Due to the elliptical shape of the Earth's orbit around the sun and the inclination of the Earth's axis, the difference between the shortest and longest solar days of the year is 30 hours and 45 seconds. Keeping time based on real solar time with its ever-changing day lengths would be impractical in everyday life, so astronomers have defined "mean solar time". The length of an average solar day (86,400 seconds), which is indicated by all ordinary clocks and watches is calculated by averaging the lengths of all real solar days in a year. The visualization of the equation, invented at the end of the second century and since then has become extremely rare, shows the difference between average solar time and real solar time. Maximum deviations occur in mid-February (+14 min. 24 sec.) and early November (-16 min. 21 sec.). The lengths of the true and average solar days are identical four times a year (April 16, June 14, September 1, and December 25). Daily differences are programmed into the circumference of a kidney-shaped cam that each year completes a rotation around its axis. An elaborate mechanism "reads" this cam and transfers the information to an additional minute hand or a second pair of hands on the quadrant equation display.