Last week I talked about a world for my next novel, where the planet’s axis is tilted at a full 90 degrees to the plane of its orbit. In other words, if you picture the sun in the middle of your living room and the planet’s orbit traced out on the floor around it, it’s rolling around on its side rather than spinning upright like a top.
When I first envisaged this, I wondered if that meant one pole would always be pointing toward the sun. I quickly realized I’d fallen into a popular misconception. A bit more thought showed that this just ain’t gonna happen. Simple physics doesn’t allow it.
Yes, some worlds do have one side always facing their sun, but their axis is the more normal perpendicular (or near enough) to the plane of the orbit.
The perpetual day/perpetual night arises if the planet’s day (one rotation on its axis) is exactly the same length as its year (one full orbit of its sun).
This may sound like one heck of a coincidence, but tidal forces between two orbiting bodies act as a braking mechanism, slowly bringing rotational and orbital periods into line. This is called “tidal locking”. A familiar example of this phenomenon is our own moon, which always shows the same face to the Earth. For many years, astronomers thought Mercury was tidally locked to the sun, although that’s now known not to be true.
Back to our topsy turvy world. Although it’s spinning on its side and swinging around its sun, if you were watching it from somewhere outside the solar system you’d see that the planet’s axis is always pointing the same way relative to the universe as a whole.
The whole planet is acting like a huge gyro compass. Again, this sounds bizarre and far-fetched, but our own Uranus is doing exactly this.
So, if anyone describing a sci-fi world proudly proclaims that their north pole always points at the sun, you can set them straight.