I recently read this article, but is quite unclear on how it got those equations and used them to find the amount of energy (J) an area receives from sunlight over a period of time (s). Does anyone have information or ways to calculate this?
The article shows this equation of calculating it:
$$mJ = (mW/cm^2) \cdot (Area/cm^2) \cdot (Time/s)$$
and subsequent other equations, which have no sources, so I am quite unsure if this is reliable.
$$\theta = 23.5° \sin (2{\partial T/365.25})$$
Solar constant for the day:
$$\sigma D = (137mW/cm^2) \cos (L - \theta)$$