Stonehenge was built 4500 years ago. It was built to observe solstice. Solstices happen in 6 months interval. It is very important astronomical shift in earth alignment in relation to sun. Summer solstice is the point where northern pole of earth tilts towards the sun to the maximum possible angle. So it is the closest position of northern pole towards the sun.
In winter solstice the opposite happens. The northern pole reaches to the furthest point from the sun. These are important turning point to understand seasonal changes in earth. In the winter solstice sun sets directly below the big stones called Alter Stones. During the summer solstice sun rises from the opposite side of Stonehenge. It rises from the Hill Stone illuminating Alter Stone. Solstices happen exact same time every year. So our ancestors use this as a reference point to map various times in a year.
Solstices are very important astronomical event that helps to track time. So many other ancient civilizations built massive monument to keep track of solstices. One of the monuments is Chaco canyon built by mysterious ancient people called anasazi.