When the spring equinox occurs, the sun rises exactly due e…

When the spring equinox occurs, the sun rises exactly due east and sets due west. In the context of Stonehenge, this alignment of the sun and the Earth is marked by the Heel stone. The ancient builders positioned the Heel stone in such a way that, on the morning of the spring equinox, the rising sun's first rays align with the main axis of the Stonehenge monument and shine through the stones, casting its light upon the altar stone within the circle.