The oldest structures that are thought to align with the equinox are the Sphinx and pyramids of Giza, Egypt. Though their precise age is unknown, it is estimated that the Sphinx was constructed around 2603-2578 B.C., and the pyramids were built around 2543-2436 B.C. On the evening of the spring equinox, the sun sets over the shoulder of the Sphinx, and between the pyramids of Khafre and Khufu—also known as the Great Pyramid of Giza. 

The Great Pyramid of Giza is almost perfectly aligned to the north, south, east and west. Glen Dash, an engineer who studies Egypt's pyramids, has suggested that the ancient Egyptians used a straight rod and the shadow it cast to track the sun’s movement on the equinox. On the equinox, the top of the shadow would run in a straight line from east to west.

"When ancient monuments are configured in alignment with due east and due west, it suggests, at the very least, that the ancient people were paying close attention to the rising and setting of the stars, as the stars rise in the east and set in the west," says Jacqueline Feke, an associate professor at the University of Waterloo whose research focuses on the ancient physical and mathematical sciences.

"If a monument is configured so that it draws attention to the sun’s position at an equinox, it suggests that the ancient people may also have been making systematic observations of the sun’s position over the course of the year. In other words, these monuments provide evidence that these early people may have been doing science."

In Jordan's ancient city of Petra, the Urn Tomb was built around 70 A.D. as a final resting place for a Nabatean king. The Nabatean kingdom, which rose as its people amassed great wealth from incense and spice trade, met its demise following a destructive earthquake in 363. Analysis suggests the Urn Tomb was designed with the sun’s alignment in mind.

“At sunset at the equinoxes, sunlight enters through the openings of the tomb and casts light onto the central apse on the tomb’s far wall,” says Feke. 

The site’s spring-oriented design was discovered by Juan Antonio Belmonte Aviles, an astronomer at the Instituto de Astrofísica de Canarias in Spain. According to Aviles, the Urn’s design suggests that the site is not only a tomb, but has served as a location for religious activities through the centuries. 

In the year 446, the Urn Tomb was converted into a Byzantine church, at least in part, Aviles says, because there were solar alignments that coincided with the Christian calendar, such as Christmas Eve and Easter.

Also known as “Temple C,” the Temple of Apollo in Selinunte was built in the mid-sixth century B.C. on the southwest coast of Sicily. Selinunte was one of the most important Greek settlements in the West, and the westernmost Greek city in Sicily. According to Aviles, the Temple of Apollo—dedicated to the Greek god of the sun—“faces a distant peak where the sun sets at the equinoxes.” Interestingly, other temples on the site don’t align with the sun during the equinox.

When ancient monuments align with the sun’s position at the fall and spring equinoxes, it suggests not only that these ancient people achieved great technological success, but also that they were paying attention to the precise position of the sun over the course of a year, says Feke. 

“From our location here on Earth, what the ancient Greeks called the ‘heavens’—consisting of the entire collection of the stars, planets, sun, and moon—appears to rise in the east and set in the west every day. But, in the meantime, the sun appears to move slowly from west to east against the backdrop of stars, so that it takes one year to complete one full revolution,” she explains.

Built between 3600 and 2500 B.C., along the southern coast of Malta, the Mnajdra Prehistoric Temples are thought to be some of the oldest surviving free-standing structures in the world. The buildings on the site are grouped into three sanctuaries—one of which functions as a solar observatory and appears to align with the sun during the spring equinox. 

“The orientation of the southern temple towards the eastern horizon indicates that the temple was constructed with astronomical phenomena in mind,” Feke says. “At the equinox, the central corridor and back altar of the temple are illuminated at sunrise.”