Writing Excuses 7.32: Astronomy 101 for Writers

Eric James Stone, Nebula winner and “graduate” of NASA’s Launchpad workshop, joins us to talk about astronomy in our world-building.

We talk about tides, habitable zones, planetary orbits and axial tilts, stellar life-cycles, and other fun factors for authors to take into account. But obviously we can’t teach you everything you need to know about astronomy in 15 minutes, so we wrap with some handy resources for you to begin your continuing education:


Your colonists are going to a world whose axial tilt is different from Earth’s. How are the seasons different?

Helliconia Spring by Brian Aldiss, narrated by Christopher Slade

39 thoughts on “Writing Excuses 7.32: Astronomy 101 for Writers”

  1. There are many programs out there that let you look at the star formations as seen from anywhere, but the one I used was a program called Redshift. I used it to figure out what was actually overhead on a certain historical date, and it works for future dates too. These are kind of like google maps for space, and if you’re using real stars in your writing then you may as well check out their positions.

    Also, nebulas aren’t going to look anywhere near as awesome in a basic telescope as their Astronomy Post of the Day false-color counterparts.

    Just a few things that I figured I’d toss out there that are astronomy related. I can’t wait until you have your Astronomy 491 podcast, since I figure that one will move beyond the absolute basics covered in this.

  2. Wait… wait! If there is no moon in the Song of Ice and Fire world, how come that Khal Drogo calls Daenerys “moon of my life”?

    Now for something more pertinent: how about rings(like Saturn’s)? Would they affect anything in the living conditions on a planet? Could they be seen from the surface?

    I imagine that a ring around the planet not only would be a great view from certain parts of the planet (closer to the ecuator), and a hazard. I guess that said planet would have lots of space debris, constantly falling towards the surface.

  3. @ggab: I was going to point you at that video. I suspect that we traded visible rings for a large moon. As Luna swept outward from it’s original position close to Earth it completely cleaned the orbit, collecting ring material into its own lithosphere, or deflecting it down to Earth.

    I don’t know how to go about doing the math (beyond asking the Hive Mind), but I strongly suspect that Luna is still too close and too large to allow Earth to have a broad set of rings. They’d get distorted and swept up.

  4. Slight correction for the show notes: badastronomy.com/index.html looks like its Phil’s old site. If you go to ‘just’ badastronomy.com it will redirect to blogs.discovermagazine.com/badastronomy/ which is the current version.

  5. Great episode.

    I knew the things you talked about here and would love a more in depth podcast on some more obscure stuff.

    I’m working on a story right now that’s set on a planet orbiting a binary star. It also features a bigger/closer moon. Besides the tidal effects discussed the binary star has some interesting effects on the moon eclipse. It’s not an eclipse but rather a snake/cat eye where the iris is the planets shadow of the binary star light. Since the moon has such a strong effect and this “eclipse” looks like a giant eye staring down at people it has resulted in strong religious sentiments. On the day it’s the god’s two eyes that lights up the world. At night there’s this huge eyeball (moon) looking away from the planet, but still giving off light and once in a while starig down with its sinister snake-like stare at the people. Obviously people will react in a barbaric way when that happens. Hopefully I’ll get the story right as well.

  6. A note for you about a habitable moon orbiting a gas giant: every satellite in our solar system which formed in place is tidally locked to its primary so that the same face always faces the primary.

    … why yes, this is a primary element of my world!

  7. There’s a really awesome simulator called Universe Sandbox that allows you to create your own galaxies or explore and edit both our own and our close neighbors. It’s a lot of fun to muck around in, and has a pretty active forum community: http://universesandbox.com/

  8. Wow this was definately the most interesting podcast of the season! Thank you very much for this and please do a few more like this one! :D

  9. If you want a great early example of how to turn a piece of Astronomy into the foundation for a story I recommend “The Currents of Space” by Isaac Asimov. He wrote it in 1952 when we knew a lot less about what causes supernovas than we do now. So in one sense it is a cautionary tale about how the science can get ahead of you in the long run. But it is a masterful job of taking a question “what triggers supernovas?”, positing an answer, and turning your answer into a wonderful tale. And for those who want to know how things really work (or, in some cases would work if they were possible) there is no better source than http://www.projectrho.com/public_html/rocket/index.php. It is put together by Winchell Chang (AKA Nyrath the nearly wise) a prolific illustrator of fun stuff and an all around interesting guy.

  10. Ok, now I’m considering what changes to axial tilt and how it works would mean for a world. I was looking for something to distinguish my world from generic fantasy worlds, so… this could be interesting. Thanks!

  11. Sorry, Dan and Mary, but the reason for Westeros’ seasons is magic. I have a link on another computer where GRRM actually says so. Also, one Dothraki dragon origin story has them hatching from the moon. So there may not be a moon over Westeros, but there is over Essos.

    Other than that, this was a good podcast. Thanks!

  12. Aaaaand my Google-Fu is weak at the moment, so…

    Anyone know if it’s possible for a planet to orbit a sun with one pole permanently tilted inwards?

  13. @Jonathan

    Uranus (stop the giggling) is tilted 90° although it’s pole is not constantly towards the sun.

    You could have the effect you talk of with a planet that’s rotating around its own axis during one year, i.e. the same side of the planet is towards the star. Our moon rotates around its own axis in one month and because of that always shows the same side to us.

    The effects becomes quite extreme with a constant day, a constant night and a constant twilight. Human life does probably only exist naturally in the twilight. It can exist in the other areas but require protection from heat/radiation (ex blast shield or underground) and cold (ex heated underground base)

  14. Now I want to ditch the moon and put planetary rings around my fantasy world.

    And @Jonathon, yes you can have the pole permanently facing the sun. The result would be one side very hot and the other side very cold with a small band around the equator where it is mixed. If the magnetic pole is also facing into the sun, you would get a lot of extra life killing radiation on that sunny side too. Not a friendly place for life generally, I’d imagine.

    You could alleviate that by placing the planet close to the sun so the cold dark side can have liquid water, or further out and have the liquid water on the sunny side. Atmospheric effects could get interesting, either way so I don’t know if there still would be a sufficiently stable and habitable environment to develop and sustain life.

    However, if it is a space-faring civilization, colonizing it, they could make the barren world work for them I suppose. Humans are great at adapting to environments hostile to hairless apes. We live everywhere from tropical islands on the equator to little science outposts in Antarctica.

  15. Interesting episode, it got my mind thinking of how my planets would be like.

    Now if only there was a website with all the equations and variables to make my own planets.

    @ Tommy: That may be true, but a few months ago io9 came out with possible ways Westeros can have the wacky seasons naturally. It is neat to think about.


  16. Ah, a cast near and dear to my heart. I spent en entire summer developing a massive spreadsheet calculating masses, orbits, seasons, and just about every other manner of astrophysical property for my planet, including local rise and set times for a second star based on longitude and latitude. Obviously, it’s more than I really needed and most of that doesn’t go into the book, but I hope it shows in the final product by making the planet feel completely believable, and that’s what it’s all about, isn’t it?

    I also came across this at last year’s WorldCon and found it pretty handy, especially for someone who wasn’t already familiar with astronomy:


  17. Thanks Tomas and Talmage. :)

    Uh, what I actually meant was not that the pole is always pointed directly at the sun. More that the world has a tilt similar to Earth’s, but doesn’t move around the sun in the same gyroscope way. My theory is that one hemisphere would have something like an endless summer, the other an endless winter. Most habitation would probably be near the equator, and there’d probably be a near-constant thaw at the edge of the winter lands, so probably very fertile lands on the summer side. Or often flooded. Further into summer you go, the longer the days, and vice versa for winter… But otherwise, I thought it might be somewhat similar to earth, but without seasons…

    But if the only way it could orbit like that would be by showing the same face to the sun at all times, oh well.

    It’s for a fantasy world anyways, so I don’t have to consider terraforming as an option, and I can probably just gloss over the exact WHY it’s always summer…

  18. This topic really fascinates me, and here’s why: I thought I had world builder’s disease when I came up with things like axial tilt for a planet I invented, and then this podcast came along, seemingly rare in subject matter, validating what I had been so fascinated with. I’ve always wanted to hear about this subject, but never thought it would ever cross anybody’s mind to explain planets, axis, and orbit in fiction.

    For my planet of an odd axial tilt, I calculated the altitude and azimuth of the sun and double moons as seen from any lateral and longitudinal location, duration of seasons, eclipse cycles, etc. I partly wanted it to be one of those Easter eggs that while some of those details weren’t relevant to the plot, people would notice if they took a closer look. I made a calendar so that if I happened to describe the sky, the moons or sun’s position to the horizon wouldn’t be made up.

    I felt extremely smart for it. Because of it, I learned to predict real solar eclipses just by looking at our own sky. I learned a lot.

  19. @Jonathon, sorry to misunderstand.

    How about having a rotational wobble or secondary axis of rotation for the world that matches the orbital period around the system’s star? It keeps the same hemisphere facing the sun all the time. If the wobble doesn’t exactly match, you will get seasonal shifts over a long period of time, dozens, hundreds, thousands, or millions of years depending on how mismatched the wobble is to the orbit.

    Now that I’ve said that, is two rotational axes possible for a planet? I think it is but I’m not sure.

  20. @Jonathon Side:
    Hopefully, Talmage’s 2x rotational axes will work (since I’ve wanted to do the same thing for a fantasy world of my own ;).

    But, if they don’t, you could always have the world’s Magic pull the planet (and just the planet – not the star) north or south. That should do the trick.
    NOTE: If you do this, make sure not to have the star also pulled – or else it will just drift down exactly as fast as the planet, thus negating the entire effect.

  21. Who ever said you needed a stable orbit for a good story? You could have quite a bit of fun writing a story about a planet being pulled out of its orbit by the gravitational force of a wandering star, or some other giant mass. Sounds like it’s time to abandon the ship or find some crazy way to save it.

    It should also be noted that high tide is also the point farthest from the moon, not just the point closest to it. There are two high tides constantly circling the planet.

  22. Wow, this one stirred up a lot of thoughts for me. the world that I am curently working in is based off a medieval Cosmology. the planets sun and moon all revolved around the ‘earth’ (though in my world it isn’t the earth, I’m not 100% sure what it is only that it is called the hub, Nexis, and grave of the stars.) I have started to wonder about things like seasons and such. this defiantly gave me a jumping off point.

  23. This podcast was most interesting and thought provoking! Whoda thunk astronomy would be an essential bit of research when writing about–well, anything but stars? Glad to hear you plan to have Eric James Stone back for more astronomy for writers.

    BTW… The moon is mentioned often in Song of Ice and Fire. Just yesterday, I was wondering why it seems it’s always a sliver or crescent (Martin uses different terminology, but that darned moon is always less than half-full). In her cell, Cersei is thinking about time: “The hour of the owl, the hour of the wolf, the hour of the nightingale, moonrise and moonset, dusk and dawn…”

  24. Fun fact, the Great Lakes are large enough to have small tides, however it’s only a few centimetres

  25. Another story that deals with a planet that has very long seasons is “Ice Rigger” by Alan Dean Foster. The planet Tran-Ky-Ky has such a peculiar orbit that its seasons last 50,000 years and the dominant life form during the ice periods differs from the dominant life form during the hot periods.

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