How is the constant "284" derived within the Solar Declination Angle equation?
The earth's axis results in a day-by-day variation of the angle between the earth-sun line and the earth's equatorial plane called the solar declination $\delta$. This angle may be estimated by the following equation [6]: $$ \delta = 23.45° \sin\left[\frac{360°}{365}(284+N)\right], \tag{2.1}$$ where $N =$ year day, with January 1 $+ 1$.