TWO/2 Eccentricity, Day & Axial Tilt

STEP ONE: Determine the orbital eccentricity of the world by consulting 2.2.1.

STEP TWO: Determine the rotational period of the world by rolling on table 2.2.2. Ignore this for tidally locked worlds.

STEP THREE: Determine the axial tilt of the world by rolling on 2.2.3. Ignore for tidally locked worlds.

Table 2.2.1 Orbital Eccentricity

1d10  Eccentricity
1-5  1d10 x 0.005
6-7  (1d10 x 0.01) + 0.05
8-9  (1d10 x 0.01) + 0.15
10  (1d10 x 0.04) + 0.25

Modification to initial roll:
+3 if a captured body


Closest approach to star (in AU) = D x (1 - E)

Furthest approach to star (in AU) = D x (1 + E)

... where: D is average orbital distance (in AU)
E is eccentricity

Table 2.2.2 Rotational Period

1d10  Chunk  Terrestrial Planet  Gas Giant / Superjovian
1-5  1d10 x 2 hours  1d10 + 12 hours  (1d10 / 2) + 6 hours
6-7  1d10 days  1d100 + 22 hours  (1d10 / 2) + 11 hours
8-9  1d100 days  (1d100 x 2) days  1d10 + 16 hours
10+  Very Long  Very Long  1d10 + 26 hours

Modification to initial roll:
Add (T x A) rounded down

... where: T is from 2.1.2
A is System Age (in Gy)

Result Modification:

MOD = T x A

... where: T is from 2.1.2 rounded down
A is System Age (in Gy)

If terrestrial planet with mass of 4 or more earths then subtract 2 (alternatively subtract world mass0.5). If gas giant with mass of less than 50 earths then add 2.

Period = generated period x [(MOD x 0.1) + 1]

Table 2.2.3 Axial Tilt

1d10  Axial Tilt
1-2  1d10 degrees
3-4  1d10 + 10 degrees
5-6  1d10 + 20 degrees
7-8  1d10 + 30 degrees
9-10  (1d100 x 1.4) + 40 degrees


Planets in very eccentric orbits tend to also be inclined towards the standard orbital plane to a significant degree. These worlds also may experience extreme seasonal temperature variations. For instance, a world might be in the life zone during the spring and autumn only, and be far too hot during the short summer and frozen over in the winter.


The orbits are inclined towards the rotational plane of the system too, but aside from very eccentric orbits the inclination is rarely above 10°, and generally less than 1/3 of that.


Is important in determining seasons. A low axial tilt indicate little seasonal change, while an axial tilt closer to 90° have quite extreme seasons. (The 90° version experience polar seasons from utter winter to very hot summers). Axial tilts above 90° show that the world has a retrograde rotation (it orbits the wrong way).


Primarily important in determining the temperature variations over a local day. Long days have greater difference between day and night temperatures and may influence wind patterns (see PART II). The day determined here is the solar day (time between sunrises), not the actual rotation period (the so-called sidereal day). The relation between solar day and sidereal day is

Solar Day = 1
(1 / Sidreal Day) x (1 / Year)

and generally easiest to calculate by using standard days.


In these cases, the local day is either very long (1d1000 + 100 days), almost infinite (counted in years, decades, centuries or longer) or stopped (tidally locked). If the rotation is very slow one gets sort of a tidal lock situation, where an Earth-like world gets a "hot" and "cold" pole slowly moving. However, a world with a very slow rotation and fairly close to the star is also very close to get fully locked or in a regular rotation. Life on a world with a slow rotation may cope by migration or hibernation.


It is quite likely that a world may change axial tilt over time (100Ky-1My), in a more or less random fashion. This can influence climate on a world significantly in the long run, and explain long-term climate variations. Worlds without large moons to stabilise the rotation can change tilt within a range of 25-30 degrees, stable worlds more along the range of 5 degree. The axial tilt change is likely influenced by the presence of other large bodies. (Jupiter may be responsible for Martian axial tilt changes and Earth's too, for that matter.)


Eccentricity can also be changed, generally by in-system gravitational forces. See SYSTEM DISRUPTION below.


Tidally locked worlds (and moons) will have a slight "wiggling" effect which will allow them to not be exactly locked to the primary. This is caused by the eccentricity of the orbit of such moons or planets. If the eccentricity is very small the libration effect will also be very small.


The big effect is to create large potential temperature difference between poles and equatorial regions, and as seasons won't moderate this worlds with low axial tilts often have very cool polar regions due to low solar infall, unless they have a thick atmosphere. This can allow polar caps on otherwise hot worlds, for instance.


The larger the axial tilt, the larger the seasonal differences will be. For worlds with axial tilts above 45 degrees, the polar regions actually receive more solar infall than the equator, though on a very seasonal basis.


This illustration shows how a system can change over time. In (A) two massive gas giants have ended up in orbits a bit too close to each other during formation. The outer, smaller gas giant is ejected and the ejection leaves the remaining gas giant (B) on an eccentric orbit which in turn can disrupt the inner system. In (C) a large gas giant, perhaps a superjovian, is in fairly close orbit. The terrestrial planet inside it is continually influenced by its large neighbour, and in (D) the orbit of that planet has become highly eccentric. Eventually it may be ejected. This increasing eccentricity can have large influence of the evolution of an otherwise Earth-like world.


Systems can be disrupted in several ways, but all of them induce distinct changes to the generated system. Basically, gravitational action serves to throw away system bodies ... perhaps in to a more distant orbit, perhaps into the primary, and perhaps out of the system altogether. This has already been considered somewhat when generating binary systems, but long-term action can still disrupt systems. What happens is 1: that a planet's orbit is affected and the orbital eccentricity changes or 2: the presence of another body inhibits the planetary formation, as the planetesimals are disrupted. A big jovian planet can thus prevent the buildup of terrestrial planets.

By Companion: In binary or multiple systems, planets are susceptible to being disrupted and thrown out by the influences of the stars.

By Other Planets: Continual interaction with other planets can also serve to affect the orbital eccentricity (Mars has an eccentric orbit due to Jupiter influence, for instance) and in the end throw out planets. Two close large gas giants could interact and finally lead to an ejection of one of them and an eccentric orbit of the remaining. The remaining planet could in turn with its eccentric orbit affect other worlds.

Jovian and superjovian worlds may prevent the formation of smaller planets nearby. The larger a planet is the more able it is to influence and disrupt other worlds. On the other hand, large worlds tend to also be able to remove stray asteroids and chunks from the system and thus lessen the impact rate in the system.

By Close Encounters: When a star system passes close to another star planets can also be thrown out or eccentricity changed. While such close passes uncommon, they are more common in more crowded stellar neighborhoods. Distant worlds are more susceptible to being disrupted this way.