Animation: the white figure sees the hexagram 'growing'. The left one from midsummer to midwinter, the right one from midwinter to midsummer.
In this website, "Chinese Fortune Calendar" the TaiJi symbol is explained.
All lines dark at winter solstice. All lines bright at summer solstice. Three bright, three dark at spring and autumn equinox.
The sun casts the shadow. In the course of one year, drawing a line along their lengths in a sun-dial (see also the animation above) creates a spiral. So the sun creates the TaiJi.
The moon creates the hexagrams out of the shadows of the sun.