See a graph of the sun's declination near solstice. While sometimes true, quite often we find the light interacting with the edge of the glyph. Interactions can often be much more complex than expected. One should always continue observing well after what seems to be the peak of the interaction. The surface casting the shadow should be examined.
Many solar markers appear to have been placed where naturally occurring shadows fall.
Since many glyphs are quite old, people often wonder if effects such as precession might cause changes in how sunlight interacts with glyphs over time. When considering solar interactions, we are concerned only with the motion of the sun, or from a heliocentric point of view, the orbit of the earth around the sun. For our purposes, changes in the orbit of the earth are negligible for more than a thousand years. A common misconception is that indirect solar marker interactions occur at sunrise or at noon. In fact, they can occur at any time of the day.