The declination of the sun changes most slowly around the solstices, which are the summer solstice (around June 21st) and the winter solstice (around December 21st).

Here's why:

* Solstices mark the points of the sun's furthest north and south declinations. At these points, the sun's apparent movement in the sky slows down before it reverses direction.

* The sun's declination changes most rapidly during the equinoxes, when the sun crosses the celestial equator. This is because the sun is changing its direction of movement at a much faster rate.

Think of it like a swing. At the highest point of the swing's arc, the swing slows down before changing direction. Similarly, at the solstices, the sun's declination slows down before it reverses its direction.