It must first be remembered that to regulate their timepieces watchmakers of yesteryear have no alternative but to refer to the true noon, that is to say, to determine when the Sun is at the zenith. A practical way to find out is using a sundial. Available in many forms, they are commonly used until electricity brings, in the second half of the 19th century, this valuable information. Today, the talking clock or the beep on radios allows everyone to adjust their watch day and night.

The equation of time is in astronomy the quantity which must be added or subtracted in order to pass from true time, that given by the Sun, to the mean time; our time that is arbitrarily divided into a 24-hour day. The equation of time varies from day to day, its value oscillates between about - 16 and + 16 seconds per day. Combining these differences we obtain a variation between the true noon and the mean noon of plus or minus 15 minutes. The most important differences are, depending on the year, around February 12 (+ 14 minutes and 59 seconds) and around November 3 (- 16 minutes and 15 seconds). The difference is to zero around April 15, June 15, September 1st and December 24. It should be known that today with the summer hours and the winter hours we live with a shift of two or three hours compared to the sun; in Geneva, the daily noon corresponding to the solar noon of Central Europe.

The equation of time also tells us about the Spring equinoxes (March 21-22) and Autumn (September 22-23) and the Summer solstice (around June 21) and Winter solstice (around the December 21). The equinox is the time when the Sun is in the plane of the equator, giving the day equal to the night. The solstice is the time when the Sun is farthest from the equator, giving the longest day and the longest night. These dates determine the seasons of the year.

Jobst Bürgi (1552-1632), Swiss watchmaker, astronomer and mathematician, built before 1600 the first instruments and clocks indicating the equation of time. Mechanician of the Margrave William IV of Hesse-Cassel (1532-1592), clockmaker of the Emperor Rodolph II (1552-1612), then collaborator in Prague of Tycho Brahe (1546-1601) and Johannes Kepler (1571-1630), he is in many fields a forerunner.

In 1657, after the invention of the pendulum by the Dutch scientist and mathematician Christiaan Huygens (1629-1695), the clocks could be adjusted to a few seconds a day. The question of the difference between solar time and mean time was then raised. Erudite like Huyghens or John Flamsteed (1646-1719) write tables of equation of time. Sundials with equations are created, notably by the London clockmaker Thomas Tompion (1639-1713).

It is considered that the oldest equation clock – which thus simultaneously indicates the mean time and the solar time – is imagined by the English mathematician, member of the Royal Society of London, Nicholas Mercator (1640-1687); the London watchmaker, of Flemish origin, Ahasuerus Fromanteel (1607-1693) made it, around 1666, under his leadership.

As a result, the scientists and watchmakers of this time begin to manufacture them. In England, there are the names of Dr. Robert Hooke (1635-1703) and Thomas Tompion, followed by his followers Daniel Quare (1649-1724) and George Graham (1673-1751). On the continent, at the turn of the 17th and 18th centuries, other scientists and watchmakers are at work: abbé Jean de Hautefeuille (1647-1724), Hans Georg Enderlin of Basel (1678-1754), Henri Sully (1680-1729), Julien Le Roy (1686-1759), Dom Jacques Alexandre (1653-1734), Charles Le Bon (1678-?) and Antoine Thiout (c.1694-1761).

The middle of the Age of Enlightenment is marked by the invention of the meridian which allows to determine the real midday to the eye. The meridian of the mean time is then imagined. It is like an elongated “8” curve, divided in two by a line through which the mean midday is calculated. In Geneva, astronomer Jacques-André Mallet (1740-1790) calculated that which was placed on one of the walls of the St. Peter Cathedral. Displaced during the Swiss National Exhibition in Geneva in 1896, it’s today on the south facade of the Tour de l’Ile, above the statue of Philibert Berthelier (c.1465-1519), the ardent defender of the liberties of Geneva.

In the second part of the 18th century and in the first part of the 19th century, we see appear watches with equation of time. The greatest watchmakers of the time make regulators, clocks and watches: Ferdinand Berthoud (1727-1807), the Lepaute brothers, Jean-André (1720-1789) and Jean-Baptiste (1727-1802), Jean-Antoine Lépine (1720-1802) -1814), Robert Robin (1742-1799), Abraham-Louis Breguet (1747-1823), Louis Berthoud (1754-1813), Charles Oudin (1768-1840), Nicolas-Mathieu Rieussec (1781-1852), Jacques-François Houdin (1783-1860), Henri Robert (1795-1874) and Louis-Constantin Detouche (1810-1889). In Great Britain, John Ellicott (1706-1772), Thomas Mudge (1715-1794) and Edward John Dent (1790-1853) also perform. In the principality of Neuchâtel (today in Switzerland), Samuel Roy (1746-1822) and his sons make some as well.

In the second part of the 19th century, most watches with equation of time are produced or sold by the most prestigious workshops and watch brands in the Vallée de Joux and Geneva: Louis Audemars (founded in 1811), Audemars Piguet (founded in 1875) , Victorin Piguet (founded in 1880), etc., Vacheron Constantin (founded in 1755) and Patek Philippe (founded in 1839).

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