In addition to the geometric (space) and chronologic (time) relationships between collector surfaces on Earth and the sun, the intensity of the sun's irradiation changes based on the distance between the Earth and the sun because of the eliptical orbit of the Earth. While the average solar radiation flux is 1361 W/m2, the annual fluctuation due to the Earth's eliptical orbit is greater than $\pm40$W/m2, resulting in peak extraterrestrial radiation flux of about 1410 W/m2 in January and lows of about 1320 W/m2 in June. The following equations can be found in the D&B text section 1.4.
Gon = Gsc (1 + 0.033 cos(360n/365))
Gon = Gsc (1.000110 + 0.034221 cos B + 0.001280 sin B +0.000719 cos 2B + 0.000077 sin 2B)
where Gon is the extraterrestrial radiation incident on the plane normal to the radiation on the nth day of the year and B is given by
B = (n − 1) *360/365
The Earth's atmosphere is the source of the greatest uncertainty in predicting solar radiation incident on a solar collector. Particulates, aerosols, clouds, moisture, etc. are the drivers behind the intermittency of incident solar energy due to atmospheric attenuation. Some meteorologists and atmospheric scientists have spent their entire careers focused on the problem of modeling the atmosphere to develop predictive tools for increased accuracy in energy models. The application of such atmospheric modeling tools is enabled with an accurate level of solar radiation known outside of the Earth's atmosphere (extraterrestrial).