Spring has sprung!
This weekend, the world is celebrating the spring (or fall, for those in the southern hemisphere) equinox, the time the sun crosses the celestial equator as it moves northward, providing Earth’s hemispheres with an equal amount of sunlight and creating days and nights of approximately equal length around the world.
This year’s spring equinox is especially unique because, due to a quirk of the Gregorian calendar, it's the earliest it’s been since 1896. As The Old Farmer's Almanac explains, any year divisible by 4 is considered a leap year unless the year is also divisible by 100, so century years (e.g., 1800) skip the leap year. The exception to this rule kicks in when a century year is divisible by 400—as it was in the year 2000, which, per the rule, was a leap year. This exception has caused solstice and equinox dates to move slightly earlier each year since, and this year’s spring equinox—officially at 4:30AM UTC on March 20—culminates the trend.
All this to accommodate the fact that an Earth year is 11 minutes shy of being exactly 365.25 days.
The quirks of measuring time are also why astronomical events such as the equinoxes were so important in ancient cultures. Equinoxes and solstices are a reliable way to mark seasons, and human civilizations throughout history have monitored such astronomical phenomena, incorporating these events into their cultural and religious practices. Today, evidence of these practices can be observed through the buildings and structures our ancestors left behind. To celebrate today’s equinox, we’ve compiled a list of ancient structures designed to interact with the astronomical arrival of spring.
El Castillo, Chichen Itza
El Castillo, also known as the Pyramid of Kukulkán, is the 79-foot pyramid at the center of the ancient Mayan site of Chichen Itza. Built around 1000 AD, the structure was built to create a fantastic visual effect during the spring and autumn equinoxes. During the equinox sunset, light hits the pyramid at an angle creating what appears to be the shadow of a massive snake as it slithers down the pyramid steps. It could be a coincidence, but many argue that the pyramid’s many sculptures representing the feathered snake god imply that the effect was intentional.
The Mnajdra temples on Malta consist of three structures built over a period of a thousand years beginning around 3600 BC. The buildings are the remnants of what was once a much larger complex, and its lowest temple may have been used as an astronomical and calendrical site, interacting with the equinoxes and solstices. According to observers, the spring equinox sunrise bisects the entrance to the Lower Temple, shining light through the main passageway and into a small shrine within the complex. Details of the civilization that built Mnajdra are scarce, so it’s difficult to determine the exact motivations behind the design.
In Cambodia, Angkor Wat was constructed and oriented to interplay with astronomical events throughout the year. A 1976 article in Science detailed the discoveries of a group of University of Michigan researchers who numerically and astronomically analyzed the temple, constructed between 1113 and 1150 AD. On the spring equinox, the researchers wrote, “an observer standing at the southern edge of the first projection on the causeway (just in front of the western entrance gate) can see the sun rise directly over the top of the central tower of Angkor Wat.”
The University of Michigan team emphasized the significance of the orientation, pointing out that the spring equinox “marks the beginning of the sun’s annual journey, regardless of the exact date of the lunar-solar new year,” which would have been significant to 12th-century Cambodians, who based their calendar on lunar and solar cycles.
Hovenweep, situated on what is now the Colorado-Utah border, was built between 1200 and 1300 AD and is part of a settlement that once housed over 2,500 people. According to a slide deck prepared by the University Corporation for Atmospheric Research, the castle was likely built as a solar observatory, using small ports to track the movement of the sun, a method of observation still used by the Pueblo people today.
Hovenweep actually reveals a slight quirk in the solar calendar developed by its builders. According to UCAR, “[T]he equinox port at Hovenweep Castle points to the sunrise azimuth 4 days after the vernal equinox. This is precisely what one would have expected if the equinox azimuth was established by counting and halving the number of days between the winter and summer solstices.”
Basilica San Petronio
Bologna’s Basilica San Petronio actually played a role in the establishment of the Gregorian calendar we use today. In 1575, cosmographer Egnazio Danti arrived in Bologna to teach mathematics and astronomy. In order to continue his work on the commission charged by Pope Gregory XIII with the development of a new calendar, he constructed a meridian line in San Petronia. The meridian line—an astronomical instrument invented by Danti—consisted of a small hole high on the wall of the church; the position of the spotlight created when the sun shined through the hole allowed Danti to define and analyze the sun’s position and movements. This technique was later used by Giovanni Cassini to confirm the elliptical orbital model proposed by Johannes Kepler.
Of course, we can’t talk about ancient equinox observations without mentioning Stonehenge. The world’s most famous prehistoric monument is still a destination for those looking to celebrate the equinox; according to the BBC, nearly 1000 people visited Stonehenge to welcome spring last year. English Heritage, which manages the site, opens access at dawn for each equinox and solstice event, so that druids, pagans, and other early-risers can watch the sun alight the ancient stones.
These are just a few of the ancient observatories that still track astronomical events purely by their orientation and physical features. Perhaps you’ll welcome next spring in the manner of their builders.