The two equinoxes (vernal and autumnal) of year have an number of interesting properties. The equinox itself is defined as the moment the sun passes directly over the equator. In locations along the equator there is a true high noon with the sun directly overhead which is impossible for places outside the tropics. Yet in the same day another related phenomenon occurs, that is, when the days and night are of equal length. In ancient sites both celebrated, (Stonehenge, Carnac in France, etc.) and relatively unknown ( Sun Dagger of Fajada Butte, Chaco Canyon, New Mexico), their creators chose to make special signifers for the equinoxes as well as the winter and summer solstice.
Why does the equinox have any significance for this particular sculpture, Equilibrium? It is not a gnomon, which is the angled rod sundials use to measure time. Equilibrium is not so much about counting time as mediating on time and by extension as all processes that are both cyclic and linear. The quiet dance of its shadows give no special mark for the equinox, but rather the equinox is moment when the outside world (i.e., everything outside of the sculptures boundaries) is in alignment with the sculpture.
Many contemporary artists (James Turrell, Nancy Holt, and Robert Morris to name but a few) from the 1960's on have sought to incorporate the solar cycle into their sculptures. It is hardly coincidental. The reasons for this tendency are so numerous as to outstretch the confines of this paragraph. For myself personally, as an artist, the most pressing reason is that life in our highly wired world tend to disassociated all us from our bodies, spatially and temporally. I remind myself about what I am feeling and where I am heading through my work. Who, What, Where, and Why?. These are basic human questions. Sculpture, particular outdoors, can be the bridging answer to the question of what and where. The who and the why are the personal province of each viewer.