What is an equinox? 10 need-to-know facts

El Castillo, a pyramid in Mexico, was built in such a way that the “snake of sunlight” would slither down its steps at the dawning of each equinox, as the sun rose into the sky.

How we think about the Earth and how it actually is aren’t necessarily the same. In a way, we’re victims of our own education. For instance, while any globe or on any map it’s round and spherical, scientists tell us the shape of the Earth is actually and oblate spheroid—a sphere that’s bulging out at the center (equator) and squished down at the poles. Sir Isaac Newton was the first discovered this. And our planet isn’t exactly straight up and down, either. It’s tilted by approximately 23.44°, which accounts for seasonal changes.


When the Earth is leaning toward the sun, its summer in the northern hemisphere and winter in the southern one. While when tilted away, the situation is reversed. The summer solstice is when the northern hemisphere is at its most extreme angle toward the sun, while with the winter solstice, it’s at its farthest away. What’s left are the spring (vernal) and fall (autumnal) equinoxes.

At these two times a year, always around March 21 or September 21, the amount of sunlight and darkness in a day is about equal, no matter your latitude. That’s because the tilt is neither toward nor away from the sun. Rather, the equator gets the lion’s share of its rays. This year, the vernal equinox occurs on March 20. While its arrival heralds spring north of the equator, it starts off fall south of it.

Here are 10 need-to-know facts about equinoxes:

Equinox spring tide at Mont Saint-Michel, France. Wikipedia.

1. The word equinox comes from two Latin words, aequus—meaning equal and nox—meaning night.

The sun rising at the spring equinox at the equator. Credit: Getty Images.

2. If you went to the equator on either of these days, you’d see the sun straight overhead at noon. This isn’t its actual position, however. It’s below that point. This optical illusion is due to the refraction or bending of the sun’s rays as they travel through the atmosphere.

The Earth’s axis. Credit: Wikipedia.

3. An image illustrating the Earth’s axis can be different, depending on the illustrator. While some prefer a left-to-right tilt, others portray it right-to-left. Both are accurate, depending on the positions of the Earth and the sun.

Comparison between the size of Mars and the Earth. Credit: NASA.

4.  How did the Earth get tilted in the first place? It’s thought that an object the size of Mars crashed into it some 4.5 billion years ago, while our planet was still young.

Credit: Wikipedia.

5. The Earth’s axial tilt, also called its obliquity, isn’t a static number. The Earth wobbles on its axis, as the result of earthquakes, but also different gravitational forces acted upon it by the moon, the sun, and the other planets in our solar system. Therefore, Earth’s obliquity varies at times between 22.1° and 24.5°.

Credit: Pintrest.

6. Other names for Earth’s wobble include the precession of the equinoxes and the axial precession.

Hipparchus of Nicea (left) and Ptolemy. The School of Athens by: Rafael, 1509. Credit: Wikipedia Commons.

7.  In 130 BCE, Hipparchus of Nicea discovered the Earth’s axis shifted slowly over time. He came across this by comparing astronomical observations separated by 100 years.

The Heliophysics System Observatory (HSO). Even though it has many parts, it’s considered one observatory. Credit: NASA.

8. Although we’ve looked into this for a long time, we didn’t really get highly detailed data until we went into space. And we’re still learning. Consider NASA’s Heliophysics System Observatory (HSO), which started in 2013 and is wrapping up in 2022. The HSO includes 20 missions and 23 spacecraft. It’ll investigate “space weather,” how our star interacts with the Earth and other planets as well.

A modern-day druid ceremony near Stonehenge during the vernal equinox. Credit: Wikipedia Commons.

9. Cultures all over the world celebrate holidays and festivals on the equinoxes. One of the most notable is the Japanese celebration of Higan, which takes places on both equinoxes. During each Buddhist holiday, it’s said that the spirits of the dead pass into Nirvana. It lasts for six days. During that time, the observant visit the graves of loved ones, clean and decorate them and reflect on and share memories of them.

El Castillo pyramid in Chichen Itza, Mexico. Revelers come even today to see the “snake of sunlight” making its way down the steps. Credit: Getty Images.

10. In the ancient Mayan culture, a ritual including human sacrifice took place at the spring equinox. It happened at El Castillo, a pyramid located in Chichen Itza, Mexico. The pyramid has four staircases. The angle of each was carefully calculated so that the “snake of sunlight” would slither down the stairs at the dawning of each equinox, as the sun rose into the sky.


Big Think
Sponsored by Lumina Foundation

Upvote/downvote each of the videos below!

As you vote, keep in mind that we are looking for a winner with the most engaging social venture pitch - an idea you would want to invest in.

Keep reading Show less

Essential financial life skills for 21st-century Americans

Having these financial life skills can help you navigate challenging economic environments.

Photo by Jp Valery on Unsplash
Personal Growth
  • Americans are swimming in increasingly higher amounts of debt, even the upper middle class.
  • For many, this burden can be alleviated by becoming familiar with some straightforward financial concepts.
  • Here's some essential financial life skills needed to ensure your economic wellbeing.
Keep reading Show less

New study finds the egg may actually 'choose' the Sperm

Here's the first evidence to challenge the "fastest sperm" narrative.

popular
Keep reading Show less

New fossils suggest human ancestors evolved in Europe, not Africa

Experts argue the jaws of an ancient European ape reveal a key human ancestor.

Surprising Science
  • The jaw bones of an 8-million-year-old ape were discovered at Nikiti, Greece, in the '90s.
  • Researchers speculate it could be a previously unknown species and one of humanity's earliest evolutionary ancestors.
  • These fossils may change how we view the evolution of our species.

Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.

A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.

Rethinking humanity's origin story

The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)

As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.

David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.

The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.

Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"

He recently outlined this idea at a conference of the American Association of Physical Anthropologists.

It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.

"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."

Migrating out of Africa

In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.

Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.

The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.

The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.

Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.

Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.

Did we head east or south of Eden?

Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.

Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.