Dial, case and movement signed. DIAM. 42 mm. THICKNESS 13.5 mm. Equation of time indicates the time difference between the true solar day and the mean solar day (or time told by a clock or watch). It has two major causes. The first is that the plane of the Earth's Equator is inclined to Earth's orbital plane. The second is that the orbit of the Earth around the Sun is an ellipse and not a circle. Equation of Time due to Obliquity (the Earth's tilt). If the Earth's rotational axis was not tilted with respect to its orbit around the Sun, the apparent motion of the Sun along the Ecliptic would fall directly on the Equator, covering the same angles along the Equator in equal time. However, this is not the case, since the angular movement is not linear in terms of time because it changes as the Sun moves above and below the Equator. The projection of the Sun's motion onto the Equator will be at a maximum when its motion along the Ecliptic is parallel to the Equator (at the summer and winter solstices) and will be at a minimum at the equinoxes. Equation of Time due to Unequal Motion (the Earth's elliptical orbit). The orbit of the Earth around the Sun is an ellipse. The distance between the Earth and the Sun is at a minimum around December 31 and is greatest around July 1. The Sun's apparent longitude changes fastest when the Earth is closest to the Sun. The Sun will appear on the meridian at noon on these two dates and so the Equation of Time due to Unequal Motion will then be zero. The mean solar day, calculated by averaging all the days of the year, was invented by astronomers for convenience so that the solar day would always be 24 hours. True solar time and mean solar time coincide four times a year, on April 16, June 14, September 1, and December 25. On these days, the equation will equal zero. During the other 361 days, the equation of time must be used to indicate the difference between the two times, amounting over 16 minutes at certain times of year. The minimum difference occurs on November 1 with a loss of 16 minutes and 23 seconds and the maximum occurs on February 11 with an increase of 14 minutes 20 seconds. This positive and negative value is offset in the time of the local noon and those of sunrise and sunset. Equation of time, often represented by a fi gure eight, called an "analemma", can be approximated by the following formula: E = 9.87 * sin (2B) - 7.53 * cos (B) - 1.5 * sin (B) Where: B = 360 * (N-81) / 365 Where: N = day number, January 1 = day 1.