The following essay is a shameless attempt to entice you into buying and reading my books, the Chronicles of Tharia. It is a collection of facts that do not appear in any of the books I’ve written so far, so there are no spoilers here. It is the background against which the stories take place.

     Tharia is an earthlike planet with a diameter of about ten thousand miles, making it a little larger than earth, but its lower density means that it has only a slightly higher surface gravity (1.1g).  Its atmosphere is about half as thick as earth’s, meaning that the atmospheric pressure at sea level is about equal to that at an altitude of five thousand feet on earth. About two thirds of its surface is covered by oceans, most of which form one single ocean covering an entire hemisphere, with most of the major land masses on the other. There are three main continents, only one of which is inhabited by civilised life. Its day is 24 hours long. I would have liked to make the length of its day different, maybe as little as 15 hours or as long as 100 hours, since it would be pretty coincidental for its day to be the same length as ours, but I thought that describing and explaining all the changes this would cause in Tharian culture would slow down the storytelling too much. A different day length would also have an unknown effect on the planet’s weather.

     I made Tharia alien by giving it two suns and three moons, therefore. I know that gravitational interactions between the three moons would make their orbits unstable, so only the outer, largest moon is natural. The other two were put there by an advanced non-human civilisation that existed millions of the years in the past, to be converted into space stations, but the civilisation was destroyed by war before the conversion process could be begun. One day the gravity of the largest moon will pull the smaller moons out of orbit, either to be flung out into space, to crash into the largest moon or crash into Tharia, sterilising the planet, but none of these things will happen for millions of years so there’s no need to worry just yet.

     The moon movers are just one of several nonhuman races that have inhabited the planet during its history. Between them, these races have almost completely exhausted Tharia’s natural resources, including its coal, oil, natural gas and most of its metals, although there are still plenty of precious metals and jewels, since these things only have a special significance to humans. Iron and steel are rare, therefore, and are used only by governments and wealthy individuals. Most people use substitutes such as slennhide, a kind of leather taken from a large, reptilian creature similar in appearance to a rhino, for armour and ironwood for weapons, saws and axes. Ironwood is an extremely dense form of wood that can take an edge sharp enough to cut bronze. The ironwood trees grow so hard as a defence against the very large herbivores, flightless dragons as much as 500 feet tall and weighing 35,000 tons, that feed on them.

     No industrial revolution is possible for the present human civilisation, therefore. They have had a medieval culture for at least the past ten thousand years, and will still be living that way thousands of years in the future. The prehuman civilisations have left other marks on the planet, most significant of which is the vast network of tunnels and caverns that honeycomb the planet’s crust, which has become inhabited by a large number of subterranean races.

     Tharia has almost no axial tilt (only about two degrees). It gets its seasons from the ellipticity of its orbit, which means that winter is much longer than summer and happens all over the planet at the same time. It also means that the seasons are more extreme than the seasons on earth. On our world enough sunlight reaches the poles in summer to stop the temperature dropping below -60 centigrade. On Tharia temperatures at the poles regularly drop below -150 centigrade. This is cold enough for co2 to freeze out of the atmosphere, forming a layer of dry ice on top of the two mile thick water ice sheets. It is also cold enough that no life of any kind can exist within a thousand miles of the poles.

     The frosting out of co2 at the poles in winter causes the concentration of this gas in the atmosphere to drop from 500 parts per million in summer to only around 200 ppm. 150 ppm is the minimum necessary for plants to grow. All land plant life would die if co2 levels dropped below this. 200 ppm is comfortably above this, but still low enough for plant growth to slow significantly in winter, even more so than that caused by the lower temperature. In spring, when co2 levels rise again, plant life all over the world experiences a massive growth spurt.

     Lower co2 levels also means less greenhouse warming. Like Earth, over 90% of the greenhouse effect comes from water vapour, which is replenished by evaporation from the oceans at the same rate as it freezes out at the poles, but the drop in co2 in winter is enough to cause global temperatures to drop two degrees celcius more than the planet's greater distance from the sun alone causes.

     Co2 is heavier than air. In spring, as the co2 evaporates from the poles, it takes a few days for it to mix properly with the atmosphere. In that time, if there is very little wind, it can collect in low lying areas, displacing the oxygen and asphyxiating anyone caught there. Fortunately, the co2 has usually mixed fully with the atmosphere by the time it reaches those latitudes warm enough for people to live.

     I briefly toyed with the idea of letting it get cold enough for oxygen to condense out of the atmosphere which, at the low air pressure existing on top of the ice sheets, would happen at around -190 centigrade. I liked the idea of rivers of liquid oxygen flowing south, boiling and evaporating as they reached the warmer lower latitudes. I abandoned the idea when I realised how much the atmospheric pressure would change between summer and winter, enough that thousand mile an hour winds would scour the whole planet down to the bedrock. I wanted to keep the planet as plausible as possible, which might seem strange when talking about a world that has magic.

     I wanted magic to be a force of nature on Tharia, like gravity and electricity, that follows rules that limit how it can work and what it can do. It is carried by massless particles that decay into photons with a half life of around a million years. Magic once permeated the entire universe, and there is still a very faint background magical field that fills all of space, but now it can only be generated in places of very high energy density, such as the centres of neutron stars. Some neutron stars have jets of energy, including magic, shooting out of their poles, reaching millions of lightyears into space. If that jet of energy bathes a planet it can be used by the inhabitants of that planet to do things that would otherwise be impossible. Magic cannot pass through dense matter, though, and so cannot reach caves miles below a planet’s surface. It is normally formless and unstructured, and needs to be arranged into coherent structures before it can be used to do something useful, something that wizards do in their bodies as they read the spells in their spellbooks. Spell effects cannot travel faster than light, so teleporting to and communicating over great distances, such as to other planets, takes time. Casting spells leaves a residue of randomised magic that can interfere with other spells cast later in the same place, causing them to fail or misfire, and two spells cast in the same place at the same time can interfere with each other. Magic can be collected and stored, in the magical equivalent of capacitors.

     When I started writing these novels, thirty years ago, I casually made reference to souls and an afterlife, something that I regretted as I aged and matured. There’s no way to remove them, they’ve become too much an integral part of the story, so I had to find some kind of rational explanation for them. I had to figure out what souls are and what they are made of. The explanation I eventually came up with is a little desperate, and for that I apologise, but it’s the best I could come up with. If you believe in the literal existence of souls and an afterlife, I hope you’re not too offended by what follows and I remind you that this is a work of fantasy.

      I decided that the very faint background magical field that fills all space, left over from the big bang and occasionally refreshed by highly energetic events such as the collision of two neutron stars, can hold the impression of a thinking mind, like the impression left in soft mud by a man’s boot. This impression is so perfect that it is conscious and aware, even after the mind that made it has died. They are very fragile, though, and can be destroyed by the slightest interference, like another boot erasing the first bootprint, which is fortunate because a bare soul, unable to communicate with anyone, living or dead, and unable to sense its surroundings, becomes insane very quickly. A long extinct civilization, a race of beings called the Heavenmakers, took steps to protect these impressions, though. Millions of years ago they built machines in another universe, a totally empty universe in a higher dimension. These machines, which came to be called ascension machines, generate a field of energy that attracts soul impressions, dragging them into this higher dimension in which the natural laws make them much stronger, able to survive for any length of time, until that soul grows tired of life and chooses to fade out of existence. It also gave the souls the ability to communicate with each other and create virtual landscapes for themselves. The Heavenmakers ruled this realm, they could allocate resources and control the parameters under which the ascension machines operated, and they could even extend their influence into the mortal planes, where they came to be worshipped as the first gods. Since then, though, most of the Heavenmakers have faded from existence, and the gods worshipped by humans today are souls gathered in by the ascension machines that have grown to power over millions of years.

     Not all souls are raised from the mortal planes, though. Some souls somehow gain a static charge of magical force that protects them from erasure and shields them from the attractive force of the ascension machines. These go on to become ghosts and other forms of undead creatures. Some undead creatures are supposed to drain the life force from the living. The idea of there being a ‘life force’ is ridiculous, so I decided that they drain heat from the living, that they kill by inducing hypothermia.

     During the course of my books we encounter human life on many worlds in many universes. I wanted to take the story to these places, but didn’t want to have to keep describing alien races, their cultures, biologies, ecologies etc. Doing this once and twice over a twelve volume series is fun, but doing it every time our heroes visit a new world would soon become tedious. The aliens had to be human, therefore, or close to human, but explaining this was a problem. Many authors cite convergent evolution or something, which is ridiculous when you remember the millions of forms that life on earth takes. I decided that all human life everywhere is descended from an ancestral civilisation, our own civilisation. My stories are, in fact, set far, far in the future, at least a million years. We colonised the stars, and found that some worlds have magic, which allowed us to colonise other universes (travel between universes is only possible by means of magic, no matter how high the technology a civilisation possesses, which is why space age cultures can’t visit medieval Tharia. Those worlds that do have magic tend not to become industrialised, as their inhabitants use magic to meet their needs rather than technology). Sometimes civilisation falls and the survivors fall back to a primitive state, taking centuries to rise again. They usually believe that human life originated on their world (some might deduce that they came from elsewhere, because of the absence of fossilised ancestors), they rediscover space travel and find worlds all around them already occupied by human life. Civilisation had risen and fallen many times before humans came to Tharia, and human life is now to be found on millions of worlds in thousands of universes.

     Well, there we are. I hope this brief description of Tharia and the universe in which it resides has whetted your appetite for more. Tharia is waiting for you. Come and explore!