The Disc World

Campaign Cartographer
Campaign Cartographer

You may remember a disc world from Terry Practchett‘s Discworld series but disc worlds go way back in human history: many cultures imagined the world as a disc, including my favorite, the Norse. It’s hard to imagine how a disc world could exist in our universe but that’s the beauty of fantasy: we’re allowed some suspension of disbelief. So, the world is a disc: what would it be like on this world?

As one might imagine, there are a lot of variables. Let’s posit a disc 6000 miles across with mountains on the edge of the disc to form a rim (why? I like the aesthetic of something to hold the air in but one could certainly do without them; this is a fantasy exercise afterall). Finally, for heat and light nothing says you can’t have a light on top of a tall tree something like Tolkien’s early world but let’s go with a classic sun.

What would days and seasons be like? Well, it depends entirely on the relationship between the sun and disc. If the disc orbited the sun with a period of hundreds of days, you would have your year, much like Earth. But what about days and seasons?

First let’s image the disc always pointed the same way in space, regardless of where it is in its yearly orbit (just like the Earth maintains the same axis giving us seasons). If we start off with the disc facing the sun at the New Year, then half way through the year the disc is facing away from the sun. A quarter of the way through the year, the sun strikes the disc in parallel. This would mean that half the year there was no light on one side of the disc and the other half of the year you went from dim to very bright and back to dim. Plus there would be no day/night variation (or put another way, a day is the same as a year).

That might make for a very different place of long winters and long days, something like the Polar Regions on earth but for the entire disc world, not just the planetary extremes. Certainly a very different setting for your project.

I like days, though, so what if we spin the work around an axis through the disc? Let’s start with the disc again facing the sun at New Year’s and the axis of rotation perpendicular to the orbit of the sun. On New Year’s day you would have day and night, something like a tropical day on Earth. In fact, as the year progressed, nothing would change. You would always have a day and night, the noon would always have the sun straight up. Now you have a world of days and nights but no seasons. Could be an interesting place for a desert or Arabian type world.

One point about this arrangement: half way through the year, the disc world’s day starts a half-day later then it does at the start of the year: each day the sun rises and sets a little later (or a little earlier depending on which way the disc spins relative to the orbit around the sun). If yearly orbit is an exact multiple of daily disc rotation , the disc-worldians see one more or one less days per year than you might otherwise expect. A minor nit but if a day is 24 hours and a year is 24 * 360 hours, the disc world would have 359 or 361 sunrises per year depending on whether which way the disc spin with respect to the orbit around the sun.

Instead, let’s keep the spinning disc but tilt the axis. So now the world faces the sun at New Year’s but it is tipped forward. What does this do? At the start of the year, you have a day where the sun is not directly overhead but a quarter way through the year sun is directly overhead (repeating for the second half of the year). Now you have seasons. The amount of season would be determined by the degree of tilt.  With a little tilt you would vary from a sun a little off vertical at noon in the winter; 90 degrees of title and mid-winter would be a day of twilight.

Now you have days and seasons similar to earth but there are differences:

  • Seasons are identical everywhere on the surface of the disc unlike Earth where it is winter in the north when it is summer in the south and the equatorial regions have no seasons at all.
  • Day and night are always equal length (except for the 90 degree tilt case where twice a year the day is all twilight)
  • There are two progressions of a seasons in a year: from winter at New Year’s to summer at the quarter year back to winter at the half year then summer at the three-quarter point.

So, that’s a basic disc world but can you do more with it? Most certainly, stay tuned for the next post when we imagine a fixed disc and a sun that orbits around it just outside the edge of the disc. Now our disc world will stop being something Earth-like and start looking very different…