The Equinox: A Celestial Phenomenon
Introduction
The equinox is a celestial event that occurs twice a year, on March 20 or 21 (spring equinox) and September 22 or 23 (autumn equinox). On these days, the sun crosses the celestial equator, an imaginary line that divides the Earth’s Northern and Southern Hemispheres. As a result, the day and night are of equal length all over the world.
Ancient Observatories and the Equinox
Ancient cultures around the world recognized the significance of the equinox and built observatories to mark its occurrence. These observatories often featured alignments with celestial bodies and were used to track the sun’s movement throughout the year.
Machu Picchu: The Intihuatana Stone
One of the most famous equinox observatories is located at Machu Picchu in Peru. The Intihuatana stone, a 26-inch-tall granite slab, is positioned at a precise angle that causes its shadow to disappear at noon on both the spring and fall equinoxes. This effect was likely intentional, as the ancient Incas believed that the sun was “hitched” to the stone on these days.
Chichen Itza: The Kukulcán Pyramid
Another notable equinox observatory is the Kukulcán pyramid at Chichen Itza in Mexico. Due to its unique geometry and construction, a dramatic snake-like shadow undulates down the pyramid’s staircase on the spring and fall equinoxes. This effect is believed to represent a giant serpent descending the pyramid, symbolizing the renewal of life.
Chaco Canyon: The Sun Dagger
In Chaco Canyon, New Mexico, the Sun Dagger site is an ancient observatory that marks the summer and winter solstices, as well as the equinoxes. Three stone slabs are positioned against a south-facing rock wall, and sunlight passing through the cracks between the slabs creates blades of light that migrate across etched spirals. On the equinoxes, a complex pattern of light appears on the rock wall.
Hovenweep National Monument and Burro Flats
At Hovenweep National Monument on the Utah-Colorado border, sunlight crosses spiral rock etchings on the summer solstice. Similarly, at a Chumash site in Burro Flats, California, a sliver of light crosses the center point of five concentric rings on the winter solstice.
The Hitching Stone in Yorkshire
In Yorkshire, England, the Hitching Stone is a large glacial boulder that marks the equinox. Observers standing near the stone at sunset will notice that the sun sets directly behind Pendle Hill, directly west of the boulder.
The Pyramids of Egypt
The pyramids of Egypt are also aligned with celestial bodies. The sphinx faces due east, taking in the rising sun on both equinoxes. Within the Great Pyramid, a skyward shaft is believed to have once pointed directly at the Alpha Draconis star at midnight on the autumn equinox.
Observing the Equinox at Home
If you can’t travel to an ancient observatory, you can still observe the equinox at home. Tilt a broomstick at the angle of your location’s latitude and hold it steady. At noon, the broomstick will cast no shadow.
The Aurora Borealis and the Equinox
The aurora borealis, or Northern Lights, is a natural light display that occurs in the high-latitude regions of the Earth. Sun flares send clouds of electrified particles toward the Earth, which collide with atmospheric molecules and create brilliant displays of colorful light. Sun flares occur most often around the equinoxes, making this a prime time to witness the aurora borealis.
Global Sharing of Sunlight on the Equinox
On the equinox, readers in every hemisphere share equally in the light of the sun. At every point on Earth, the sun rises from due east and sets at due west, spending 12 hours in the sky. The equinox is a day of global sharing.