Vision: A Resource for Writers
Lazette Gifford, Editor
Vision@sff.net

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 seasons reversed.

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...