&gt; So &quot;astronomical summer&quot; oddly starts with the day that the incoming solar energy is highest in the northern hemisphere?<p>In a way, but you&#x27;re looking at the effect, not the cause. The solstice is the day where either day or night is the shortest, which is caused by the earth either being at the aphelion or the perihelion (i.e. nearest or farthest points from the sun).<p>One could reasonably suspect that the solstice dates better represent a period just before the centre of the season rather than the beginning of it, since seasons take a bit of time to move in (at least up here in Canada). Snow won&#x27;t stay until the ground has cooled enough, for example.