Worldbuilding For Writers Number Four: The Rotation Period
Welcome back to the weekly blog series Worldbuilding for Writers, Gamers and Other Creatives, where we look at how to build an Earth-like planet for use as a setting in your storyworld. We're taking a more-or-less top-down approach. So far we've covered selecting a star suitable for an Earth-like planet, finding the habitable zone around that star where a planet has the best chance of supporting life as we know it, and calculating the orbital period of our planet. Now that we know the length of the planet's year, it's time to figure out the length of the planet's day.
Different Periods of Rotation
Put simply, the period of rotation is the time it takes for a planet to revolve around its axis. In fact, we commonly refer to two different kinds of rotation: Sidereal and solar.
Sidereal rotation is the time it takes any point on the planet to revolve around its axis relative to a distant point of reference, like the planet's star. When we speak about the rotation period of planets, we usually mean sidereal rotation. However, the rotation period that usually matters more to inhabitants of the planet is the solar rotation, which is simply the time between sunrises.
Why the difference? As a planet orbits its star, it has to rotate slightly farther to directly face the star. In other words, the solar day has to take both solar and sidereal rotation into effect. For planets with a fairly fast rotation period, the difference isn't all that much — on Earth, the sidereal rotation is about four minutes less than the solar rotation.
Figuring out the difference is fairly simple, and depends on the following formula:
solar day = sidereal day / (1 – (sidereal day / orbit period))
Let's plug some real numbers into that formula. The Shaper's World, my storyworld that stands as a running example in this series, has a sidereal day of 25.6382 Earth hours and an orbital period of 8608.4808 Earth hours. So:
1 – (25.6382 / 8608.4808) = 0.9970
25.6382 / 0.9970 = 25.7153
The solar day of the Shaper's World is 25.7153 Earth hours.
How Fast Should It Be?
The rotational speed of a planet is affected by many different factors over time. Collisions with other bodies could change the speed of rotation in much the same way a billiard ball's spin is changed by the strike of the cue ball. The close orbit of a large satellite could mutually affect both bodies. Even the gravitational interaction with other planets could make a difference.
So… there's no clean and easy way to calculate what your planet's rotation period should be. You have some creative leeway here, but there are things to keep in mind:
The Coriolis Effect
The atmosphere of your world moves along with the planet as it rotates. At the equator, the rotational velocity of the atmosphere is roughly the same as the planet. As you move farther north or south from toward the poles, the ground below the atmosphere moves slower while the atmosphere itself retains most of its original velocity. That excess speed pulls the atmosphere to the side, which results in whirling patterns (gyres) that we know as hurricanes.
All things being equal, a world roughly the same size of the Earth with a rotation period twice as fast will have twice the Coriolis effect. Storms will spin twice as tightly and contain more energy. This rapid distribution of warm energy from the equator to the poles (and cold energy back down from the poles to the equator) will have a profound impact on a planet's weather systems.
On the other side of the spectrum, a very slowly rotating world will also have violent weather as warm air from the day side clashes in convection with cooler air from the night side.
The Coriolis effect also influences the surface currents close to the surface of the oceans. A swiftly spinning planet will have faster, churning currents compared to a world with a slower rotation period. This could have an impact on climate and on the capabilities of sea-going vessels.
The Rotation Period's Impact On Life
Consider the influence the length of the day / night cycle might have on your planet's ecosystem, life forms and cultures. The circadian rhythms — the body clocks, if you will — of your life forms will reflect the relative speed of the day. Perhaps metabolisms are faster on swiftly spinning planets..? It's something to think about when designing your storyworld.
Next
In the next installment of Worldbuilding for Writers, Gamers and Other Creatives, we'll examine the relationship between your planet's density, mass and gravity.
Attention Astronomers
As I prepared this installment of Worldbuilding for Writers, Gamers and Other Creatives, in my research I failed to find an easily digestible formula to determine the upper limit of a planet's rotation before it tears itself apart. Is such a formula readily available? For that matter, would the rapid spinning of a world actually tear the world apart, or is that a false assumption? Thanks, in advance, for your input… I look forward to catching your attention and hearing from you in the comments!
Matthew Wayne Selznick - Telling stories with words, music, pictures and people.
