Equation of time ET--Algorithm 1.12

The Equation of Time describes variation between clock time and solar time due to eccentricity in the earth�s orbit. ET is positive when sun time is before clock time. Like solar declination, ET depends on the leap year cycle, but the following equation is accurate to within 40 seconds.

Day number, J
J=1 on 1 January, J=365 on 31 December. February is taken to have 28 days.


The argument of the sine function is in radians.

IES (1)
  1. IES Calculation Procedures Committee Recommended practice for the calculation of daylight availability Journal of the Illuminating Engineering Society of North America 13 (4) 381-392 (1984)

Go Back to Subtask C Contents