Earth’s inclination causes our seasons. It would be hard to find a more ordinary fact, or one less valued. Yet everything about the Earth derives from that inclination, even our religions and our philosophy . . .

Those are the words of Gus Leinhoff from the upcoming novel Cyan.

I like axial tilt as a means of individuating planets, so much so that I have run the bases, hitting all the possible extremes. Cyan, the planet from the novel coming out in January, has no axial tilt and no seasons; Stormking, around Sirius in the as yet unwritten sequel Dreamsinger, lies back at a Uranian inclination and has seasons you wouldn’t believe. Harmony, from the novel Jandrax has a tilt of 32 degrees resulting in heavy glaciation with a narrow habitable band around the equator; it has two summers and two winters each year.

So does Earth – at the equator.

I’ll bet you didn’t know that. One of the great pleasures of world building is finding things you should have realized, but missed. This is one of them.

Let’s imagine the changing tilt of the Earth as the seasons progress. Of course I know the tilt doesn’t change; it only appears to do so from an earthbound perspective. But twenty-seven years of teaching science to middle schoolers has taught me that casual language gets the message across better than an excess of formality.

Today is the equinox, autumnal in San Francisco, vernal in Sydney. The sun lies above the equator at noon, and will (seem to) move southward in the coming weeks. I won’t waste your time telling you what you already know, but consider what you know from a new perspective.

Today at the equator the sun is overhead (call it summer) and for the next three months it will move southward until it gets as low and ineffective as it will ever be (the equivalent of winter), then it will come north for three months until it is overhead again (summer), and continue northward to its other lowest position (winter again), and so forth. Two “summers”; two “winters”.

Earth’s dual seasonality is masked by local conditions, at least in its oceanic regions. The world in the novel Jandrax has a stronger tilt and its oceans are tied up in glaciation. The refugees naturally settle at the equator, where summer and winter really do come twice a year.