I’m lying awake in bed, unable to get to sleep. It’s those graveyard hours, the wee hours after midnight when the veil between worlds grows thin. Sharp, blood-encrusted claws of demons are scratching at the window and a long, pale arm, bony fingers tipped with cracked, yellow nails, reaches out from under my bed.

Cold sweat beads my brow. Why I didn’t pick an easier world to build than Hollow?

A hollow world sounds great, right? It’s unusual, interesting and fits nicely with being a world nobody can ever leave.

In my head, it’s almost complete. The inside surface, on which everyone lives, is a little bigger than Earth’s surface, and it has seas, continents, islands, mountains, plains and… well, you know, everything that will make a great backdrop to set my stories in: things like deserts, plains, jungles, forests, tundra, glaciers and all that icy stuff we have at the poles.
All Hollow needs is a tiny sun located in the dead-centre of the hollow globe, and there you are: all the light and warmth a world needs.

But by then, the demons have opened a window, and they’re in the room. One, slightly bigger than the others, says, “How will night happen in your hollow world? The sun would be hanging there in the centre shining all hours.”

I think a bit and answer, “It could switch off at night. You know, like a light bulb.”

“Then… it would be night all over the world at the same time?”

“Yes.”

The demon smiles in triumph. The other demons behind him chortle. They wink, and their glowing yellow eyes are like tiny suns switching on and off.

“Ha! Your world would freeze every night.” He shakes his head, and his necklace of finger-bones rattles like a hundred death-knells. “And even though the sun switches on again the next morning, by the time your world thaws, it will be night again. Doomed to eternal ice.” A ball of smoke puffs from his mouth and fades into nothingness. “So much for your ideas.”

“But… What if…?” My voice trails to a halt.

He was right. Half the world needed to be day while the other half was night. But how?

The pale arm reaching up from under the bed is groping over the covers towards me. I slap it and it stops moving. I get the feeling it’s watching me.

“Ah!” I smack my fist into my palm. “Only half the sun will turn off at a time.”

“Don’t be stupid.” The demon rolled his eyes. “You can’t turn off half a sun.”

“Oh yes, I can. Technically, the sun won’t actually turn off. There’s a shield curved around half of it. The shield rotates slowly so that darkness – nighttime – moves around the world. One rotation every twenty-four hours. Half the world will be in light and warmth while the other half is in darkness.”

I hold my breath while I watch the demon. His pupils narrow to slits, like a cat’s.

His lips peel back from his fangs. “I thought you wanted glaciers and frozen north and south poles. The sun will pour the same amount of heat across every inch of whichever part is in daylight.”

“Well, obviously there’s magic,” I mumble. “Magic that sucks the heat out of some areas… Freezes them. That sort of thing.”

“You poor fool.” The demon makes no effort to hide his glee. “Your readers aren’t going to trust you if you merely throw magic at every problem to solve it. Admit it. You’ve failed.”

Pale under-the-bed-fingers wrap around my wrist. I yank away my arm.

A random spark of inspiration from the cosmos pings into my brain.

“The sun won’t be round. It will be rod-shaped… like one of those tubes from a fluorescent light, but shorter. The regions directly under the sun will get the full light and warmth of the sun. A person standing there looking at the sun will see it fully, but if they travel to the side, the further they go, the shorter the sun will appear to them, and the less light and heat they’ll experience. It’s like if you look at a baseball bat from the side, you see the whole thing, but if you hold it up with one of its ends towards you, all you see is a tiny part of it, a disc.”

The demon looks flummoxed for a second, then his eyebrows lift.

Before he can speak, I say, “Don’t ask about seasons. The rod-sun solves that problem too. If it tilts a few degrees one way, then the other way over the course of a year, it will bring different amounts of warmth to each hemisphere in turn. So when it’s winter in one hemisphere, it will be summer in the other!”

I’ve finally got it. It’s my turn to gloat.

The demon leans forward and seizes my throat. His claws sink into my skin. “Nobody will believe it, idiot!”

I wake up with my head under my pillow. Blurry eyed, I reach for my laptop and begin to type.