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Worldbuilding for Writers, Gamers, and Other Creators: Star, Planet, Moon

Storyworld: None | Series Name:
Reading Order: | Stand Alone? Yes
Genres: Non-Fiction, Writing Aid | Editions: E-Book

With Worldbuilding For Writers, Gamers, and Other Creators Volume One: Star, Planet, Moon, you can create a realistic world for your story, game, or other creative endeavor!

This concise volume provides straightforward guidance on how to model the astronomical elements of an Earth-like planet for your storyworld setting. You’ll get the science and help with the (basic) math required to…

  • Select an appropriate star
  • Figure out the best distance for your planet to orbit that star
  • Work out the details of gravity, day length, and atmosphere for the planet
  • Determine when your planet’s moon is full

…and much more… all with just a calculator. No advanced degree required!

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An Excerpt from
Worldbuilding for Writers, Gamers, and Other Creators: Star, Planet, Moon


Worldbuilding — sometimes called sub-creation or secondary world creation — is the task / art / craft of designing and representing an internally consistent and realistic setting for use in fiction, video or tabletop role playing games, movies, comics, a transmedia experience… really any type of storyworld.

In each concise volume of “Worldbuilding for Writers, Gamers and Other Creators,” and in the omnibus edition, you’ll find explanations, tools and resources to give your setting a level of verisimilitude that inspires the willing suspension of disbelief on the part of your audience.

We’ll work with a “top down” approach both literally and in terms of the subjects we cover. As we select an appropriate star, muse on the size, mass, and orbit of your world, its geography, climate, weather, biomes and habitats for life (including the sentient variety!) and more, I’ll use one of my own storyworld settings, the Shaper’s World, to provide examples.

Since the ultimate goal is to create a fictional representation of an Earth-like planet, we’re going to keep a broad focus by necessity… but you’re strongly encouraged to delve deeply into each topic via Wikipedia (http://www.wikipedia.org/ ), Wolfram Alpha (http://www.wolframalpha.com/ ) and other Internet resources.

Yes, there will be math, but what’s required is more basic than you might expect, and there are lots of tools (hello, again, Internet) to help us in that regard. It’s worth it!

What Is Meant By “Earth-like Planet”

For the purposes of Worldbuilding for Writers, “Earth-like” means the following:

  • Size and density close to that of the Earth to allow for a gravity suitable for an ecosystem similar to Earth’s
  • Orbiting in the “habitable zone” of a star and having appropriate orbital characteristics to allow for a climate similar to that of Earth
  • Having an active, self-replenishing crust to allow for geologic processes similar to Earth
  • Having an atmosphere similar in composition and density to the Earth
  • Being an age sufficient to allow for the development of complex organisms, including at least one sentient species

If you’re interested in creating a setting that is anything but Earth-like, you’ll still find the information in Worldbuilding for Writers, Gamers and Other Creators useful — it’s important to understand what’s normal before you push beyond!

Size, Gravity, Mass and Density

Our goal is to create a realistic representation of an Earth-like planet — that is, a world that will support creatures with biological demands similar to those of Earth life. Given that a planet’s size, mass, density and gravity are all interdependent qualities, we have to consider them all in concert.

Since we have to start somewhere, let’s begin with the planet’s size and mass.

If your planet is too small, its weaker gravity will make it less likely to maintain an atmosphere rich in the molecular gasses necessary to support Earth-like life. Too thin an atmosphere could result in wide variation from too hot to too cold.

Larger planets are friendlier to the development and sustaining of Earth-like life. A large planet, especially if it has a fairly swift rotation period, is more likely to have a magnetic field, which serves as a shield against radiation from the primary star. Larger planets tend to maintain their internal heat, which sparks the engine of convection that drives volcanic and tectonic activity. A constantly changing planetary crust perpetually stirs up the essential minerals and elements necessary to fuel life. The constantly changing land masses are thought to be essential to driving diversity and evolution.

Of course, a large planet usually possesses a stronger gravity, and this helps maintain a rich and complex atmosphere. Too much atmosphere, though, and a runaway greenhouse could result, driving surface temperatures past the point of support for Earth-like life.

Given all the variables, scientists suggest a range of about 0.75 to 1.25 times to radius of the Earth as suitable for Earth-like life. The Shaper’s World, our example setting, is 0.9788 Earth radii, or 6,243.2 kilometers (3,879.3446 miles) in radius.

Calculating Horizon Distance

One nifty result of knowing the radius of your storyworld planet setting is the ability to calculate the distance to the horizon on a uniform surface, like a vast plain or desert or the surface of the ocean. For something approximately at human eye level, the horizon is the square root of (height * radius of the planet.) Make sure you use the same units of measurement in your equation!

So on the Shaper’s World, the distance to the horizon is:

0.0016764 * 6,243.2 = 10.4661

Square root of 10.4661 = 3.2351 kilometers (2.0101 miles)

Similarly, we can determine the maximum distance at which an object of a certain elevation can be seen by taking the square root of (radius * elevation). This is handy for knowing just how close you have to be before those mountains show up. So a mountain 0.9144 kilometers (3,000 feet) high would be seen at a distance of 75.5564 kilometers (46.9486 miles).

How Massive?

Regardless of the size of the world, its chemical composition will have a strong bearing on its gravity. The ratio of heavy elements like iron and uranium to lighter elements like calcium and carbon has a direct relationship to your world’s tectonic activity and, by extension, ecological diversity. Even technology is affected: a planet where metals are very rare will likely never support an industrial society, and that, in turn, has implications for scientific understanding and social sophistication.

Mass and density are inter-related. A massive (large) planet might be poor in heavy metals and therefore be less dense (and have a weaker relative gravity) than a similar planet rich in heavy metals.

An extreme example is the planet Jupiter. Even though it’s eleven times larger than Earth and 300 times more massive, because it’s composed entirely of light elements Jupiter’s density is just 25% that of the Earth’s.

It’s suggested that a range of mass between 0.4 and 2.35 Earth masses is acceptable for an Earth-like planet. The Shaper’s World has a mass 0.92 that of the Earth.

Finding Density

Your planet’s density can be calculated once you know the radius and mass. The formula, expressed with Earth values equaling “one”, is mass divided by (radius to the third power.) For the Shaper’s World, we find:

0.92 / (0.9788^3 = 0.9377) = 0.981 Earth density


Gravity is an expression of the planet’s size and mass or density. If you know the radius of the world and either the mass or the density, you can calculate the gravity. Here’s how it works:

Gravity = density * radius or Gravity = mass / (radius^2)

The gravity of the Shaper’s World is therefore:

0.981 * 0.9788 = 0.96 Earth’s gravity or 0.92 / (0.9788^2 = 0.958) = 0.96 Earth’s gravity

Gravity is an interesting story element if your storyworld’s planet is visited by folks from other worlds. If your storyworld is, perhaps, a fantasy setting with no “offworld” visitors to offer a different perspective, then for your characters, it’s irrelevant if their local gravity is greater or less than that of the Earth.

However, knowing the gravity of your world helps you calculate a variety of other things, most notably the likely composition of your world’s atmosphere, the density of that atmosphere (and how rapidly that density thins with elevation) and the escape velocity required to escape the gravitational hold of your world.

The Shaper’s World: Star, Planet, Moon

Let’s take everything we’ve determined about the Shaper’s World and put it all together.

The Shaper’s World orbits Tah, a G3 star with a temperature of about 5,690 Kelvin. Tah is slightly less massive that Earth’s star: .973 of Sol. Tah’s luminosity – the amount of heat energy it radiates – is .9086 compared to Sol.

Tah’s habitable zone is between .6911 and 1.1820 astronomical units from the star. The Shaper’s World orbits at an average distance of .979 astronomical units. From this, we know that it takes 358.6867 Earth days for the Shaper’s World to orbit Tah. Since the orbital eccentricity of the Shaper’s World is very small (.0192) it has no meaningful impact on the planet’s temperature through the year.

The Shaper’s World rotates on its axis once every 25.6382 Earth hours. The average time from sunrise to sunrise is slightly longer: 25.7153 earth hours.

The Shaper’s World is slightly smaller than Earth: 6,243.2 kilometers (3,879.3446 miles) in radius. For a human standing in a relatively flat environment on the Shaper’s World, the horizon appears to be 3.2351 kilometers (2.0101 miles) away.

The Shaper’s World is a little less massive (.92) and less dense (.981) than Earth, which leads the planet’s gravity to be likewise less (.96 the gravity of the Earth.)

The atmosphere of the Shaper’s World is similar to the Earth’s: 72.7% nitrogen, 21.2% oxygen, around 1% carbon dioxide and a variety of trace elements. Humans (of a sort) can and do live there comfortably up to about 5,500 meters above sea level.

The Shaper’s World seems to be, overall, a colder planet than the Earth – about 11 Celsius (52 degrees Fahrenheit.) Since this could make for an environment too far from our creative intentinos, it’s likely we’ll play with both the albedo and greenhouse effect numbers of the Shaper’s World in the volume of “Worldbuilding For Writers, Gamers and Other Creators” dealing with climate and weather.

The Shaper’s World has a single moon, the tiny satellite Tala.  Tala is just 243.125 kilometers in radius (a tiny fraction of the size of the Earth’s Moon), with a density of 2.8 grams per cubic centimeter.  Tidal forces have “locked” Tala’s rotation period long ago – it appears to always show the same face to the planet.

Tala is tiny (and because of that size it’s unlikely that its a spherical object) but it orbits very close to the Shaper’s World.  Because of that, it takes up as much as .54 degrees in the sky, and would look larger than Earth’s moon in the sky.

Tala orbits the Shaper’s World every 1.2984 local days, or 257.895 times per local year.  The dance between the orbit of Tala around the Shaper’s World, and the Shaper’s World’s orbit around Tah, results in a Tala “moonrise” every 4.3504 days and a “full” Tala every 1.3034 days.

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