Twice a year, the sun reaches an extreme point in the sky. It is the day with the least amount of sunlight or the most amount of sunlight, depending on where in the world you happen to be.
It is a day that almost every early culture around the world recognized because it was the one day a year they could document by following the path of the sun.
Learn more about the solstice, how it works and how it has been celebrated on this episode of Everything Everywhere Daily.
Let me start out by noting that the solstice is not a day. While we recognize it as a day because it happens to result in the longest or shortest day of the year, in reality, the solstice is just a moment.
In fact, if you wanted to get really technical, you could pinpoint the moment of the solstice down to the second.
So, what exactly is this moment which is called the solstice? There are two solstices each year, a winter solstice and a summer solstice. Which is which will depend on what part of the world you live in.
A solstice is when either pole of the Earth is pointed at its maximal angle towards or away from the sun. If a pole is pointed away from the sun, that hemisphere is having its winter or hibernal solstice. If it is pointed towards the sun, it is having its summer or estival solstice.
When it is the winter solstice in the northern hemisphere, it is the summer solstice in the summer hemisphere, and vice versa.
The solstice has everything to do with the tilt of the Earth.
The Solar System has an imaginary plane running through it called the ecliptic plane. This is the plane which is the average location of the Earth’s orbit.
The Earth’s axis is tilted approximately 23.4 degrees off the ecliptic plane. If you really want to get technical, the tilt of the Earth is 23.4365472133 degrees.
The poles are orientated at the same point in the sky, which is why the North Star is the north star all year long.
The tilt of the Earth’s axis, plus the fact that the poles point at the same point in space, is what results in the seasons.
If the Earth had no axial tilt, there would be no seasons. If one pole were always orientated towards the sun, one pole would be in perpetual light, the other in perpetual darkness, and there would be no seasons.
The date on which the solstice occurs isn’t always the same. There are a couple of reasons why this is the case.
The first is that because the solstice is a moment in time, what time it is can differ on different parts of the Earth due to time zones.
The other reason why it doesn’t occur at the same time every year is that the number of days doesn’t divide equally into a year. That is the reason why we have leap years.
So, if we just go by universal time or UTC, the winter solstice in the northern hemisphere occurs on December 21 for three years and then on December 22 every fourth year.
The axial tilt of the Earth being responsible for the seasons and solstices is something I’m sure most of you probably already know.
What most people don’t know is that the solstice and the seasons are slowly changing over time.
In a previous episode, I talked about Milankovitch Cycles and the idea of precession.
If you think of a spinning top, as the top is spinning, it also may have a slight wobble to it. The orientation of the axis of the top will move as it is spinning. This is precession.
Just like a top, the Earth’s spin has a wobble in it, but it wobbles very slowly.
The Gregorian Calendar is designed to track the tropical year, so equinoxes and solstices always occur at roughly the same time.
However, and I’ll refer you to my episode on the Gregorian Calendar, while it fixed the big problem with the Julian Calendar, it still isn’t perfect. Over time, unless there is a future correction, solstice dates will continue to creep backward.
For example, in the year 2048, the northern hemisphere winter solstice will actually take place on December 20th. Over thousands of years, without a calendar correction at some point, the dates will keep moving back. This isn’t a pressing issue, but around the year 4000, and multiples thereof, it might be necessary to not have a leap year, whereas it normally would be a leap year.
Likewise, it might be necessary to have an exception to that rule every 20,000 years, but that isn’t something we have to worry about.
Assuming calendar issues aren’t a problem, the solstices will still occur at the same time on the calendar, but they will in a different part of the sky.
Right now, the date of perihelion, which is the point at which the Earth is closest to the sun, takes place around January 2 to 4th, reasonably close to the northern hemisphere winter solstice.
However, in the year 15021, that same solstice will occur at aphelion, which is the point where the Earth is furthest from the sun.
There is one other thing that is changing slowly over time, the tilt of the Earth.
The tilt of the Earth changes over a period of 41,000 years from 22.1 degrees to 24.5 degrees. Currently, the Earth is in a period where the angle is decreasing, meaning over the next several thousand years, seasons will become less extreme.
This will also mean a change in the location of the tropics of Cancer and Capricorn and the locations of the Arctic and Antarctic circles.
These lines are determined by the axial tilt of the Earth. The tropical lines are the same number of degrees from the equator as the tilt, and the polar lines are the number of degrees from the poles.
So, OK, the solstices are determined by the tilt of the Earth, which is moving, but that is something that we’ll never have to worry about in our lifetimes.
A solstice isn’t just a geophysical phenomenon, it is also a cultural one.
In fact, a solstice has the unique distinction of being a nonreligious, non-cultural specific event that is almost universally recognized and celebrated.
Some of the very earliest evidence we have of human construction, and by extension civilization, are structures built to align to the winter solstice.
If you remember back to my episode on Gobeki Tepe, this is probably the oldest structure that we know of in the world, dating back 11-12,000 years. There are some who think that it might have been, at least in part, designed to track the winter solstice.
The Intihuatana Stone at Macchu Pichu in Peru tracks the solstice.
A stone ring was found in Australia built by the Wadda Wurrung people, which tracks both solstices.
In Egypt, the Temple of Karnak is aligned with the winter solstice, as is the Mayan temple at Tulum.
One of the most interesting ancient sites designed to be aligned with the winter solstice was the site of Newgrange in Ireland.
Newgrange is basically a large 5,200-year-old mound with a single opening. You can visit the site pretty much any day of the year and go inside. There isn’t room for that many people.
However, the singular entrance to the mound is aligned perfectly such that the interior of the mound is illuminated by sunrise on the winter solstice and the two days surrounding it.
If you want to experience the inside of the mound on the solstice, you have to enter a lottery that tens of thousands of people enter every year. 10 people are allowed inside on each of the five days.
So, why build these structures to track the solstice?
In addition to just calendar planning and tracking, there were probably celebrations that were tied to the solstice.
Midwinter and midsummer celebrations are almost ubiquitous for any culture which isn’t located very close to the equator.
Ancient Rome had the festivals of Saturnalia and Sol Invictus. Saturnalia was a wild celebration where social roles were reversed.
The Incas had a winter solstice celebration known as Inti Raymi. They would fast for three days before the solstice, then greet the dawn with gifts and sacrifices for the gods. They celebrated in June, of course, because they were in the Southern Hemisphere.
Ancient Persia had their celebration for the god Mithra who was their god of light.
Ancient Scandanavians had a celebration called Yule, where they would burn logs when the days were short. These were the basis of the yule log, which some people still use today.
Solstice or midwinter celebrations aren’t just a thing of the past, they are still celebrated around the world today.
In Scandanavia, they now celebrate St. Lucia’s Day. It involves the lighting of fires, and girls wear white dresses with wreaths on their heads with lit candles.
In China, it is known as Dong Zhi. It is a time for families to get together, and they commonly eat glutenous rice balls.
In Iran, they celebrate Yalda Night, which involves eating foods such as watermelon, nuts, and pomegranates, staying up late, and reading poetry.
In Japan, they have Toji, which involves the lighting of bonfires, especially on Mount Fuji as well as taking hot baths, often in hot springs.
The amazing thing, of course, is that all these cultures and civilizations, despite celebrating in radically different ways, all did so on the same day.
Before I end, I want to address one common misconception people have about both the summer and winter solstice. People often assume that if a solstice is the longest or shortest day of the year, then it must be the hottest or coldest day of the year.
This is not the case. While this would make some sense at first, the reason why this isn’t the coldest or hottest time of the year is because it take time for the atmosphere to warm up and cool down.
Extremes in temperature usually take place about a month after each solstice, but there is obviously a lot of variation in that depending on specific weather circumstances.
A solstice is a unique event in that it is universal experience. Not only is it something everyone on the planet can share in, but it is something that has been experience throughout all of human history.
The Executive Producer of Everything Everywhere Daily is Charles Daniel.
The associate producers are Thor Thomsen and Peter Bennett.
Today’s review comes from listener moyerjr7 from Apple Podcasts in the United States. She writes:
Keeps my tired mom brain sharp (ish)
I’ve enjoyed this podcast for many months but finally am leaving my 5 stars and a review after your episode on Parícutin. I saved it to listen to with my almost six-year-old Luke who is fascinated by volcanoes. While I’m not yet part of the Completionist Club, he and I have listened to the Parícutin episode at least ten times. What higher praise than “this podcast keeps an energetic kid sitting still and fully enthralled for ten minutes”? Thank you from a very tired but very proud mom – you help my brain wake up every morning as well as my kids.
Thanks, moyerjr7! Even though you are not yet a member of the completionist club, if I was giving out merit badges for listening to the show, you’d have to earn one for listening to the same episode 10x. As far as I know, that is a new record.
Also, if you have a five-year-old who is already interested in volcanoes, I’d say you are doing a pretty good job as a mom. You’d also get a merit badge for that. I hope that you and Luke keep listening.
Remember, if you leave a review, you, too, can have it read on the show.