Due to the eccentricity of the earth’s orbit around the sun, the intensity of the solar radiation just outside the earth’s atmosphere is not constant. In fact, the solar constant, Io, is the annual average intensity. Between January 2nd when the earth is closest to the sun and 3rd July when it is furthest away, the intensity varies ±3% from the average.
with n denoting the day of the year, n = 1…365. As a consequence, the Southern hemisphere enjoys more irradiation during its summer than the Northern hemisphere.