Winter is coming. It’s not just a catchy motto from Game of Thrones, you know. No, winter really is on its way, as the seasons move on their eternal cycle. And this change from fall to winter can make you wonder. We know why the seasons change: our planet’s tilt, combined with its movement around the sun. But what does that truly mean? And, from a worldbuilding perspective, does it have to be that way? Well, let’s take a look.
Reason for the season
The Earth is tilted on its axis. Anybody past about the third grade knows that, and it’s patently obvious just by looking at the sky at different points in the year. Right now, our world has somewhere in the vicinity of 23° of axial tilt, and that’s a fairly stable number. It hasn’t changed much at all in written history, and only within about a degree either way throughout all of human existence. In the distant past (millions of years ago), there were periods where it was much higher or lower, but things are much more settled in this modern era.
Now, the axis doesn’t move, at least on scales of a single year. (We’ll ignore precession and other effects for the moment, as they tend to work on much larger periods of time.) What does that mean for us? Only that different parts of the world will get more sunlight at different times of the year. And that’s what causes the seasons to change.
Summer, of course, is when your part of the world gets the most direct sunlight, and that happens when your half of the world points more towards the sun. Winter is the exact opposite, and it’s on the other side of the year. Spring and fall (autumn, if you prefer) are in the middle, when the planet’s tilt is roughly perpendicular to the sun’s rays. But the Earth has two hemispheres: northern and southern. They can’t both be pointed at the sun, thus the complementary seasons that make Christmas a summertime holiday in Australia.
Tropical highs and lows
There’s a lot more to it than that, though. Because of the Earth’s tilt of about 23°, we can divide the world into a few sections. First, we have the tropics, the area around the equator, from the Tropic of Cancer in the north, to the Tropic of Capricorn in the south. Coincidentally enough, these lines are at exactly the latitude equal to the axial tilt. (It’s not a coincidence at all; it’s the whole reason why they exist.) Every point in the tropics will have the sun directly overhead at some time in the year.
The polar regions are also defined by the tilt. The Arctic and Antarctic Circles are at a latitude of about 67°, or as far from the pole (90°) as the axial tilt, or in math terms: $90° – a$. Everywhere in a polar region will have a time when the sun is at the nadir, and a day where it doesn’t rise at all. But it will also have days where the sun doesn’t set, giving us the “midnight sun” of Alaska and Scandinavia.
In between the polar and tropical regions lie the temperate zones. In these, the sun will never be directly overhead or directly below, and it will rise and set every day. And it’s here that seasonal variation has the most visible effects.
Day and night
If the Earth wasn’t tilted, there wouldn’t be any seasons. Every night would be 12 hours long, no matter where you were. But we don’t live in that world, we live in one that is tilted. Thus, our nights change in length. At the equinoxes, the lengths of day and night are equal, hence the name. At the solstices, they’re as far apart as can be. In between, there’s a gradual shifting that gives us the feeling that days are growing longer or shorter.
As you get farther from the equator, the variation grows. Thus, at my latitude of around 35° north, I might only get about 9 hours or so of daylight on the winter solstice, but summer nights will also be that short. Up in New York, it might be split 16/8, while London might be 17/7 or 18/6. Helsinki, up near 60°, is going to have some long winter nights, but there will always be a sunrise. Barrow, Alaska and McMurdo Station in Antarctica are both inside the polar region, so they’ll have days without nights, or vice versa.
An added complication
The whole thing would be perfectly symmetrical but for one little detail. Earth’s orbit around the sun isn’t a perfect circle. It’s an ellipse. That ellipse doesn’t move any more than the axis does. (Again, we’re ignoring precession.) As of right now, the perihelion, the point closest to the sun, comes around in January, during the northern winter. Orbital mechanics dictates that the aphelion, then, is six months later.
As anyone who has played Kerbal Space Program knows, things move more slowly at apoapsis. (“Aphelion” is just the apoapsis of something orbiting the sun.) Therefore, since our apoapsis occurs in July, northern summer is a little bit longer than winter, while the southern hemisphere is the other way around. It’s not much of a difference, only about one or two days, so it doesn’t affect the climate that much. But it’s something you may have to keep in mind.
So all that works for Earth. How about a different planet? How would the seasons work? The answer: about the same. Earth is simply the most convenient example, since we’re already living here. Mars has seasons, too; the Phoenix lander was killed by the rigors of a Martian polar winter. For the rest of the solar system, things get dicey. Jupiter doesn’t have much tilt, for example, while Uranus is practically lying on its side. Mercury has its resonance-lock thing going on, which screws everything up. And moons don’t really work the same way.
But for your ordinary, habitable, terrestrial world, seasons are going to be like Earth’s. Summer and winter, spring and fall, they’re all going to be there. They may be different lengths, based on the planet’s orbital period and eccentricity. The tropical and polar zones may be larger or smaller, if the tilt isn’t our 23°. The division of day and night might scale differently, due to these same factors. But from a scientific point of view, that’s all you have to worry about. The years-long summers and winters of Westeros are scientifically implausible; you need magic to account for them.
Summer is always going to be the hottest part of the year, with the most sunlight and shortest nights. Winter will be the coldest; the sun will hang low in the sky, and its rays will strike more glancing blows on the world. Spring and autumn will both be marked by equinoxes, days when the periods of daylight and darkness are the same length. Spring tends to get warmer as you go through it, while autumn cools down.
In the tropics of your fictional world, there won’t be as much seasonal variation, especially close to the equator. The poles, by contrast, will be marked by long summer days, cold winter nights, and periods of total darkness or everlasting sunshine. In between will be the temperate zones, where civilization tends to flourish. And the southern hemisphere will always be backwards when it comes to the calendar.
But this is all speaking from the view of orbital mechanics. On the ground, there is a lot of room for change. Latitude only determines the kinds of seasons you have, whether tropical, temperate, or polar. A location’s climate is certainly affected by this, but many more factors come into play, so many that I’ll dedicate a future post to them.