Just like the minute repeater, Tourbillon and perpetual calendar, Equation of Time is a rare and important complex function. It is true that the function is mysterious, it can not herald the time, and resist the influence of gravity, either. But it is rooted in watchmaking history, therefore, it is meaningful for watch lovers.
The equation of time tells the solar time – The time needed for the sun to return to its original position in the sky every day. On average, the difference between it and real time is not obvious (no more than 16 minutes). For practical reasons, people divide every year into 365 and 1/4 days, every day into 24 hours and per hour into 60 minutes. However, the orbit of the earth is elliptical rather than circular, so the time associated with the sun changes every day. April 15th, June 14th, September 1st and December 24th, only these four days of the year were exactly 24 hours. The changes always appear on the same date, which can be incorporated into the movement mechanism, depending on the year rotation of the cam can be achieved. The cam is usually directly linked to the calendar, so the display of the time equation always corresponds to the current date.
In the two way of presenting time equations, the most desirable is to add a sub dial or arc which marking -16 to 14 scale and indicating how many minutes needed to add or subtract for the flat sun to get the true sun? In other words, it is an equation or a formula. The recent example is the Radiomir 1940 and Luminor1950 time equation special edition watch launched by Panerai in 2015 SIHH. Both of them use the linear scale instead of arc to show the changes. Panerai located it at the bottom of the dial to replace the conventional storage display.
One of the reasons that why time equation is so precious and valuable for collection is the rarity. There are many brands that can manufacture Tourbillon and minute repeater watches, but there are few brands that can make time equations. There are only a handful of Vacheron Constantin, breguet replica watches, Patek Philippe, Panerai, Zenith, Athens and Audemars Pigeut.