Vision: A Resource for Writers
Holly Lisle's Vision
Axial tilt and things
By Bob Billing
2002, By Bob Billing
Axial tilt for a world is actually very easy to work
out. Your imaginary planet has an axial tilt, which is a number that you can
choose to suit the story. Think of the planet as going around the sun in a big,
flat ellipse. Then think of the axis about which the planet rotates. The axial
tilt is the angle between the axis of rotation of the planet and an imaginary
line sticking straight up out of the plane of the orbit.
Now imagine that you are standing at the North Pole.
The sun runs around the horizon clockwise every day and at the same time it bobs
up and down every year. At midsummer it is above the horizon by a certain angle,
which is exactly the same as the axial tilt. At midwinter it is below the
horizon by the same amount. At the equinoxes it is exactly on the horizon. At
the South Pole the same thing happens but the seasons are reversed and the sun
runs around the horizon in the opposite direction.
At the equator the sun goes more or less overhead
every day, but the path it takes through the sky swings right and left by the
same angle - the axial tilt - every year.
You can now add arctic and Antarctic circles. These
are separated from the poles by - you've guessed it - the axial tilt, the same
angle again. Within these circles you get both midnight sun and continuous
darkness at different times of the year. The further you go towards the pole the
longer the midnight sun and continuous darkness phases until at the pole the sun
rises and sets once per year. In this area you could get - if you choose the
right numbers - five months of darkness, a month of lengthening days, five
months of continuous daylight and a month of shortening days. The sun would
never get very high above the horizon so it would be perishing cold.
Now add a moon. This will orbit the planet in another
ellipse, with its own tilt. If the moon orbits the planet in the same plane as
the planet orbits the sun it'll be lower and lower on the horizon as you get
closer to the poles, behaving more or less like the sun. But if you tip up the
moon's orbit so that it orbits in a different plane it'll move between the
northern and southern hemispheres. This won't be quite at the rate of once a
month; in fact it'll either do one more north-south cycle or one less than it
does cycles through its phases in a year.
The easiest way to understand this is to get a globe
of the world and an electric torch (flashlight). Put the globe on a table, turn
off the lights and rotate the globe - this will give you the daily cycle. Now
walk around the table carrying the torch and you'll see the yearly cycle.
What would happen on a planet with a 90 degree axial tilt - in other words, a planet with the axis of rotation pointed through its sun?
Imagine you are at the South Pole in December (I'll
use our months to illustrate the point, even though our calendar wouldn't really
apply). The sun would be the South Pole star and would hang motionless overhead.
Shadows wouldn't move at all, so the sunlight would burn what are effectively
photographic images of them into the vegetation.
Then as the year advanced the sun would begin to do
small circles in the sky, slowly spiralling down to the horizon. Around the end
of March it would begin to graze the horizon all the way around (rather like
Earth's midnight sun). Allowing for variations in ground level it would then
flicker on and off for several days. Most of April would be twilight, and then
the stars would come out and stay visible until about the middle of August when
the twilight would start again, followed by a sunrise in late September. The sun
would then spiral up the sky again, coming to rest as the pole star in December.
At the north pole the same thing would happen with the
At the equator in December the sun would hang on the
southern horizon, never quite setting due to refraction. Then it would begin to
spiral outwards clockwise until it began to rise and set. It would rise higher
every day until late March, when it would pass directly overhead. It would then
move north until late September when it would spiral anticlockwise to a halt on
the northern horizon. It would then hang motionless for a while before
spiralling outwards again.
At the equator the sky would be light all day from
November to mid-February and from May to the middle of August, stars only being
visible outside these periods.
I'd suspect that the seasonal temperature changes would make life outside a narrow band around the equator very difficult. However given a natural heat source, such as a volcano, it might be possible to survive the dark winter. The limited habitable area could lead to population pressure problems. I can see dissidents being driven out to die in the temperature changes, then building a civilisation around a heat source. When the heat source begins to fail...