At sunrise on the Northern Hemisphere's longest day of the year — the summer solstice — thousands of modern-day druids, pagans and partiers gather in the countryside near Salisbury, England, to cheer as the first rays of light stream over a circular arrangement of stones called Stonehenge. The original purpose of the ancient monument remains a source of academic debate. The large stones erected about 4,000 years ago are aligned with the summer solstice sunrise, leading scholars to suggest a link to an ancient sun-worshipping culture.

Egyptologists continue to debate who the approximately 4,500-year-old Sphinx is and what it represents, but there's one thing you certainly get when you visit the limestone monument at Giza: a great view of the sunset at the summer solstice. As the longest day of the year bleeds into night, viewers at the Sphinx can watch the sun disappear exactly between the Khufu and Khafre pyramids.

The pyramids themselves, according to scholars, are thought to symbolize the sun's descending rays. The pharaohs buried within would ascend the rays to the afterlife.

Ancient cultures throughout the Americas have a long history of marking the seasons with solar observatories. The first known to do so was a sun cult on the Peruvian coast, which built a solar observatory about 2,300 years ago at the archaeological site of Chankillo. The observatory consists of 13 towers rising on a mound that span the arc of the rising and setting sun from the summer to winter solstice when viewed from special platforms on the east and west sides of the mound. In this image, made from the western observing point, the rising sun on the June solstice is seen just to the left of the leftmost tower.

From the year 500 to 1300, the Pueblo marked the passing of the seasons in Chaco Canyon with daggers of light that pierced spiral petroglyphs etched into a cliff wall, right behind three slabs of rock high up on Fajada Butte, the outcrop shown here, in New Mexico.

On the summer solstice, for example, a lone dagger of light — called the Sun Dagger by artist Anna Sofaer, who discovered the phenomenon in 1977 — slices through the center of a large spiral. On the winter solstice, two daggers bracket the large spiral. On the spring and fall equinoxes, the dagger is shifted to go right through the large spiral, and another dagger pierces a smaller spiral. Unfortunately, the effect disappeared in 1989 when the slabs of granite shifted, perhaps due to erosion caused by too many people scrambling up the butte to check out the light show.

From about the year 700 to 1050, a Native American culture mushroomed into what became the largest metropolis of its time anywhere in the world, on the banks of the Mississippi River in present-day Illinois. Known as Cahokia, the city had a regional population that peaked at around 40,000 people, according to archaeologists' estimates. The Cahokians are known for their mounds, on top of which they constructed buildings and buried their dead. They also built a solar calendar formation that is today known as Woodhenge, due to its similarity to the better-known Stonehenge in England. The calendar consists of tall timbers in a circle. Two timbers mark the solstices, and a third marks the spring and fall equinoxes. The role of the other timbers are less certain, but they might have marked festive dates in the calendar.

El Castillo, a Mayan pyramid built between 1000 and 1200 on Mexico's Yucatan Peninsula, is most famous for the shadow of a serpent that slides down the western flank of the northern staircase at sunset on the spring and fall equinox. Another nod to astronomical timekeeping at the temple, built to honor the god Kukulcan, is an axis through the northeast and southwest corners of the pyramid that points to the rising sun on the summer solstice and the setting sun on the winter solstice.

What's more, the staircases on each side of the square pyramid have 91 steps. When the top platform is counted as one step, the total is 365, the same number of days in a non-leap year.

In the centuries before European colonists and their diseases reached the Americas, the Amazon forests teemed with networked settlements as sophisticated as any medieval European city, according to archaeologists working in the region. The networks of towns and hamlets were each organized around a central plaza.

Each settlement had an identical formal road connected to the plaza that was always aligned northeast to southwest — following the direction of the sun at the midyear solstice.

Archaeologists say the organization and planning required to construct the networks is a hallmark of urban society. Shown here are charred remains of a house in one of the settlements.

The Inca's sacred relationship with the sun shines at Machu Picchu, a mountaintop citadel that is thought to have been built in the mid-1400s for the ruling elite. On the June solstice, which is winter in Peru, the sun pours through the central window in the Temple of the Sun and lights up a ceremonial stone. The Temple of the Three Windows in the site's Sacred Plaza is positioned so that it faces the December solstice sunset.

At the spring and fall equinoxes, the sun lies directly over Intihuatana, the hitching post of the sun, so that it casts no shadow.

Copyright 2012 by The Associated Press. All Rights Reserved.