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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 $+ 1$.

from ScienceDirect Topics: Solar Declination

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This is to get the correct phase, so that for $N=81$ (which is March 21, the March equinox) the result will be $\delta=0$, as shown in the graph below.

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Image from REUK.co.uk: Solar declination

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