What is an Equation of Time?The equation of time was developed to explain and compensate for the discrepancy between true solar time and mean solar time. The true solar day is the length of time between one local noon, when the sun is highest, and the next. The length of this day is not always 24 hours, as the hours between sunrise and sunset are not always the same. The variable rate of the true solar day is due to two factors: 1) the Earth's rotation around the Sun is an irregular elliptical course and 2) the tilt of theEarth's rotation axis relative to its orbit around the Sun. Our yearly calendar that we rely on is based on an equal length of days throughout the year. The mean solar day (or time told by a watch or clock), 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. The true solar time and mean solar time coincide four times a year, April 16, June 14, September 1 and December 25. On these days, the Equation willqual zero. During the other 361 days, the equation of time must be used to indicate the difference between the two times, amounting to about 16 minutes at certain times of the 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 fourteen minutes and 20 seconds. These positive and negative values are the offset in the time of the local noon and those of sunrise and sunset. The equation of time is oftn represented by a figure 8, called an 'analemma'.The equation of time 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