There is an astronomical context for understanding mysterious structures like Stonehenge. Keeping a calendar was a vital job for any ancient culture. We can imagine that an enormous effort would be put into measuring and marking the changing seasons. In the absence of true understanding, we can also imagine that these structures would have a ceremonial purpose. Today, most astronomers and archaeologists agree that Stonehenge was built as a marker or ceremonial site, dedicated to observing the date of summer solstice. The avenue points from the center of Stonehenge to the position of sunrise on June 21st. Someone standing in the center of Stonehenge could see the Sun rise over a big stone, called the heel stone, on the day of solstice. (The mysterious name, "heel stone," may have come in ancient times from the Greek root, helios, for Sun.) Still more astronomical purposes have been suggested for Stonehenge, including lunar eclipse predicting, but these controversial.
The investigation of Stonehenge’s timekeeping functions helped to create a new field of astronomy called archaeoastronomy — the study of astronomical practices in ancient societies. Astronomical temples, which were used to help calibrate the calendar for agricultural and ceremonial purposes, have been found in many parts of the world. Various examples show the rich aspects of culture that are woven into the stone monuments.
The long span of human observations of the sky allows subtle motions to be discovered. The astronomer Hipparchus studied two centuries of star maps and realized that the direction of the north celestial pole has moved slowly across the sky. A spinning gyroscope that is not pointed straight up will wobble — its rotation axis sweeps out a circle. This conical motion is called precession, and the Earth precesses with a long 26,000-year cycle. From day to day or year to year this motion is far too small to detect. The north celestial pole has not always pointed at Polaris, and ancient cultures were aware of this steady shift. In one legend, the precession was ascribed to a great whirlpool in the Mediterranean Sea, which slowly twisted the heavens.
The Great Pyramid in Egypt — constructed around 2650 B.C. from over two million limestone blocks weighing two tons each — is aligned almost perfectly with the north-south axis. Two airshafts point directly from the pharaoh’s tomb to the brightest star in Orion's belt. The Egyptians identified Orion with the underworld god of rebirth, and it is likely that the airshafts were intended for the passage of the pharaoh’s soul on its journey to afterlife in the heavens. One airshaft does not point at any bright star now, but if we account for 4500 years of precession, it used to point at the bright star Thuban.
The ancient city of Chichen Itza rises out of the dense rainforest in the Yucutan peninsula of Mexico. Several buildings dating from around 1000 A.D. incorporate astronomical alignments, most notably of Venus since it was important in the Mayan religion. The Mayan calendar was based on the cycles of both Venus and the Sun. On the morning of summer solstice, the rising Sun casts a shadow on the corner of the pyramid at Chichen Itza that climbs up the structure like a snake.
In Arizona, the Hohokam native Americans built a large ceremonial structure called Casa Grande in 1350 A.D. The rising Sun aligns perfectly with holes on opposite sides of the building only on the longest day of the year. Further north, at Chaco Canyon, the Anasazi tribe created a structure on a high promontory where light is admitted through a space between rock slabs. The gap lets in a "dagger of light" that projects onto a spiral pattern carved on the opposite wall. The placement of the Sun dagger marks both the solstices and the equinoxes. These apparitions — snaking shadows, projected beams, and light daggers — are vivid demonstration of the diverse ways that cultures have marked astronomical time.
Not everyone uses the sky in the same way. We know that at our northern latitude, the stars move on slanting paths in the sky and they all appear to orbit in slow circles around Polaris. However, near the equator the stars rise straight out of the east and set directly in the west. Polaris may be low on the horizon or even below the horizon. In the Tropics, which is the zone between 23.5° latitude North and 23.5° latitude South, there are two days each year when the Sun is directly overhead at the zenith. The Incas of Peru and the Javanese of Indonesia both chose to fix their calendars around these notable days, when a tree or a vertical stick casts no shadow. Meanwhile, at far northern latitudes above the Arctic Circle, the Sun spends large chunks of the year below the horizon and is no use for timekeeping. The Inuit and other tribes have always used the tides to keep track of time, which is a kind of lunar calendar.