The Equation of Time measures the difference between the Mean Solar Time, and the Apparent Solar Time. Mean Solar Time is based on the location of the Sun in the sky assuming that it moves each day at an average rate of 360 degrees/365.24 days = 59.1 arc minutes per day to the east along the celestial equator. But we all know that the Earth doesn't move at the same speed in its orbit. In January when it is closest to the Sun, it is traveling faster than in June when it is farthest from the Sun. There is also an effect caused buy the 23.5 degree inclination of the ecliptic relative to the Earth's equator. The actual motion of the Sun, which marks the Apparent or True Solar Time, changes relative to the average location of the Sun for the same day. The difference can be as much as 16 minutes.
Month Minutes .................... Jan -12 Feb -15 Mar -10 Apr - 3 May + 3 Jun - 5 Jul -10 Aug - 7 Sep - 3 Oct + 5 Nov +13 Dec - 3 .......................