The Reasons for the SeasonsDiagram showing the Axis Tilt of the Earth

Let’s start with why we have seasons at all. If you’ve ever seen a globe of the Earth they’re usually shown with the Earth being tilted. This is not artistic license, this is scientific accuracy. Earth is tilted, by an angle of 23.5 degrees.

What we consider straight up-and-down in our solar system is determined by the Sun. Straight up would be a line coming right out of the Sun’s north pole, while down is a line coming right out of the Sun’s south pole. A similar line through Earth’s axis, from its north pole down through its south pole, wouldn’t point in the same direction as one going through the Sun’s axis. It would look tilted, and the angle of this is what we call Earth’s axial tilt—23.5 degrees.

Fun fact, this tilt is a relic of a gigantic impact. Earth got whacked hard by something the size of Mars back in the solar system’s early days. This impact resulted in the formation of the Moon (a story for another day) and left Earth knocked over by 23.5 degrees.

The tilt is where seasons come from. A planet with no tilt would have no seasons, and the more extreme the tilt, the more extreme the seasonal variation (more on this later). So the reason Earth’s northern hemisphere experiences its winter solstice on December 21 has to do with axial tilt.

The Extremes

This tilt means that different hemispheres of the Earth are angled more towards or away from the Sun at different times of the year. Which part of the planet is most angled towards the Sun changes during Earth’s orbit around the Sun. During December, for instance, axial tilt angles the Northern Hemisphere away from the Sun and the Southern Hemisphere towards it.

A person in Earth’s Northern Hemisphere in December will observe this angling away from the Sun as it rising later and setting earlier than other times of the year. It will rise farther south of east and set farther south of west than at other times. Its highest point in the sky will be lower than at other times. The light coming from it isn’t as intense as at other times.

And the farther north you go, the more pronounced these trends are. In fact, if you go farther north than the Arctic Circle, latitude 66.5 (which, incidentally, is 23.5 degrees below the north pole—not a coincidence) then there are portions of the year where you don’t see the Sun at all because it never makes it above the horizon. The farther north above latitude 66.5 you are, the bigger the portion of the year with no sunrise. At the pole itself you’ll be going half the year without a sight of the Sun.

The day we mark as the winter solstice is really a single moment. That’s why this year it’s occurring on December 21 for the US and December 22 for Europe and Asia. It’s happening at 10:27pm Eastern time. It’s the moment when the Northern Hemisphere reaches its maximum tilt away from the Sun. This is when winter officially begins. After this moment, though the change starts out realllly slow, the Northern Hemisphere will begin to be angled back towards the Sun.

Six months later, in June, the opposite happens. The summer solstice (occurring at 4:50pm Eastern on June 20 in 2024) happens when the Northern Hemisphere is most tilted towards the Sun, when we’re getting early sunrises and late sunsets and the Sun is getting high in the sky and the sunlight is hot and intense. Those spots that didn’t get a sunrise around December now have portions of the year without a sunset. The north pole will go six months with the Sun in the sky at all times.

Stuck in the Middle

Then there’s the in-betweens. As Earth’s orbital motion moves it between the two extremes represented by the solstices, there are times when things are more or less equal. At these times the planet’s poles are angled sort of off to the side, rather than towards or away from our local star. There is, in fact, a single moment in March and another in September when it’s all balanced and no part of the Earth is more tilted towards the Sun than any other part.

These are the equinoxes, the official beginnings of spring and fall. Whether you are in the Northern or Southern hemisphere you will see the Sun rise perfectly in the east and set perfectly in the west. “Equinox” means “equal night”, and refers to the idea that on these days the length of day and night are the same for the entire globe. This turns out not to be quite true, but it’s very close.

These are also big days for the poles. At the March equinox the Sun rises at the north pole after six months of darkness to start six months of light, even as it sets at the south pole. And, of course, the opposite happens at the September equinox.

Seasons Around the Solar System

If you look around the solar system at the other planets, you’ve got a whole wide variety of axial tilts, meaning our fellow planets experience different intensities of seasons.

At one super extreme is Mercury, which for all intents and purposes has no tilt. I mean, it technically has a 0.03 degree tilt, but who counts that? Jupiter also has very little tilt, at only 3.1 degrees. These planets really don’t experience seasonal variations on their surfaces.

Venus also doesn’t have any seasonal variation, really, but it’s because it took tilting to a stupid extreme. It technically has an axial tilt of 177.4 degrees. We’re really not certain why. Functionally this is the same as having an axial tilt of 2.6 degrees, so Venus doesn’t have seasonal variation, but it does spin backwards. On Venus you’d see the Sun rise in the west, or would if you could ever see the Sun through the clouds and if you weren’t busy dying one of the many horrible deaths Venus would subject you to.

If you want a planet with crazy seasons, the place to go is Uranus. It’s the other extreme, all the way over on its side with an axial tilt of 97.8 degrees. While we don’t know for sure how that happened, we suspect Uranus also suffered some huge impacts in its youth.

This tilt means that during Northern Hemisphere summer on Uranus it’s not just the region near the pole that experiences constant daylight. It’s basically the entire northern half of the planet! Meanwhile the entire southern half languishes in constant darkness. Only a narrow strip around the equator will see sunrises and sunsets. And of course this then flips during Northern Hemisphere winter.

This leads to intense seasonal variation in Uranus’s weather. It’s actually something we’re still learning about because it takes 84 years for Uranus go through an entire orbit’s worth of seasons, and we haven’t had good observations of Uranus’s atmosphere for that long. But it would definitely be a lousy place to build a vacation home, or would be if you could actually build anything on Uranus.

And just to round it out, Mars, Saturn, and Neptune have axial tilts fairly similar to Earth’s, at 25.2, 26.7, and 28.3 degrees, respectively.

Party Time

Here on Earth, the solstices and equinoxes are a great excuse to throw a party, and many cultures had or have some sort of festival or celebration centered on these days. Famously, many of the traditions we now firmly associate with Christmas, from decking the halls with boughs of holly to referring to this time as Yuletide to kissing under the mistletoe, originated in pagan celebrations intended to mark the time of the winter solstice.

And it’s no wonder people would want to celebrate this day—it’s the day the Sun starts coming back! If you’re one of those folks who finds it annoying when it’s dark by 4:30 (guilty. I realize as a person obsessed with space I should relish the chance to have extra hours to stargaze, but it’s cold and I’m a wimp) then the solstice marks the moment you can start looking ahead to longer, brighter days.

So this December 21st curl up with your sweaters and fuzzy socks, pour yourself a mug of something warm, and raise a toast to the shortest day of the year as the Earth begins to tilt us back towards the Sun. Unless you’re in the Southern Hemisphere, in which case enjoy your sunny days while you can you lucky dogs.