The mean amount of sunlight at the top of earth's atmosphere is called the solar constant. Measurements made by several satellites show that the solar constant is 136.8 watts per square centimeter.
The sunlight intensity at earth varies because earth's orbit around the sun is slightly elliptical.
The difference in the intensity of sunlight between perihelion (closest point) and aphelion (farthest point) is about 6.7%. Use the solar constant table to find the solar constant for the first day of any month.