mean solar day is taken as 1 mean solar hours. The so-called "day" and "hour" are usually abbreviations for mean solar day and mean solar hour.

　　One 1/86400 of a mean solar day = 1 mean solar seconds, which serves as the basic unit for measuring time. Modern clocks and watches are all designed and manufactured according to this principle. In 1820, French scientists held a meeting and decided to represent the length of a day by the average value of one rotation of the Earth, which was unanimously recognized by the scientific community as the standard for time measurement. This time standard established back then continued until 1960, representing a fairly long historical phase. The watchmaking industry has continued to use the mean solar time standard as the basis for measurement to this day. Developing to the present, people habitually divide the 24 hours of a day into two 12 hours. 0:00-12:00 is represented by AM; 12:00 to 0:00 the next day is represented by PM. This is the most common usage. Some industries and departments prefer to use 24-hour continuous calculation. For example, astronomical departments, navigation, aviation, and other industries usually represent PM6:30 as 18:30, which is due to the nature of their professional work.