Creating an Earthlike Planet by Geoff Eddy

Creating an Earthlike Planet

Written by Geoff Eddy
All text in green, added by The Astrographer

Last updated: 30 January 2007

[Brian Davis, if you’re reading this and wondering why you
haven’t heard from me: I lost your email address and all your emails
when my computer died. Aargh.]

This page is intended to help role-players and authors of sci-fi and
fantasy to, as the title suggests, create a world which resembles the
Earth – that is, a world in which humans could live and develop
societies similar to those with which we are familiar.

The key ingredient here is familiarity. Worlds which are
substantially different from the Earth are certainly interesting, but
they need more effort and imagination to create, and I’d personally
prefer to channel my imagination into the story rather than the
setting. People with alien tastes may nevertheless find some material
of interest here anyway, particularly in the Astronomy and Geology sections.

The wildly different lengths of the sections reflect three factors:

  • What I know and have got around to writing;
  • How much freedom the subject allows you;
  • The amount of relevant information elsewhere on the Web which I
    see no point in repeating here.

Thus Astronomy is very long, but Flora and fauna is short. Further contributions,
particularly to the shorter sections, are actively sought and will be
gratefully received.

Geoff Eddy would like me to add that Mark Rosenfelder’s Planet Construction Kit is superior in every respect. 🙂

This should provide more reliable hosting for the page, but the formatting suffers a bit in adaptation to wordpress. I’ll do what I can, but I’m not really a wp guru…


Introduction, or Why I Wrote This

This page originated in discussions I had with various people, both
online and offline, about the world I was creating for my
novel-in-progress. It emerged quite clearly that you can’t create your
world at random; there are a lot of interacting scientific principles
you need to know to guide you, and if you’re not careful you could end
up making careless yet avoidable mistakes which a pedantic reader will
take great pleasure in pointing out later. I am an (aspiring!) author
rather than a role-player, for which reason role-players should
substitute players where I say readers.

There is plenty of relevant information out there on the World Wide
Web, but of course you need to find it first, which takes time and
effort; moreover, I’m not aware of any source which contains
everything you need to know in sufficient detail. This page is
ultimately intended as a source of enough information, or failing
that, pointers to further information, to take the hassle and effort
out of finding and using it. Most of the page is based on my
experiences and discoveries while creating my own world. Some of you
will no doubt find some sections irrelevant, while others may well
find these same sections to be a source of hitherto unsuspected
interesting ideas.

I can’t claim to know everything about every subject I touch on here;
the sign “[*]” indicates that, due to such a gap in my knowledge, I’m
soliciting for more information from someone who knows more. Nor can I
in all conscience claim infallibility; for which reasons I will
gratefully accept corrections of errors. All contributions will be
appropriately credited.

Note: to simplify the maths, all equations are given using
normalised quantities, where the quantities are relative to the
Earth, the Sun or the Moon as appropriate. This gets rid of several
universal constants.

The Easy Way, or Things You Can Assume To
Start With

Robert Louis Stevenson once said that every adventure should start
with a Map. If, like him, you’re assuming a planet similar to
the Earth in size, composition and density which orbits around a
Sun-like sun at more or less the same distance and has days of
approximately the same length and similar seasons, you can skip the
mathematically-oriented parts of the Astronomy and
Geology sections and start drawing your Map.

On the other hand, particularly if you’re writing sci-fi, you might
well want to consider what happens if you have a different type of
Sun, or a planet with the average density of foam rubber. In this
case, carry straight on; there are a lot of surprising restrictions
which crop up where you might not expect them. Certain calculations
have been simplified, although their conclusions are not particularly
affected; there’s little point in trying to be too exact.

Astronomy, or Your Universe and What You
Can See In It

This is the subject with the most material, and the most mathematics;
be warned!

I’ll start this section big and steadily work inwards. The first
assumption is that your world will be set in either this universe or
one with the same laws of physics; if it isn’t, many of the following
equations will need to be changed.

Useful astronomy-related links include Curious about Astronomy,
Phil Plait’s Bad Astronomy
and the Planetary
. The Voyager
Project website
(one of many, so I believe) has lots of stuff
about our own Solar System. For further equations pelating to
planetary mechanics, don’t miss The
World Builders’ Cookbook

My source for all these links also adds that “A good reference on
resonance effects in planetary mechanics is at
which is part of a very good online book on the Solar System formation
and state at
which is in turn from the NASA online histories page at
— this has so much good stuff that space history geeks (that’d be me)
could be there for weeks.” What you make of all these is up to you,
but if you’re paranoid about violating the laws of physics, they’re
well worth reading.

Have a look at StarGen,
a program which creates “moderately believable planetary systems
around stars other than our own”.

The Stars

From the point of view of an Earth-bound observer, the stars remain
fixed with respect to each other but appear to move en masse
across the sky as if fixed to the inside of a celestial
. The exact locations and brightnesses of the stars will
matter more if you’re writing SF than if you’re writing fantasy; but,
if you want to randomly generate an interesting night sky resembling
the Earth’s, you could do worse than the following equation, which I
empirically found to be useful:

magi = 2 × log10i + B – R × rand(100) / i

where magi is the magnitude of the i‘th
brightest star, R is a randomizing factor (the larger it is,
the greater the deviation from a true logarithmic scale), and B
is the magnitude of the brightest star in the sky. For Earth, R = 1
and B = -1.4.

This should give you a naturalistic distribution of the stars by
brightness; now you need to place them on your celestial sphere. The
right ascension (the celestial equivalent of longitude) can be totally
random; the declination (analogous to latitude) should be the inverse
cosine of a random number between -1 and +1.

Another addition to the celestial sphere is the Milky Way; as
visible from the Earth, it forms a great circle in the celestial
sphere because the Earth is in the galactic plane. If your planet is
some way removed from the galactic plane, the Milky Way will form a
smaller circle. In general, the density of stars will be greater
closer to the Milky Way and less further away from it; the area
looking towards the galactic centre (on Earth, this is in the
direction of Sagittarius) will be particularly rich. This does not
preclude placing bright constellations away from the Milky Way; you
can place the brighter stars where you like, so the above equations
should still apply.

Bear in mind that constellations are apparent groupings of
stars which are really at widely differing distances; this is why it’s
meaningless to talk about “the Sagittarian Sector” (sci-fi writers
please note), since in any given constellation there are stars which
are closer to Earth than to the other stars in that constellation.

As far as other night-sky objects are concerned, external galaxies are
more visible when you’re looking away from the galactic plane;
naked-eye galaxies are thus most commonly found far from the Milky
Way. Globular clusters are generally found within or near to the
galactic plane.

The colour of a star is of course a function of its
temperature. The hottest stars are white or blue-white, the coolest
are orange or red, and those in between are yellowish. The colours are
actually only noticeable for the brightest stars; faint stars all look
white. In general, the brightest stars tend towards the hotter end of
the temperature range (classes B and A, with very occasional O); as
the stars get fainter, types G and especially K become more common.

Because the Earth is rotating on its axis, the celestial sphere
appears to rotate just over once per day. The “just over” – a result
of the Earth moving along its orbit – causes the night sky to appear
the same at any given time as it does slightly later the preceding
night and slightly earlier the following night. The difference in time
is calculated by dividing the length of the day by the length of the
year; for the Earth it is 236.5 seconds per day.

The sun

Your planet will in all probability orbit round a single sun, which
will essentially be a rather ordinary star. It’s very tempting to
orbit your planet planet in a figure-of-eight path around a binary
star (i.e.) two suns, but unfortunately such an orbit is unstable. If
you have more than one sun, you’ll encounter the “n-body problem” [*],
which is insoluble; in general, an orbit around two suns is only
stable if:

  • The planet is at least five times as far away from both suns
    as they are from each other.
  • The planet is at least five times as far away from one sun as
    it is from the other.
  • The two suns are in a very nearly circular orbit around their
    barycentre and the planet makes an equilateral triangle with
    them. This is, I am informed, “the Lagrange points L4 and L5
    cases; Donald McLean provided a reference
    explaining Lagrange points
    if, like me, you don’t know what
    they are. The planet orbits the common centre with the same
    period as the suns.

Added to which, one correspondent mentions that “binary star systems
will generally have too great a fluctuation in temperature to be

For this reason I will assume one sun only here.

Our own Sun (spelt with a capital) is a main sequence star of
spectral type G2 (yellow), which is pretty average in star
terms. Its diameter is 1.39 million km, and the Earth orbits it at a
mean distance of 149 million km (1 astronomical unit, or AU) in
365.25 days (1 Earth year) to complete one orbit. Note that 1
AU = 216 Sun radii.

Life on Earth has evolved because the Earth is at the right distance
from the Sun to ensure that it receives the right amount of heat from
the Sun. None of the other planets in the Solar System have developed
“life as we know it” because they’re either too close and thus too
hot, or too far away and thus too cold. If you want your sun to be of
a different spectral type from the Sun, there will be several knock-on
effects to consider. There are several physical quantities which are
relevant here, two of which are fundamental:

  • M, the mass of the sun.
  • R, the distance between the planet and the sun.

From these can be derived:

  • L, the sun’s luminosity, i.e. how much light it
    gives out.
  • D, the diameter of the sun.
  • I, the insolation, or amount of heat energy the planet
    receives from the sun. This is equivalent to the sun
    apparent brightness, i.e. how bright it appears when
    viewed from the planet.
  • T, the orbital period, or year, of the planet around the

which are related as follows:

L = M3.5

D = M0.74

I = L / R2 (inverse-square law)

M × T2 = R3 (Kepler’s third law)

Note also:

Surface temperature = M0.505

Lifetime = M-2.5

Now, for Earthlike planets I must be close to 1; according to Brian
Davis, “recent work suggests very conservatively 1.1 > I >
0.53”. Thus the feasible limits for R and T can be calculated:

Rmin = sqrt(L / 1.1)

Rmax = sqrt(L / 0.53)

Tmin = 0.53 × M2.125
Tmax = 1.1 × M2.125

From these can be calculated, for a star of any spectral type,
reasonable year-lengths for a planet with Earthlike life orbiting
around it. Using data from Norton’s 2000.0 (18th edition), we
get the following table. [I might redo this table sometime when Ihave
the time to bring it in line with the new equations].

Type L       M      D       Rmin    Rmax    Tmin (days) Tmax (days)

(main sequence)
O5 500000 40 14.72 674.20 971.29 1010981.52 1748158.60
B0 20000 18 6.01 134.84 194.26 134797.54 233087.81
B5 800 6.5 3.91 26.97 38.85 20063.49 34693.18
A0 80 3.2 3.02 8.53 12.29 5084.97 8792.77
A5 20 2.1 2.07 4.26 6.14 2219.26 3837.48
F0 6.3 1.7 1.53 2.39 3.45 1037.11 1793.35
F5 2.5 1.3 1.22 1.51 2.17 592.96 1025.34
G0 1.26 1.1 1.02 1.07 1.54 385.59 666.75
G5 0.79 0.93 0.96 0.85 1.22 295.48 510.93
K0 0.4 0.78 0.88 0.60 0.87 193.66 334.87
K5 0.16 0.69 0.77 0.38 0.55 103.56 179.08
M0 0.06 0.47 0.68 0.23 0.34 60.13 103.98
M5 0.01 0.21 0.42 0.10 0.14 23.47 40.58

G0 32 2.5 6.01 5.39 7.77 2893.60 5003.52
G5 50 3.2 9.34 6.74 9.71 3574.36 6180.67
K0 80 4 14.70 8.53 12.29 4548.13 7864.49
K5 200 5 32.47 13.48 19.43 8087.85 13985.27
M0 400 6.3 66.12 19.07 27.47 12117.71 20953.57

B0 250000 50 18.49 476.73 686.80 537669.89 929722.47
A0 20000 16 32.69 134.84 194.26 142974.38 247226.96
F0 80000 12.5 193.19 269.68 388.51 457518.01 791126.27
G0 6300 10 80.98 75.68 109.03 76041.58 131488.79
G5 6300 12.5 112.49 75.68 109.03 68013.66 117607.15
K0 8000 12.5 177.43 85.28 122.86 81359.49 140684.36
K5 16000 16 339.94 120.60 173.75 120941.60 209128.55

Sun 1 1 1.00 0.95 1.37 340.05 588.01

So, theoretically, your year may vary over a range of 23 days to a few
thousand Earth-years; note that years of Earthlike length are only
possible with Sunlike suns, and shorter years imply redder suns.

Brighter stars, giants and supergiants have shorter lifespans (3
billion years for F0, compared to 10 billion for the Sun). There’s
presumably a lower limit for the lifetime, below which the planet’s
atmosphere won’t be able to become breathable before the star turns
into a giant, but nobody seems to know what it is [*].

Stars dimmer than about K2 have tidal forces strong enough for the
planet’s rotation to be slowed down or stopped. This is what’s
happened with Mercury and Venus, but for different reasons; research
at Weather on
Tide-Locked Planets
suggests that the day side might be able to
support life.

A correspondent says:

“… if you want to create a group of stars with masses distributed
the way you would see in a real-world group of stars,
-ln(1-x)/ln(1.35), where x is a random number between 0 and 1, will do
the trick. Most stars that come out of this are larger than the sun
(2.3SM is about average), but the larger stars die so much more
quickly than the smaller ones that there are already far more small
stars in the galaxy than big ones.”

The Solar System

Now it’s time to consider the other planets which orbit your
sun. Our own Sun has eight of these: Mercury, Venus, Mars, Jupiter,
Saturn, Uranus, Neptune and Pluto; they appear in the sky as moving
stars. There’s no obvious limit to the number of planets you can have
around your own sun, but there are limitations on where they can
go. Moreover, planets too far away will be too faint to see, and
planets too close to the sun will be very hard to see in the sun’s
light. Uranus, for example, is just at the edge of human visibility,
but was not actually recognised as a planet until 1781; and Mercury is
very hard to see except just after sunset or before sunrise.

I’d really like to know what effects the processes of planetary
formation have on the distances of the planets from the sun [*]. In
the meantime, the best I can offer is a method based on Bode’s
. This law relates the distances of the planets from the Sun to
a simple formula, by which the distance of the i‘th planet is
given by:

Ri = 0.4 + 0.3 × 2i – 2

i.e. the distances in AU are ideally 0.55, 0.7, 1.0, 1.6, 2.8. 5.2,
10.0, 19.6, 39.2 and so on. Note however that Mercury’s distance is
0.4, not 0.55; there is no planet at 2.8 AU from the Sun (we have the
asteroids instead); and the Law puts Pluto where Neptune should
be. Whether or not Bode’s Law is a genuine physical law or the product
of coincidence, you can still use it to generate a workable set of
planetary distances by twiddling the numbers to your preferences. (I
am informed that “Bode’s Law works because planets tend to settle into
orbits whose periods are in simple fractional relations:
e.g. Neptune:Pluto::2:3 and Venus:Earth::8:13”.) You can now work out
the orbital periods of your planets with Kepler’s third law:

T2 = R3

Having done this, you will also need to re-twiddle your distances to
eliminate the possibility of two planets disturbing each other’s
orbits at the same point within the orbits. What this means is that
the ratio of no pair of orbital periods must be close to the ratio of
two small integers (e.g. 4/3, 3/2), unless the planets are far enough
apart (how far? [*]). Once that’s done, you can work out the
synodic period (S) of each planet, which is the time
taken by the planet to reach the same position relative to the sun and
your own planet:

1/S = 1 – 1/T, or S = T / (T – 1)

This doesn’t mean that every S years the planet returns to the same
part of the sky (except as seen from from the Sun), because the home
planet has also moved in that time; instead it means that the planet
will be best visible every S years, and will have moved across the sky
by an amount equal to the fractional part of S.

For example, consider Mars as viewed from Earth. For Mars, R is 1.52;
T is thus 1.877, or 685 days, or one year and 10.5 months, giving a
value of S of 2.14 years, or 781 days. This means that successive
oppositions of Mars, when it is opposite the Sun as seen from
the Earth, occur every 781 days, during which time it has moved 0.14
(the fractional part of S) of the distance across the sky from the
previous opposition.

In general, it must be said that our own Solar System is believed to
be typical of most solar systems. Thus it’s highly probable that the
outer planets of all solar systems are gas gaints, all with ring
systems and large numbers of satellites. Note, too, that celestial
mechanics dictate that neighbouring planets cannot approach each other
closer than a certain limit without becoming perturbed and breaking
up; this is the origin of the asteroids, and probably several of the
moons of the planets beyond Earth.

Finally, you can work out how bright your planets will be in the sky;
the equations here come from some pages from the
National Solar Observatory Sacramento Peak
. First of all,
calculate M0 and M, the absolute and apparent
magnitudes of your sun, from its luminosity (L) and distance
from your home planet (R, which must be in kilometres):

M0 = 4.8 – 2.5 × log L

M = M0 – 5 × log (R / 308.6 ◊ 1015),
or M0 + 5 × log R – 72.447

For the Sun, these values are 4.8 and -26.8 respectively. Next
calculate a useful constant C (for the Sun, 14.10):

C = M + 5 × log R, or 10 × log R – 2.5 × log L – 67.647

You can now calculate the magnitude m0 of a planet
at 1 AU:

m0 = C – 2.5 × log (a × r2)

where r is the planet’s radius in km and a is its albedo
or reflectivity. The albedo depends on what the planet is made of; for
rocky planets a is around 0.15, and for gas giants it’s between
0.4 and 0.6. Venus, which is covered in highly reflective clouds, has
an albedo of 0.65; the Earth’s is about 0.4; and that of an icy planet
would probably be 0.6 to 0.8.

At last! The magnitude of your planet is given by:

mmax = m0 + 5 × log(d1 ×
d2) – 2.5 × log (0.5 + 0.5 × cos phase)


  • d1 is the distance from the planet to the
    viewer in AU
  • d2 is the distance from the planet to the sun
    in AU
  • phase is the phase angle of the planet, i.e. the
    angular proportion of its visible disc which is illuminated.

For inferior planets, those closer to the sun than your planet,
the phase angle is 180 degrees at inferior conjunction, i.e. when the
planet is directly in front of the sun, and zero degrees at superior
conjunction, when it’s directly behind the sun. Obviously, you won’t
see inferior planets at either of these times; they’re best seen
around greatest elongation, when they’re at their maximum
distance from the sun in the sky. This distance, and the phase, are
given by:

emax = sqrt(1 – d22)

phase = 180 – arccos d2

For superior planets, i.e. those outside the orbit of your home
planet, the phase angle is rarely far from 180 degrees. Superior
planets make complete circuits of the sky, including the interesting
phenomenon of retrograde motion at opposition. This is
particularly noticeable with Mars; as it reaches opposition, it slows
down and stops, then moves backwards through its opposition, then
stops and moves forwards again. Experimenting with a night-sky viewer,
such as my Night Sky Applet, should help
you to understand the process.

Note that planets with apparent magnitudes less than 6 will be
invisible without optical aid, as is the case with Neptune and Pluto
from the Earth. The value for Uranus is 5.5.


From the point of an observer on the planet, moons differ from planets
in that they are larger, brighter and cross the sky more
quickly. Moons orbit the planet rather than the sun, and are very
important because their gravitational pull helps stabilise the
planet’s axis of rotation and thus the climates. Put another way, a planet without a moon
is unlikely to have stable enough climates to develop in an Earthlike

Earth has, of course, only one Moon; there’s no reason why your
planets can’t have many more. Of all the topics in this section, this
one offers the greatest number of possibilities which are
interestingly different from the Earth.

However many moons you have, you need to know the following about each
of them:

  • Diameter, which will affect how big they appear, and
    thus whether they can cause total or annular eclipses of the
    sun and of each other. The Moon has a diameter of 3475 miles.
  • Brightness, which is similarly a function of their
    composition. It also determines how many of the fainter stars
    will be drowned out when the moon is above the horizon.
  • Distance from the planet, which affects the apparent
    size and the orbital period, which in turn affects the planet’s
  • Orbital inclination relative to the planet’s orbit,
    which will affect how far from the ecliptic (the path of
    the sun across the sky) the moon will appear in the sky. This
    in turn affects the frequency of eclipses.

And, of course, there’s the moon’s colour, which is largely a
function of what the moon is made of. Moons may be:

  • Rocky, like the Moon and several other moons in the
    Solar System. Rocky moons appear greyish and cratered.
  • Icy, like Jupiter’s Europa. Icy moons will be white and
    bright, since ice reflects light much better than rocks.
  • Volcanic, like Jupiter’s Io. These moons will be red,
    orange and yellow.
  • Gaseous, like Saturn’s Titan. Titan itself is orange,
    although most other colours are possible.

Additionally, the moons will have gravitational effects on each other,
which means that certain combinations of distances from the planet
will be impossible. The unsolvable n-body problem rears its ugly head
here again, and it’s difficult to give precise details; as an example,
though, the four main moons of Jupiter can never form a line on the
same side of the planet.

The apparent diameter of a moon (i.e. its diameter as seen from the
planet) is proportional to its actual diameter and inversely
proportional to its distance from the planet; thus a moon half the
size of the Moon and twice as far away will appear one-quarter the
apparent diameter. This is why the Sun and the Moon appear about the
same size: the Sun is roughly 400 times the diameter of the Moon, but
also about 400 times further away.

Kepler’s third law can be used to calculate the moon’s distance from
the planet given the length of the moon’s orbital period, or vice
versa. The formula here needs to be used in its full form:

G x (M + m) x T2 = 4 x pi2 x R3

in which all the quantities are used directly, rather than being
normalised; thus M is the mass of the primary body (the planet)
in kilograms, m is similarly the mass of the secondary body
(the moon), R is the distance to the secondary body in metres,
G is the universal gravitation constant (6.673 x
10-11 N m2 kg-2), and T is in
seconds. (If you’re dealing with a planet orbiting your Sun, the mass
of the Sun is large enough that the mass of the planet can be

There is a lower limit to R – it’s called Roche’s Limit
– below which the effect of the planet’s gravity will pull the moon
apart and form rings, as with Saturn. Roche’s limit equals 2.45
◊ r x (P/p)1/3, where P and p are the
densities of the planet and the moon respectively and r is the
planet’s radius. (The quantities don’t need to be normalised, since
the ratios of the densities are used, not their actual values, and
Roche’s limit is relative to the planet’s radius anyway.)

The phases of your moons depend on the relative positions of
the moons and the sun:

  • New moons occur when the moon is on the same side of the
    planet as the sun. If the moon is directly in front of the sun,
    there is a solar eclipse: total if the moon is larger in
    the sky than the sun, annular if it’s smaller. Partial eclipses
    occur if the moon does not cross the sun completely. New moons
    are always invisible.
  • First quarter occurs in the evening when the moon is at
    90 degrees to the sun and moving way from it.
  • Full moons are best seen at midnight and occur when
    the moon is opposite the sun. If the moon passes through the
    planet’s shadow, there is a lunar eclipse.
  • Last quarter moons are best seen in the morning, and
    happen when the moon is at 90 degrees from the sun and moving
    towards it.

Intermediate phases can be interpolated as desired. Crescents
occur between new moon and quarters, gibbous moons between the
quarters and full moon. Note that the further the moon is in its cycle
of phases, the later it will be at its highest in the sky; crescent
moons are rarely prominent at midnight, for instance. Because full
moons are opposite the sun in the sky, they are highest in the sky
when the sun is lowest, i.e. in the winter. A bit of thought, or
simple observation, should convince you that first quarter moons are
best seen in spring and last quarter moons in autumn.

A moon will return to the same place in the sky relative to the stars
once every orbital period, but with a different phase. The phases
repeat after a period of time different from the orbital period,
because during the moon’s orbit the planet is also moving around the
sun. If a year is y days and the moon orbits in m days,
then the phases of the moon will repeat in t days, where 1/t =
1/m – 1/y. For the Moon, m = 27.1 and y = 365.25, so p = 29.27 days –
about one month, which shouldn’t be surprising.

For complicated reasons your moons will always be tide-locked
to the planet, with their rotational periods the same as their orbital
periods. This essentially means that you will always see the same side
of the moons, as is the case with our own Moon; in other words, phases
aside, the moons will individually always appear the same throughout
their orbits.

One correspondent asked if it’s possible for moons to have sub-moons
orbiting around them. It turns out that the answer is “no”, because
this would require an impossible set of three separate
tide-locks. Note that tide-locks only occur within a certain radius;
thus the moon is tide-locked to the Earth, but the Earth isn’t to the

Tides are caused by the gravitational pull of the sun and moons
on the planet; more moons will produce more complicated tides. Here’s
a good
page explaining how tides work
. The magnitude of the tide a body
causes at a point equals:

t = D3 × P


t = magnitude of tide

D = apparent diameter of body as viewed from the point

P = density of body.

To finish with, here are some interesting phenomena of some moons in
the Solar System, which you might want to emulate:

  • Phobos, the larger moon of Mars, is so close in that it
    has an orbital period of 7.7 hours. Since a Martian day is
    about 24.5 hours, this means that, to a Martian, Phobos would
    rise in the west, race across the sky in 3.85 hours and set in
    the east, rising again 3.85 hours later and repeating the
    process just over 3 times every day. During each passage across
    the sky, it would go through half a complete cycle of phases,
    and you could even see it moving.
  • Triton, Netpune’s largest moon, has a retrograde orbit;
    i.e. it would also rise in the west and set in the east, but of
    course over a longer period of time (5.8 days) and for
    different reasons from Phobos.
  • Nereid, another moon of Neptune, has a highly
    eccentric (non-circular) orbit which takes it alternately close
    to and far away from Neptune. This would cause its appearance
    to change from very small and faint to large and bright.
  • Pluto’s moon Charon has an orbital period equal to one
    Plutonian day, and thus appears in the same place in the sky at
    all times. This is because Pluto and Charon are so close
    together that they are tidelocked.

Retrograde orbits are unstable; a satellite in one will steadily orbit
closer to its primary until it either breaks up or crashes into it.

The day

An Earth day is the length of time it takes the Sun to make one
complete journey across the sky; it is divided into, of course, 24
hours. Dividing this by 365.25 days gives 236.5 seconds, which is the
extra time added to the length of the day by the Earth orbiting the
Sun; this means that the Earth rotates on its axis in 23 hours
and 56 minutes. The Earth’s axis of rotation is inclined at an
angle of 23.5 degrees to the plane of its orbit around the Sun.
The direction of rotation is from west to east, which means
that the Sun and other celestial objects appear to move from east to

The length of the day on your planet is affected by one factor only,
the speed of the planet’s rotation about its axis; faster rotation
results in shorter days, and slower rotation causes longer days. The
speed of rotation has several other knock-on effects; for example,
faster rotation will have the following effects, which I am unable to
provide equations for all of as yet [*]:

  • Faster winds.
  • More atmospheric cells, which affect the climate. Jupiter, for example, rotates on its
    axis in less than 10 hours, and has several clearly visible
    bands in its atmosphere.
  • A greater oblateness, or flattening at the poles. The
    Earth, for example, has a polar diameter of 26 miles less than
    its equatorial diameter, an oblateness of about 1/300.
  • Some effect on the gravity; see below.
  • The nightly views of the sky will change more slowly. In
    general, the night sky presents the same appearance slightly
    earlier each night; the time difference is the “extra time”
    calculated earlier – 236.5 seconds for the Earth.
  • Since the days will be shorter, the amount of sunlight per day
    will be less, which will affect anything which needs sunlight
    to live (i.e. plants and animals).

Moreover, there’s a lower limit to the length of your day; below this
limit the planet will be spinning too fast and will thus disintegrate.
The Alien Planet
gives an equation for this. Elizabeth Viau’s online
mentions about 3 hours; this limit is, I suppose, unlikely
to be a problem in practice. One correspondent suggests “about 84
minutes for earth density; somewhat higher if you want to hold an
atmosphere”. The upper limit of the rotation speed increases with the
planet’s density, which is why neutron stars can rotate so fast.

For longer days, of course, all of these effects are reversed; Jordi
Mas informs me that there is no upper limit to the length of the
day. In particular, days which are too long will produce enough heat
from the sun to kill off certain flora and fauna.

If you want to be precise, here’s the maths. ob, the
oblateness, is:

ob = (re – rp) / re

where re is the equatorial radius and
rp is the polar radius. The upper limit for
ob is:

obmax = (5 × pi2 × r3) / (G × M x


pi is, of course, 3.14159

r is the equatorial radius of the planet in metres

G is the universal gravitational constant, 6.67 x

M is the planet’s mass in kilograms
T is the length of the planet’s day in seconds

The lower limit for ob is:

obmin = obmax × 0.315

You may want to experiment with retrograde rotation – i.e. what
happens when the planet rotates “backwards” with respect to its orbit
around the sun. Aside from making the sun and other objects move in
the opposite direction, this would mean that the night sky would
repeat its appearance slightly later each night, not earlier.

The axial inclination affects the heights above the horizon of all
heavenly bodies; the greater the angle, the greater the variation in
their positions. If the axial inclination is i degrees, at
latitude L the height of the sun above the horizon will vary
between i-L degrees and i+L degrees. Its maximum height in summer will
also be i+L, while on the shortest day its maximum height will be
L-i. The sun’s changing height has a significant effect on climate, for which see later.

Geology, or What Your Planet Is Made Of

The Earth has a diameter of nearly 8000 miles and thus a circumference
of nearly 25000 miles. Roughly 70% of its surface is covered with
water. The acceleration due to gravity, i.e. how fast things
speed up when falling freely, is 9.8 metres (32 feet) per second per
second. The mean relative density, i.e. the density relative to
that of water, is about 5.5.


The surface gravity of your planet will affect everything which moves
upon it and around it; in particular Brian Davis says that surface
gravities greater than 3 times Earth’s are “probably not long-term
survivable from a biomechanics viewpoint”.

Gravity also affects the atmosphere, but here the upper atmosphere
temperature is also important; Saturn’s moon Titan, for example, has
an atmosphere with a surface pressure 1.5 times that of the Earth. The
surface pressure of a breathable atmosphere should probably be within
0.1 and 4 times that of the Earth.

Time for some more equations. The values here, all taken relative to
the Earth, are:

  • g, the acceleration due to gravity at the surface at the
  • M, the planet’s mass.
  • R, the planet’s radius.
  • P, the planet’s density. This is determined by what the
    planet is made of.

Density is defined as the mass per unit volume, and volume is
proportional to the cube of the radius, therefore:

M = P × R3

while surface gravity is related to mass and radius thus:

g = M / R2

Eliminating M, we get:

g = P × R

In other words, a planet with a radius twice the radius of the Earth
will have to be half as dense to have the same gravity, and vice
versa. An Earth-sized planet made of polystyrene will have a relative
density of about 1, and thus a surface gravity one 5.5th that of the
Earth’s. If you were to jump on such a planet, you’d rise and fall
very slowly. You’d also probably die trying to breathe the tenuous
atmosphere, but that’s another matter.

Here’s a gravity-related mistake in a popular work of fiction, which
only a pedant like me would notice. According to Karen Wynn Fonstadt’s
excellent Atlas of Pern, which accompanies the books written by
Anne McCaffrey, ten degrees of latitude on the planet Pern equals
about 80 miles, which indicates an equatorial circumference of 80
times 36 = 2880 miles, or a radius of 917 miles – 1/8.64 that of the
Earth. Assuming that Pern has the same surface gravity as the Earth,
this indicates that Pern has a density of about 43, twice the density
of the densest known element, osmium, and thus physically
impossible. Oops!

The gravity at the poles is always greater than that at the
equator. For Earthlike planets, Jordi Mas provides the following

The variables, again normalised relative to the Earth, are:

  • P = density of planet, as above
  • T = period of rotation (i.e. length of day) in Earth
  • re = equatorial diameter
  • rp = polar diameter
  • ge = gravity at the equator
  • gp = gravity at the poles
  • K = tweak factor; see below.

The value of K depends on the composition of the planet, and
can be interpolated from this list:

  • 0.5: bodies with their mass concentrated at the centre, such as
    supergiant stars (or, interestingly, bodies with mass
    distributions like a chunk of rock surrounded by many
    expanded polystyrene balls)
  • 0.73: gas giants, such as Jupiter
  • 1.0: Earthlike planets (actually, Earth = 0.97; Mars = 1.09)
  • 1.25: planets with uniform density

Then the oblateness is given by:

ob = K × 0.00346 / (T2 × P).

If this is greater than 0.2, you have a very oblate planet for which
the following formulae are not appropriate.

The polar radius and gravity are thus:

rp = re × (1 – ob)

gp = ge × (2.5 – K) × (1 – ob)

You can also work out the shape your planet will have, although it
gets complicated! First of all, calculate its angular momentum using a
formula somewhere within this
. There are four cases to consider, based upon the momentum
relative to two values X and Y:

  • Zero momentum: the planet is spherical. This only happens if
    the planet isn’t rotating.
  • Momentum less than X: the planet is an ellipsoid with the two
    greater axes equal, and will rotate around the shortest
    axis. The Earth is an example of this.
  • Momentum greater than X and less than Y: an ellipsoid with
    three different axes. The relative sizes of the axes depend on
    the momentum.
  • Greater than Y: the mass will split in two egg-shaped pieces
    called Roche lobes.

So you can work out the upper limit for the rotation speed, and you
can calculate the planet’s shape as a function of its rotation speed
if the density is uniform.

Surface composition

This heading refers to the proportion of the planet’s surface which is
covered by water. This affects quite a number of factors, such as climate and culture; a planet
with no water at all, such as Frank Herbert’s classic Dune,
will consist entirely of desert, since it won’t rain. Going to the
opposite extreme, a planet whose surface is almost entirely water will
have very small continents and very little opportunities for different
cultures to advance by sharing ideas. (Think of Polynesia, for

The proportion of water to land on a planet’s surface affects the
carbonate-silicate cycle. Too much or too little water will
cause this cycle to be unstable, which in turn will decrease the
likelihood of a stable climate over geologic time, and thus the
likelihood of Earthlike life.

Plate tectonics come into play here, too, although you don’t need to
worry about them too much; if you’re interested, Wikipedia’s
is very good. The areas where two plates meet are highly
likely to feature mountain ranges (e.g. the Himalayas or Andes),
volcanoes (the Mediterranean, Japan) and earthquakes (California). A
correspondent points out that: “plate tectonics has one result worth
remembering: you can only get high mountains on one side of a
continent, since the newest mountains will be on the ‘leading edge’:
compare the Rockies and the Appalachians”.


Geology also encompasses the actual elements, metals, and minerals
which may be mined in your planet. Three useful links here are A Short
History of Metals
, this list
of essays
, and Zompist’s History of Chemistry. As
Zompist points out, biology may differ, but chemistry is the same

The Map, or What Your World Looks Like

The Map is the most important element in the creation of your
world; it tells you, and your readers, where everything is in relation
to everything else. Opinions differ widely concerning how much freedom
you have in designing your Map; at one extreme is “anything you do can
be explained in some way”, while the other has “things can only happen
in a limited number of ways”. The best compromise seems to be “you can
do what you want as long as it’s explicable and not too

If you haven’t already done so, decide on a scale, so you know
the size of the area the Map is supposed to represent. Start with your
coastlines and the neighbouring islands, if you have
them; offshore islands are usually formed by the same processes as the
nearby coast, and so should have roughly similar-looking

Next, draw your mountains and rivers. Unless you’ve got
a good reason to do otherwise, mountains form irregular parallel
chains, and are often continued offshore as islands. And don’t forget
that rivers always start high and flow downhill; and that most
rivers are created by rainfall, which is highest on the windward sides
of mountains.

For supplementary reading, author Holly Lisle has a workshop
about mapmaking
; it’s oriented towards expediency rather than
scientific accuracy and rigour, but might be useful if you’re in a
hurry. Take a look at her own Map, too. Mark
Rosenfelder has created some lovely Maps for Virtual Verduria, with
instructions for
drawing Maps of similar quality; if this seems too much like like
work, have a look at my 3d mapping

Once you’ve decided on the shapes of the land and sea, virtually
everything else on your Map is dictated by the climate. Climates are affected by both large-scale
and small-scale factors, for which reason it’s probably a good idea to
establish what the major land masses and seas are in the areas
adjacent to your Map.

Climate, or What Weather You Should

“Climate is what we expect; weather is what we get.” – unknown wit.

The climate of an area is defined as the weather conditions
experienced by the area averaged over a long period of time; it is
most conveniently described in terms of the yearly amount and patterns
of two important and easily-observed factors, rainfall and
temperature. These factors dictate the plants which grow, and
in turn the animals which are found; these factors influence what kind
of human cultures develop in the area. A desert society will be very
different from one which inhabits a region with a cool temperate
climate, for example.

Establishing your climates

Climates don’t occur at random, but can be predicted from a variety of
factors. My Climate Cookbook provides a
step-by-step guide to working out your climates; to learn about the
theory in more detail, have a look at this
good online course
, especially chapter 7, section (o) onwards; the
most relevant pages are the ones about the global
circulation of the atmosphere
and climate

Effects of climate on the land

One obvious way in which the climate affects your Map is the
rivers. Rivers which flow through areas with seasonal rainfall
will be much higher in wet seasons than in dry seasons; a river which
flows through a savanna climate, for example, will be low in the
winter and high in the summmer, giving rise to the possibility of
seasonal flooding. Exactly this happens with the Blue Nile.

By contrast, because there is so little rain in dry climates, rivers
in areas with such climates will have formed elsewhere. In general,
too, rivers are less frequent on the drier leeward sides of

Local Winds

Local winds are those which depend on a particular set of geographic
circumstances, and it’s a good idea to be aware if your landscape will
create any. For example, parts of the south of France are subject to
the Mistral, a cold wind which is caused by cold air “spilling”
off the nearby Massif Central and Alps in winter and is drawn south by
low-pressure areas above the Meditterranean Sea. Similarly, wet winds
from the Pacific Ocean shed their moisture on the Rocky Mountains and
heat up as they descend to the Great Plains, creating the hot dry

Other factors

Thanks to Aidan Elliott-McCrea (website), who let me quote his response
to a question on the ZBB asking how
to create a planet with small seasonal variation in climate.

Small axial tilt will reduce seasonal variation in any one place by
reducing the variation in how direct the solar input is at each
place. But, that will increase the variation in the total solar input
at different latitudes, so you’ll have a stronger temperature
gradient: the difference in temperature between the equator and the
poles will be greater. Temperature gradient is what drives weather,
and you’ll get stronger winds and other weather effects (all other
things being equal).

Large tilt will have the opposite effect. The seasonal variation will
be strong as a given latitude experiences a wider range of solar
input, but this will also tend to spread heat more evenly across the
planet, and reduce the temperature gradient, and make a calmer
atmosphere (all other things being equal).

The secret to fine tuned control of your planetary weather is ocean
currents. If you want small temporal and latitudinal
temperature variation, you need strong North-South ocean currents
working to distribute heat, and weak East-West ones that cannot
effectively accumulate heat differentials.

Your planet should avoid long stretches of ocean where currents can
flow East-West without running into land, especially near the equator
and the poles. A major cause of instability in our current atmosphere
is the West Wind Drift, the ocean current that circles Antarctica; it
is almost completely flat, and precludes north-south transfer of
heat. Also, you want lots of warm water moving around, avoid very
large patches of unvaryingly warm water, which are hurricane breeding

Your planet should also avoid any very large land areas (like
Asia). Land changes temperature faster than water; not actually
because of the specific heat of water, but because in water, the heat
gets distributed through a larger vertical section, whereas in land it
all tends to accumulate in the top few centimeters. So large patches
of land will create large areas of differential heating.

Flora and Fauna, or Your Plants And

Regions with similar flora and fauna constitute a biome, and
biomes correspond more or less with Kˆppen climate areas. More
details about the fauna associated with particular biomes on Earth may
be found at the World
page. Major
Biomes of the World
may also be of interest.

Islands feature species typical of their climatic regions, but
in fewer numbers and often with idiosyncratic species. The absence of
snakes in Ireland is due not to Saint Patrick, but to the simple facts
that snakes don’t cross water and didn’t reach Ireland before it
became an island. Moles and woodpeckers are other species which are
absent from Ireland for the same reason; and distinctively Antipodean
birds such as the cassowary, emu and kiwi evolved in isolation from
those in the rest of the world.


You don’t find cactuses growing halfway up a mountain in a snow
climate, nor do you get vast coniferous forests in the middle of a
desert. The principle here is simple: the plants (and animals) which
would flourish in any given region on your planet will be similar to
those from a similar climatic region on Earth.

Taking as a starting point the obvious fact that plants need water to
grow, some useful generalisations follow. Most importantly, rainy
climates will support many more species of plants and animals than dry
climates; compare a rainforest to a desert and you’ll get the idea.

Plants which grow in dry climates will develop to conserve precious
water; this is why cacti are thick-skinned and why cork oak grows its
thick spongy bark, for example. This fact also explains why conifers
have smaller leaves than broadleaved trees, since small leaves lose
less water through evaporation.

Coniferous trees are good examples of plants adapting to their
climate for other reasons: their conical shapes allow heavy snowfalls
to slide off onto the ground, and their strong branches are able to
support the snow which remains. Their leaves, besides being small to
conserve water, are also dark to absorb as much of the Sun’s light as
possible; sunlight is in much shorter supply in the cold climates in
which conifers grow compared to the more temperate climates which
support broadleaved forests.

Less obvious is the effect of landscape on the variety of plant
species. North America has a much greater variety of tree species than
Europe for two principal reasons: the orientation of the mountain
ranges, and the effects of past Ice Ages. Essentially, as the ice
encroached southwards during the Ice Ages, the trees in North America
were able to retreat before the ice since the north-south mountains
provided no real barrier; by contrast, in Europe the east-west
mountains (the Alps and Carpathians) prevented all but the most hardy
species from retreating southwards, with the Mediterranean Sea sealing
the gaps.


This can be used to explain, in somewhat simplified form, the animals
which will inhabit a particular biome. Obviously, biomes with a
greater diversity of plant species will support a greater variety of
animal species; the rainforests are the most diverse biomes on the
Earth, for example. At the other extreme, relatively few types of
animal may be found in moorland.

Like plants, animals adapt to their environment. A particularly
striking example of this may be found in snow climates (e.g. subarctic
and humid continental), in which snow lies on the ground for periods
of several months at a time; animals in these climates, such as the
snow hare, ptarmigan and Arctic Fox, typically turn white in winter
for camouflage. Animals in cold climates also evolve ways of retaining
heat; seals and polar bears, for example, have layers of fat for this

Another good example is the fauna of the savannah climate. Here there
are vast grasslands punctuated with occasional trees which have
adapted to store water throughout the long dry season (when the
grasslands turn to semi-desert), such as the bottle-shaped baobob
tree. The grasslands support large numbers of herd animals such as
gnu, impala, wildebeest and so on; in turn these herds support
carnivores such as lions, cheetah and leopards. The wide open spaces
allow the herd species and predators to evolve the ability to run fast
to outrun each other. The huge herds of buffalo of the the American
Great Plains lived there for similar reasons.


Individual human cultures, like the local flora and fauna, are shaped
in large part by their environment. Consider what would happen in a
cold climate: the inhabitants need to have lots of layers of clothing
to keep warm, so the hunting of the appropriate furry animals (hares,
bears, wolves) constitutes a major part of their lives. Conversely,
desert cultures may paint their dwellings white to reflect the sun’s
heat and keep the interiors cool.

Cultures develop and evolve by interacting with other cultures and
borrowing their ideas and inventions. This implies that cultures
living in isolated regions, such as in mountainous areas or on
islands, won’t develop at the same speed as those on large flat
plains. Plains are also easier to conquer and integrate into single
cultural units; this explains not only why mountains make good natural
borders, but also why there are only three countries in North America
but over forty in Europe.

A very good read about the development of human cultures is Guns,
Germs and Steel
by Jared Diamond, which sets out to answer the
question of why European cultures came to dominate the world,
overtaking those of China and the Middle East. To simplify the book’s
main thrust somewhat drastically, the reason is ultimately down to the
east-west orientation of Eurasia compared to the north-south
orientations of Africa and the Americas, which provided Eurasia with
much more land in temparate latitudes than any other landmass. This
large amount of land greatly facilitated east-west diffusion of
cultural developments, since little adjustment to different
environments was necessary. By contrast, the diffusion of cultures
through Africa and the Americas was hindered by the presence of
deserts, dense rainforests, and the narrow mountainous land-bridges of
Central America.

According to Diamond, China was eventually overtaken culturally by
Europe because the more mountainous regions of Europe resisted
homogenisation and preserved many competing cultures, which developed
and, from a few centuries ago, exchanged ideas and inventions at a
faster rate than in China. In particular, one reason why the
Industrial Revolution began in Britain was that Britain was able to
exchange cultural ideas with mainland Europe but was not hampered by
wars on its soil. By contrast, it was easy for one culture to conquer
the plains of China; this monolithic culture was not conductive to
development at the same speed.

Another effect of the interaction of cultures in Europe was that
resistances to diseases were spread quickly among the various
peoples. The more isolated peoples in Central America did not share
the same resistances; as a result, when the Spanish arrived in Central
America, the native peoples suffered as much from European diseases as
from their superior warcraft.

It’s useful to know the populations of the places on the
map. In general, the population density – the number of people in a
given area – depends primarily on the quality of the soil and the
level of farming technology; good soils in areas of reliable rainfall
which can be ploughed with horse-drawn ploughs are likely to support
much higher population densities than arid areas of steppe. Another
factor is the security of the area – people don’t generally tend to
live in areas of land which are regularly ravaged by war. This page should help
you calculate population densities; it’s geared towards mediaeval
societies and RPGs, but the basic principles should still be valid.

Language, or How To Name Things

Two pages which are once again required reading are Words
Maketh the Culture
by Cheryl Morgan, and What’s
in a Name?
[Extreme pedantry: that should be “Makath” or just
“Make”; “Maketh” is an older form of the singular, “Makes”. Or you
could try “The Word Maketh…”]


Perhaps more than any other single factor, the names of the places on
your Map create a lot of its flavour and atmosphere. Consider for
example the different moods conjured up by the following lists of
place-names from various parts of Great Britain:

  • Abertawe, Llanwrtid, Betws-y-coed, Ynys MÙn
  • Littlehampton, Much Wenlock, Leighton Buzzard, Newport Pagnell
  • Satterthwaite, Kirbyunderdale, Copmanthorpe, Thirkleby
  • Auchtermuchty, Kilravock, Glenkindie, Abernethy
  • Nancledra, Tregavarah, Penderleath, Carharrack
  • Garthamlock, Ruchazie, Polmadie, Cowcaddens

or these, from assorted European countries:

  • Kortenberg, Hasselt, Nederokkerzeel, Sterrebeek
  • La Spezia, Palermo, Napoli, Brindisi
  • Gdansk, Wroclaw, Szczecin, Warszawa
  • Tampere, Oulu, Viipuri, Helsinki

or, from various fictional worlds:

  • Pelargir, Calembel, Ithilien, Emyn Arnen
  • Pyrdon, Auddglyn, Dun Deverry, Aver Peddroloc
  • Gethruva, Malottsa, Surrunguz, Hebrytcest
  • Hwitbaurg, Klenam uftra, Stainxaim, Licadal
  • Samhuomi, AzurÎmesti, Beluvaco, TrosesuÎ

Good karma and plenty of kudos to anyone who can correctly identify
all fifteen sources 🙂

Place-names generally derive from local geographical features; for
example Abertawe is “mouth of the river Tawe”, and
Sterrebeek means “stream of stars”. Sometimes the names remain
more or less unchanged down the centuries, as with these two. Other
names change to varying extents, as with Dunfermline, which
comes from the Gaelic dun fearum linn (I’m not sure of the
spelling), which means “fort by the crooked stream”, and York,
which results from various types of phonetic change affecting the
original Latin Eburacum.

Names of rivers tend to be particularly conservative; the RhÙne
in France, for example, has had the same name (subject to linguistic
changes; the Romans knew it as the Rhodanum) since at least pre-Roman

Language creation

You can go a long way with just English; if you have a river called –
say – the Foo, a town where it meets the sea could be called
Foomouth. To add a bit of spice to the nomenclature of your
world, however, there’s nothing to beat making up a language.

If all you want from such a language is a way of naming places, you
can get away with:

  • a list of naming elements: “hill”, “stream”, “pool”,
    “valley”, “king”, “chief” and so on;
  • a compounding rule which specifies whether modifying
    words precede the words they modify, as with Newport, or
    follow, as with Abertawe.

You can complicate matters a bit by adding another rule which
specifies how the individual elements change when combined; for
example Penybontfawr in Wales (“the head of the large bridge”)
comes from pen + y + pont + mawr. Of course, if you start down
this road, before you know it you’ll be creating a grammar and syntax
for your language and writing epic prose in it; if you get caught up
in this, the Language
Construction Kit
will be invaluable.


For enhanced realism, remember that languages are rarely spoken
uniformly; in any reasonably-sized area there’ll be differences in
pronunciation and meaning, and you’ll add a lot to your world by
allowing naming elements to take different forms in different
areas. Sterrebeek, for example, is the name of a village in
Belgium; its pronunciation in one village reflects its spelling,
approximately “stare-uh-bake”, but in a nearby village has been
reduced to something like “stare-beck”.

The mixture of names in any given part of your world reflects the
cultures which have lived and fought there. The Great British names
above, for example, come from areas settled by Anglo-Saxons, Vikings,
P-Celts and Q-Celts.

Other pages

In no particular order, here are some links to sites too general to
fit under individual categories.

Designing a
Fantasy World
, from everything2, is a very fine
essay which covers many points I’ve skimped on above.

The Worlds
in the Net
site. This contains a list of useful
about world-building, of which Jesper
Udsen’s experience
of designing a world and Rich
Staats’ essay
are particularly good. Hunting around this site
turns up plenty of other goodies, too.

Mark Rosenfelder’s Virtual Verduria is an
impressive constructed world, complete with attractive Maps and large
amounts of absorbing detail.

Web Blackdragon is an online
role-playing IRC channel. Don’t miss the lovely Map.

Wrede’s Worldbuilder Questions
are useful pointers to things to
think about.

Elizabeth Viau has an interesting online
course in world-building
, which contains plenty of
scientific notes
, although they aren’t complete yet. The course is
about creating planets in general, not just Earthlike ones.

The Alien Planet

The Nocturne Research
world-building website
has archives with lots of interesting
material, some of it relevant to the topics in this page.

MythoPoet’s Manual
. Very good for culture and religion.

Epona, a planet in a
constructed solar system. Very interesting and well thought out.

Here’s a Quick’n’Dirty
about science-related topics, some of which are relevant.

Occasionally you stumble across lecture notes for university courses
which contain material of interest; here’s
a set
about geography.

Not related to world-building, but rather to writing, are Holly
Lisle’s Forward Motion pages,
and the very funny “What I Would Do If” lists on Chicken Soup for the Gamer’s

Matthew White’s website
contains a lot of amusing and stimulating material, some of which
(such as Climate in
Mediaeval America
) is of particular interest to
world-builders. Other bits of it are barking mad, but good fun.

International recognition! Teresa Costa translated some of the
astronomy section into Portuguese for her own world-building pages,
which contain much else of value and interest.

Two pages of useful writer’s resources are this
one (may be defunct)
and Creating
Fantasy Worlds
by Paul Nattress. Both have many further links, and
also speak very highly of this page too :-). One link particularly
worth looking at is How to create a
fantasy world
, by the Australian author Sara Douglass.

takes the Earth as a basic model and extrapolates the consequences of
various simple but drastic changes, to quote the ZBB member who
mentioned the page.


  • Principal credit goes to Mark Rosenfelder, whose mention of a
    “World Construction Kit” during email exchanges planted the
    seed from which this page has grown.
  • My brother Iain, my cousin Bruce and my good friend Alasdair
    Swanson have all provided useful input at various times.
  • Madelaine Chapman provided me with Boltzmann’s Law, which
    appeared in an erroneous set of equations in a previous
  • Jordi Mas sent me lots of information about planetary physics,
    much of which has been incorporated.
  • Brian Davis pointed out some mistakes and provided plenty more
    data; he asks that I also credit
  • Grzegorz Gacek pointed out a couple of mistakes in my maths and
    corrected the spelling of “Szczecin”.
  • Jeremy Hussell provided some useful information, especially
    about the importance of star masses.

Thanks too to everyone who sent in information but didn’t want their
name to appear here.

3 Responses to Creating an Earthlike Planet by Geoff Eddy

  1. bastendorfevergrass says:

    There is a metric ton of broken links in this article…

    • Astrographer says:

      True. It’s a very old article. It was already pretty old when I started my blog, and some of my older posts are displaying linkrot(which… I really need to go back and fix). In fact, the reason I posted Geoff Eddy’s work here(with his permission) is because it had already disappeared off of the net more than once. This is Geoff Eddy’s work, and I don’t feel right replacing links without his input. I should at least add notation about dead links with possible alternate sources where I’m sure about what the original link contained(in green text, of course, to differentiate my vandalism from his original work), but at the rate I’m doing things here, that could be awhile. I do need to go back and try fixing the dead links on my own old posts, but even that will take time. For my own part, I still find this frequently useful, though I find his Climate Cookbook to be rather more uniquely valuable.

  2. Pingback: Favorite Worldbuilding Resources

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