Paradiso 10: Why the Earth has seasons

Paradiso 10. 7-25:

Lift up then, Reader, to the lofty wheels
With me thy vision straight unto that part
Where the one motion on the other strikes, 

And there begin to contemplate with joy
That Master's art, who in himself so loves it
That never doth his eye depart therefrom. 

Behold how from that point goes branching off
The oblique circle, which conveys the planets,
To satisfy the world that calls upon them; 

And if their pathway were not thus inflected,
Much virtue in the heavens would be in vain,
And almost every power below here dead. 

If from the straight line distant more or less
Were the departure, much would wanting be
Above and underneath of mundane order. 

Remain now, Reader, still upon thy bench,
In thought pursuing that which is foretasted,
If thou wouldst jocund be instead of weary. 

I've set before thee; henceforth feed thyself . . . 

A more recent explanation:

1. The Earth's axis is tilted 23.5^o with respect to the ecliptic and is always pointed to the celestial poles as the Earth moves around the Sun. Sometimes the northern hemisphere is tilted toward the Sun and sometimes the northern hemisphere is tilted away from the Sun.
    
2. The Earth's orbit around the Sun is NOT circular.  
3. The apparent path of the Sun in the sky is known as the ecliptic and is actually the intersection of the plane of the Earth's orbit with the celestial sphere. Because the rotation axis of the Earth (which defines the celestial sphere) is tilted at an angle of 23.5^o with respect to the plane of the Earth's orbit, the ecliptic is inclined at an angle to the celestial equator, as shown in the diagram below.
   The ecliptic and the equator intercept at two points, the vernal and autumnal equinox:  
   
   • Vernal or spring equinox: This point is defined as the point where the Sun, moving along the ecliptic, crosses the celestial equator from south to north. Occuring on March 21, when day and night are of equal length, this marks the beginning of spring.The altitude of the Sun in the sky increases from the spring equinox to a maximum (farthest North) on June 21 - the summer solstice - marking the beginning of summer.
     
   • Autumnal or fall equinox: This point is defined as the point where the Sun, moving along the ecliptic, crosses the celestrial equator from north to south. Occuring on September 21, when day and night are again of equal length, this marks the beginning of fall.The altitude of the Sun in the sky decreases from the autumnal equinox to a minimum (farthest South) on December 21 - the winter solstice - marking the beginning of winter.