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George R. R. Martin is wrong. One of his Game of Thrones characters is seven years old and has not yet experienced winter. That’s just not possible.
Sure, it’s fiction—it’s a made-up story. But even in fiction, the writer must build an internally consistent world. The term year is a measurement of time equal to one orbit, whether you’re on Earth circling the Sun or in some other galaxy, orbiting some other star. A year need not be 365 days. In fact, Venus’s year is 225 days. But one year is one orbit. A seasonal cycle, too, is tied to the planet’s orbit. If there exist discernible, regular seasons, that cycle can’t be arbitrarily decoupled from the notion of a year.
That’s the sort of issue we’re pondering this week at Launch Pad, an intensive astronomy workshop designed specifically for writers. Now in its seventh year, this year’s workshop has attracted an array of quirky, ambitious, and well-published writers with widely varying knowledge of astronomy. The fourteen participants and four instructors are gathered in Laramie, Wyoming, a town unexpectedly reminiscent to us of Galesburg, Illinois, where we earned our bachelor’s degrees at Knox College. The hope is that all of us writers will leave with a deeper understanding of astronomy and, as a result, will take the time to get the science right when we write our stories, poems, and nonfiction.
The need to get the science right was illustrated on our first day of lectures, when the predominant misconception about why the Earth has seasons was discussed. Generally (and in a documentary that asks Harvard University graduates faculty, and alums), people seem to think that winter is when the Earth is farthest from the Sun, whereas closer proximity to the Sun creates summer. This would occur if Earth’s orbit were highly elliptical, but it’s almost circular.
In other words, Earth is pretty much the same distance from the Sun all year long. In fact, Earth is closest to the Sun on January 3, when it’s very cold in the Northern Hemisphere. Besides, if Earth were noticeably farther way from the Sun during winter, all of Earth would experience winter at the same time. Instead, when people in the Northern Hemisphere—like Wyoming—experience winter, people in the Southern Hemisphere—like Australia—experience summer.
Many people, of course, know that it’s the tilt of Earth that causes the seasons. Earth’s axis is tilted at 23.5 degrees in relation to the plane of its orbit around the Sun. Some people still want the distance from the Sun to matter even if they know seasons have something to do with tilt. In other words, whichever end—north or south—of Earth is tilted toward the Sun should experience summer because it’s closer to the Sun. In fact, whichever end is tilted toward the Sun does experience summer, but the distance from the Sun is pretty much the same for the whole Earth. The smidge of tilt is not enough to account for disparate seasons. The difference in the Northern Hemisphere’s and Southern Hemisphere’s distance from the Sun is correlation—it happens to be true—not causation, when it comes to the seasonal cycle.
Instead, the tilt causes seasons because the Sun’s rays hit different parts of Earth at different angles. Hold a flashlight pointing straight down, and the direct rays create a dense, bright spot of light. Then, change the angle of the flashlight in relation to the floor so that the light strikes at an angle and the light rays cover more area of the floor. At an angle, the light is less intense at the edge. That’s the concept at play.
The tilt stays oriented the same way all year; it does not wobble. If you hold a pencil with its eraser facing your forehead and move the pencil around your head with your arm but without changing the position of your hand, the eraser will face away from the back of your head. If you apply this stable position to an orbit, Earth’s North Pole tilts away from the Sun and the Northern Hemisphere experiences winter at one position in the orbit. At the opposite side of the orbit, Earth’s South Pole tilts toward the Sun, and the Southern Hemisphere experiences winter.
It’s not the distance; it’s the angle. If there were no tilt, there would be different climates because the Sun’s rays would hit the equator straight on and be most dispersed at the poles. But any given area would experience a single season all the time because the Sun’s rays would hit that area at the same angle all year long.
So, for any particular place on Earth, the planet’s tilt accounts for variations, depending on the time of year (which is also a given point in Earth’s orbit around the Sun), in 1) how direct the sunlight is, 2) how long the daylight is, and 3) what path the Sun takes across the sky.
It makes sense for writers to know how the universe works and, when possible, to take advantage of that knowledge to enrich their stories. Some writers and some stories, of course, can get away with cheats or the implausible. And sometimes budgets and technology constrain filmmakers. That’s likely why we accept that the crew of Star Trek or Firefly walk around the ship without accounting for the lack of gravity that Apollo and shuttle astronauts faced in reality.
There exists a tradition among astronomy graduate students to write scholarly April Fool’s Day papers. This year, a group of Johns Hopkins University nerds took George R. R. Martin to task in a paper simply entitled “Winter is Coming.” Martin isn’t the first science fiction or fantasy author to tangle with inquisitive, science-minded students. In 1971, a group of MIT students who’d identified engineering problems in the Larry Niven classic Ringworld chanted “Ringworld is unstable!” during Worldcon. Ray Bradbury once had a run-in with a precocious reader of The Martian Chronicles. We’ll leave you with the video of Bradbury describing the outcome.