Diskworlds

Back in 2nd Edition D&D, there was a map released with the Spelljammer supplement that laid out the geometry of a variety of different TSR worlds; their size, relative location of continents, their position within their star system, moons, etc.  In addition to the standard spherical worlds we’ve all come to know and love, they had strangely shaped worlds, like cubes, disks, torus’, even a mobius strip.

With that in consideration, I got to thinking recently about what life would be like on a disk shaped world.  I’m not talking ‘Discworld’, the world that is shaped like a disk and rides on the back of four elephants that stand on a turtle, whose ‘star’ orbits the disk but is infinitesimal in size to our local star, but exists in a world where the laws of physics are quite different (when they stare into our world, they can’t understand out ’round worlds’).  I’m not even thinking of the diskworld of (spoiler) “Dark City”.  I’m just thinking about disk worlds in general, how they might work, and what implications their shape would have on visitors.

To start with, I looked, but could not find, a simulator or formula that would describe what gravity would be like on a disk shaped object.  I’m certain that there are ways to properly integrate and figure it out, but I wasn’t able to find someone who had already done the legwork and my grasp of university level physics and math is more on the description side of things now rather than the numerical.  As a result, I’m basing my thoughts on what seems to make sense, but may not actually work.

A disk shaped planet made of dense material (dense enough that it would produce an average of 1 G on it’s surface) would eventually accrue material such that it would be shaped like a sphere.  This is an unfortunate long term problem that the disk would face, but as we’re talking about disk shaped worlds, strange things are bound to be going on.  In the short term, I can imagine that material would tend to bunch up in the middle of each side, as it is closest to the centre of mass.  Another odd characteristic is that the centre might actually have less gravity, as while you’d be pulled down, you’d be pulled equally from the sides (from the material underneath the edges of the disk), whereas gravity might be the greatest while walking on the edges, as the entire mass of the disk would be underneath you.  Walking over the edge of the disk would be a rather smooth experience, much like David Bowie’s character in “Labyrinth”.

If the disk were to produce a gravitational field mechanically, like the deck plates on ‘Star Trek’, the field might be shaped something like a magnetic field, where one side of the disk would pull you down, where the other side of the disk would hurl you up, then over, then down onto the right side that would pull you down.  The edges of this world would not lose material (water, ground, air), as is commonly seen, as anything that ‘falls off’, would moving into an area of zero force (not slowing down), then would hit the other side and be pulled back to the right side.  Long term, this would be a very stable structure, in that it wouldn’t change much from being shaped like a disk.

Next to be considered is how the disk revolves around it’s local star.  Let’s start by imagining the star at the centre of a clock, and the disk revolving around where the numbers are.  If the disk were tide locked (always facing towards the sun) there’d be no day or night, nor any seasons, but that’s pretty typical of tide locked planets.  Now let’s get the disk spinning, but retaining it’s orientation towards the star (that is, at noon the disk is facing towards the star whether it is at 3 o’clock or 6 o’clock).  You could have your regular day / night cycles, but wouldn’t have any seasons (because seasons require a change in angle relative to the local star, and at any noon, the orientation of the disk is the same no matter where it is in its orbit).  The star could rise in the north and set in the south, the reverse, east or west, whatever.

To get seasons, we need to do what the Earth does, which is have it’s spin not related to it’s position in orbit.  This does, however, produce an interesting result.  Consider an extreme case; the disk is spinning around an axis that is parallel to it’s orbit at 3 o’clock, but perpendicular to it at 6 o’clock.  At 3 o’clock and 9 o’clock the star would be directly overhead at noon, but at 12 o’clock and 6 o’clock, the star would be either parallel to or below the horizon (depending on how thick the disk is).  Every year, or orbit, the disk would have eight seasons.  Further, in one summer, the sun would set in the north, while in the other, the sun would set in the south.  Finally, the season would be the same across the entire disk; no part of the disk would be in summer while another is in winter.

Navigation on such a spinning disk might actually be relatively easy.  Seeing as sunrise and sunset would occur at the same time no matter where you were on the disk, it is interesting to note that when the sun is directly overhead would be different for different parts of the disk.  If we continue using the disk from above, the sun would be overhead earlier in the south than it would be in the north (of course depending on the summer, it could be reverse), and so combining that knowledge with a clock that could pace out half of an entire day, you could determine how far north or south of the centre line of the disk you were.  The angle to the sun at ‘noon’ would be able to give you your east or west location.

With no central axis to work around, a magnetic field around a disk world would have interesting consequences.  The field could be oriented around an axis that goes through the edges of the disk, producing a borealis in opposite directions on the disk.  If the axis was oriented through the centre of the face of the disk, it might make a central region that might be inhospitable; the solar radiation would be funneled down to that spot, which might make a borealis that would be visible during the day.  That said, the magnetopause of the Sun / Earth system is far above the Earth’s atmosphere, and thus radiation from it’s local star may not impact the disk.