World Building 101: Today's Topic: Tilt a World!

Our Earth is tilted at a 23.5-degree angle in its orbit about the sun. We spin like a tilted top and this accounts for our seasons. When the northern hemisphere is tilted away from the sun it’s winter up there. Six months later it’s summer because the tilt is now towards the sun. In the southern hemisphere, it’s exactly six months different: When it’s summer in the United States, it’s winter in Australia.

Some assume that during the winter the sun is farther away, and in the summer it’s closer. But that has very little impact on the seasons. The Earth’s orbit is only slightly elliptical. It varies from about 92 to 94 million miles away from the sun.

This tilt of ours creates a wide temperate zone where we get four distinct seasons: Winter, Spring, Summer, and Fall. A larger tilt would create a harsher difference between a cold winter and a hot summer. A smaller tilt, or no tilt, would pretty well eliminate the seasons. If, however, the planet had a large elliptical orbit, then that would create seasons that would affect the whole planet at the same time in the planet’s year.

The planets in the solar system vary considerably in tilted-ness. Mercury isn’t tilted at all, while Uranus is at 82 degrees (almost lying on its side), and Mars is very similar to Earth at 25 degrees.

So a fantasy world could really have any tilt you’d like, and an eccentricity (a measure of elliptical-ness) greater than Earth’s. The more tilt you give it, the more extreme the temperatures are going to be. If you had a planetary tilt like Uranus’s, then for half the year you wouldn’t see the sun at all. It would be a very cold, very long, night. Then the sun would start to peek on the horizon, making little circles in the sky. As the year wore on, the sun’s little circle would get higher and higher in the sky, and then finally set on the opposite horizon. The summer would be one long, very hot, day. Such extremes would beg the question of how plants and animals would survive. Presumably, life would find a way.

The tilt of a planet slowly rotates about itself like a tilted top… very slowly. It is called the Precession of the Equinoxes, and for Earth, the loop takes 26 thousand years. This moves the seasons slowly backward in the calendar year.

All of this can play heavily in your fantasy world building. From the tilt of the planet and the eccentricity of its orbit you can have massive winters and unrelenting summers, or mild, practically non-exist seasons.

And as for the philosophical and religious implications of a tilted planet, just look at how the Mayan’s viewed it all.

Related to tilt, we have spin. A planet spins (rotates) on its axis as it orbits the sun. The rotation, over time, will slow down. The Earth’s spin is currently at about 365 spins per orbit, giving us our 365-day year. A planet could spin faster, or slower. If it slows down completely then it will rotate exactly once per year, meaning one side (the heavier, or denser side) will face the sun, while the opposite side is forever in the dark.

This allows your fantasy world to have very short days, or very long ones. A planet can even spin in the opposite direction to Earth’s, meaning the sun would rise in the West and set in the East. (Of course, if the denizens of your fantasy world know nothing of Earth, then ‘opposite’ of it means nothing. The sun merely rises and sets.)

World Building 101: Today’s Topic: Darn Those Constellations!

Look up on a clear night and you’ll see a million stars splashed across the sky – provided you’re not in the middle of a large city. But even in those light-polluted boroughs, you should be able to pick out the Big Dipper (Ursa Major) or Scorpio.

In a fantasy realm the constellations will be completely different (unless that realm is a mirror-universe of this one, of course). Your world will be sitting somewhere else, or somewhen else. That means the view at night of the neighboring universe will look different.

Our night sky has been mapped into 88 different regions. Each houses a collection of stars that are together in the sky – or at least apparently together. The only problem is, this map was drawn as if all the stars were the same distance, as if there was a celestial sphere about the Earth – a dome of stars. But we know that while some stars are close, others are very far away. It’s like looking at a tank of glowing fish, some may look like a group, but if you walked around to the side, they might be all spread out. Also, like fish, stars move. Some are moving away from us, others towards us, and many are moving across the sky. The movement is relatively slow, in that it takes hundreds, or even thousands of years before the movement is obvious. If your world is the Earth a million years ago, or a million years hence, the arrangement will have changed.

These regions – constellations – were mostly named in ancient times. Today we still retain many of the names used by Western Civilization. Orion, Draco, Cassiopeia, Leo, and so forth were so named thousands of years ago. Some are newer: Little groups of stars that didn’t really fit in the main constellations. Musca (the Fly) is a good example. (Here's a good source list of constellations, if you are curious.)

In a fantasy world, what will the characters see when they stare up at the night sky? Well, it all depends. If the world is in a galactic arm of a spiral galaxy, very likely they’ll see a sky much like our own, except all the stars will be scattered across it in its own unique way. Likely, your character’s social history included a similar connect-the-dots sort of naming convention. Then, too, your character will see constellations in the sky, but named after his or her own mythic or real heroes of the past. It has been mankind’s tendency to see the divine in star patterns, whether gods, heroes, or mythic animals. Your world’s beings may be similar, or completely different.

But perhaps your world isn’t located in the outer neighborhood of a nice spiral galaxy. Maybe yours is closer to the center, or in a small, dense cluster of stars, or in the middle of a gaseous nebula, or a dark dust cloud. These conditions would cause the sky to look very different, very different indeed.

In a dense-star neighborhood, the sky would be filled with brightness. There would be so many shining stars that it would be brighter than our own full moon. Some stars could be close enough to be shining beacons unto themselves. Which could be true even in less populated areas of the universe. But I'll talk more about close stars on another day...

Or, put your system in a gaseous nebula, and the sky would be aglow with wispy gossamer trails and whorls of faint light. Like some eerie abstract watercolor, the night sky could be a surrealistic artist’s dream.

One could combine the two, for many dense star clusters are full of gas and dust nebula.

And lastly, the world might be isolated in a dark cloud, an area of space filled with dust. Though there may only be one particle per cubic yard, if it goes on for ten light years, that’s enough to block out all neighboring light. The world’s night sky would be total darkness. There would be no constellations to name.

Choosing your night sky, and any constellations, will help to reveal the history of your fantasy culture. A sky full of animals will indicate a society where animals are highly revered. A night sky of war heroes, gods, and monsters tells us something else.

Constellations were important to the early navigators. The stars were used to tell the position of ships in the night, and to confirm the seasons of the year. And of course, constellations have been used to predict the future behavior of people (astrology). The twelve signs of the zodiac are the twelve constellations that the sun passes through during the year (It doesn’t literally pass through, it’s all relative. The Earth moves around the sun, which means the background of stars behind the sun changes throughout the year.)

Images from Portraits of Stars and their Constellations and Astronomy Picture of the Day.

World Building 101, Today's Topic: Rings are cool!

Saturn, the second largest planet in our solar system, has a spectacular ring system. Beautiful icy rings that hug the gas planet's equator. So, as a fantasy writer wanting a cool world, rings might be just the thing!

Actually, all four of our solar system's large slushy planets have rings: besides Saturn, Jupiter, Neptune, and yes, even the pun-inducing Uranus has rings (nine dark ones!). So, could our smaller, terrestrial planets have rings? Sure, but it's less likely. Mars might one day end up with rings, as its inner moon is right on the edge of something called the Roche Limit. More about that later...

Rings are formed when dirt, dust, and ice gets captured by a planet's gravity. If the stuff is outside of the Roche Limit it will slowly gather together like a snowball due to its own gravitational pull and eventually form a moon. If the stuff is inside the limit it will either fall to the planet's surface, or continue to orbit planet. The stuff can't collect into a ball to form a moon because the tidal forces of the planet pulls it apart... The explanation is a bit technical, but basically, it's because closer objects have to spin around a planet faster than farther items... So, the outer part of a moon moves slower than the inner part. If the moon is far enough away the tidal force isn't enough to break it apart. But if its close in then the inner part has to move so much faster than the outer that it literally flies apart.

So, if enough debris collects around a planet and orbits it inside the Roche Limit, a ring will form. Over time, rings will form a nice flat disc high above the planet's equator (See this explanation as to why).

The four inner planets of our solar system (the terrestrial or 'rocky' planets) don't have rings mostly because the four outer giants act as vacuum cleaners, sweeping up a lot of the debris as it falls in on the sun's gravity well. And then there's the sun itself, which pulls rather mightily on all this dust and ice. Also, our own moon is quiet large for our size of planet, so it too sweeps up debris that might otherwise fall into an orbit about the Earth, given the chance.

A fantasy world could have a ring. All it takes is enough dirt and ice to collect about the planet. It would also help to have a solar system with fewer large planets in the outer orbits, thereby reducing the amount of clean-up that occurs.

A ring can be bright or dim, gaseous or rough, banded or smooth. It all depends on what it's made up of, and whether there are any 'shepherds'. A Shepard is a large hunk of dense rock -- a tiny moon really, that is orbiting inside a ring. Since it is dense the tidal forces don't pull it apart, and it acts like a little broom, sweeping up the debris in its orbital path. These Shepard moons give a ring two or more distinctive bands (You can read about Saturn's Shepard moons here). Bright rings are composed of ice particles, dark ones of rocky debris. High iron content will give it a reddish color, while other more rare metals can give it a blue (cobalt) or green (copper) sheen. And the planet's magnetic field can have an effect on the metallic rocks as well, moving then into positions in the ring that give rise to faint dark spokes and braided banding.

Millions of ice particles in the ring can refract and diffuse the sun's light, making the ring scintillate with color. Beautiful to behold indeed!

But since a ring will form about a planet's equatorial belt, it will look differently viewed from the planet's surface, depending on what latitude (north-south) position you are at. If you are right on the equator, you'd look up to see a thin line across the middle of the sky, neatly dividing it in two east to west. Rings are quite thin, usually only a few kilometers thick. Meaning that looking at it edge-on from a thousand or so miles away would make it look like a fine demarcation, or even disappear entirely, depending upon just how dense the ring is.

The further one gets away from the planet's equator, the more of the ring will become visible. In the northern hemisphere the ring will look like a giant band going across the southern part of the sky, east to west. In the southern hemisphere one will see the ring in the northern sky.

If the planet has a tilt like Earth's then the band will rise and fall in the sky throughout the year, and the sun will cross behind the ring twice a year. Depending upon just how large the band is, the sun might spend several months behind it. If it is a mostly an ice crystal ring, then the whole band would be diffused with the sun's rays. A brilliant band across the sky for several hours before dawn to after sunset.

As for moons, your fantasy planet could still have them, they would just have to be outside the Roche Limit. The Earth's Roche Limit is about 40,000 km, and our moon is about 400,000 km, easily beyond the limit.

The Earth does have a ring of sorts. Our man-made satellites and various space trash, orbits the Earth in a band of junk... if enough builds up then we'd have our own visible ring! Read more about space trash here.

That finishes my lecture on World Building: Planetary Rings.