The speed of the sun is not always the same. It is sometimes affected behind and ahead from the apparent time. If the sun is perpendicularly overhead some time after the clock strikes 12 O’clock then the time equation will be positive (+) and if the sun is shining vertically overhead before the clock shows 12 o’clock, then the time equation is negative (-). Only four dates of the year are such, when the observable time and mean time are the same. These four dates are April 16, June 15, September 1 and December 25. During these dates, the time equation is zero (0).
