“wHILE PERPETuAL CALENDARS ARE ALREADy IMMENSELy COMPLEx, THE PINNACLE Of CALENDAR COMPLICATIONS IS, wITHOuT DOuBT, THE EquATION Of TIME”
words, nature’s time) and “mean” solar time (a manmade construct—what we, humans, count as a full day). Please bear with us as we attempt to explain.
See, the Earth’s orbit around the sun is elliptical. Not only that, the axis of the planet’s rotation is slightly tilted from perpendicular to the plane of the equator. As a result of these two factors, the length of a “true” solar day (which is defined as the interval between two “true” noons—when the sun is at its highest point in the sky) varies quite a bit throughout the year. In fact, there are only four days in the year that are exactly 24 hours: April 15, June 14, September 1 and December 24. All the other days are either shorter or longer than 24 hours, ranging from minus 16 minutes and 23 seconds (on November 4) to plus 14 minutes and 22 seconds (February 11).
Historically, sundials would show “true” solar time, and some early clocks were equipped with mechanisms that would correct their readings to match sundials. As mechanical clocks became our standard timekeeping device, “mean” time (which was technically “uncorrected”) became the accepted standard. Equation of time watches (with the word “equation” here is used in the archaic sense of “reconciling a difference”) once again allows us to follow “true” time.
Most equation of time watches show the difference between “true” time and “mean” time on a separate sub-dial (usually an arc)