The Disc World (2): a land of extremes

Last time we looked at a vanilla disc world; this time we’ll take a look at something more exotic: a world where the sun is noticeably larger in the morning than the afternoon, where the edge of the world is a place of deep, bitter cold except for brief times where it rains molten rock, a world worthy of a high fantasy setting.

We still start with a world that is a disc, as in the Norse myths. Instead of a disc orbiting a sun at an Earth-like distance, let’s go to a more cozy world, something more along the scale of what the ancient Greeks imagined to be celestial distances.

For this world, let’s take the same 6000 mile disc and let’s add Rim Mountains 200 miles high. Yes, that is quite an impressive height for mountains but it is no more (or less) implausible than a 6000 mile wide slab of rock. How is that physically possible? I don’t know; this is a fantasy exercise, let’s take that as a given.

To that, let’s add a sun only 200 miles across but orbiting at a radius of 3200 miles so that the edge of the sun is only 100 miles from the edge of the disc. As a minor note, there is also a mountain range along the east-west axis about the height of the Rim Mountains. (That doesn’t affect things much but explains why there are perturbations in some of the energy plots for this world that I’ll share in the next post).

In the system, the disc is fixed in the center of the universe. The sun orbits the disc once per day. The plane of the solar orbit at start of year is parallel to the plane of the disc. Over the course of the year, the solar plane rotates along the disc’s north-south axis. This means that at New Year’s the sun is just below the Rim Mountains. As the year progresses the sun rises higher and higher over the mountains until at the quarter of the year, the sun rises highest over the world passing exactly over the north-south line. Then it starts taking a lower path in the sky until at the half of the year, it is once more just below the Rim Mountains. For the second half of the year, the sun progresses as in the first half however this time it traverses the sky in reverse (since the plane of the sun is “flipped” over).

Let’s assume that the sun rotates clockwise in the plane as looking down at New Year’s. This means for the first half of the year the sun rises in the north and sets in the south. For the second half of the year, the sun does the opposite: it rises in the south and sets in the north. Day and night are essentially equal except for the effect of the Rim Mountains. When the sun is very low in the sky around New Year’s and half way through the year, the mountains block the sun much of the day and the days are therefore shorter.

Okay, that’s quite a bit of a set up but what does that get us?

For starters, with the sun orbiting just beyond the radius of the world disc, the apparent size of the sun depends a great deal on where you are on the world and where the sun is in its orbit. How much? Well if you are on in the Rim Mountains themselves the effect is huge: the sun could take 45 degrees of sky (for comparison, the moon and sun on Earth are very close to half of a degree. At its farthest distance when the sun is rising on the far side of the disc, the apparent size would be less than half a degree. For some at the north pole, at sun rise, the sun would extend from horizon to halfway to overhead. That’s immense.

Moreover, when the sun is close it would be scorching hot. In the world of Calyx, it would be hot enough to melt rock, sending rivers of lava off the edge of the world to circulate back onto the Rim Mountains as a rain of lava and ash. When the sun is distant, it would be frigid in the mountains, a land of ice. The Rim Mountains, at least in the east and west (the north and south are different as we will see) would be briefly hot beyond anything on earth then ice-locked the rest of the year. You would have a half a year of hard ice ended by a brief rain of fire. How’s that for an exotic land of fire and ice? Kind of reminds you of the land of frost and fire giants of Norse myth, doesn’t it?

The north and south poles are different at the edge of the world because the sun always rises and sets at these poles. Therefore the sun blisters the Rim Mountains at the North and South Pole every single day. Instead of mostly frigid days and a few days of melting rock, you would have molten rock and lava every day of the year: a land of lava flows.

Partway between east and north on the Rim Mountains, it grades from one extreme to the other. For those looking for a setting, this provides a nice continuum to plop your characters into.

At the center of the world, things are much more stable. For one thing, the sun is always 3200 miles away. Around New Years it is very low in the sky and warms the land little, as the season progresses it rises higher until the sun is directly overhead and hottest at quarter year and so on through the rest of the year. Hottest is still relative: with the sun 3200 mile overhead, it is still distant and cool. The only change is that the sunlight beats straight down onto the land rather than glancing. This makes for a chilly place.

Moving from the center, things become stranger. Go north along the north-south axis from center and you find the sun closer and therefore larger in the sky and hotter in the morning (at year start) and dimmer, colder and smaller in the afternoon; the closer you are to the rim, the larger the effect. In the second half of the year, it all reverses: large sun in morning, smaller one in afternoon.

Move east from the center along the east-west axis and the sun will be same size in morning and afternoon. But now in the first quarter of the year, the sun is charting a path over the eastern half of the disc and it is very warm (how warm depends how close you are to the rim but remember as we get very close to the rim it is hot enough to melt rock). In the second quarter of the year, the sun is tracing a path over the western portion of the disc and the sun is distant and cold.

So: along north-south, warmth and size of sun varies over the course of the day. Along the east-west, warmth and size of the sun varies over the course of the seasons. Of course, if you are in between true north and true east, you get a mixture of the two effects.

One might ask how I can speak so authoritatively. Well, some of it is just a mental exercise but in the end it does get quite tricky and it is one thing to mentally figure out the general flavor of things, another thing to really quantify how things would behave. Therefore, being the uber-geek that I am, I wrote a C++ program to produce all sorts of cool pictures and images to help visualize what life on this disc world might be like. A taste of these images are in the post.

More in the next post….

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