This is more a list of simple mistakes than things I think should be there. I don’t mind worlds with unusual numbers of moons (or suns), or neat star-patterns. But since the astronomy in many books is apparently supposed to be similar to Earth’s, it irritates me when these things happen.

1. Make sure you keep track of the moon phases.

I’ve read several books in which it was apparently full moon for months or even years. You’d think someone would notice and start proclaiming the end of the world, but no one seems to.

If you aren’t sure how to calculate moon phases, you might want to borrow them from an Earth calendar. (This is how Tolkien calculated the moon phases in LOTR, using the calendar for the years he was writing the first draft). And then keep careful track of time. If your book begins on a full-moon day and the next time you mention the moon is a week later, it shouldn’t be either still full or completely new, unless you’ve already established that your world runs on different astronomical rules. If you don’t mention the exact time, but it’s safe to assume that several days have passed, then descriptive phrases like “a sliver of a moon” will move you safely in the right direction. Since the phases of the moon are so widely known, this is not just another hole through which the reality of your fantasy world can pour if you let it; it’s more like a gaping wound.

2. If you have multiple moons, their phases should be the same—unless you also have multiple suns.

Say you have a world with two moons. For one to be full and one to be in its first quarter, they would have to be reflecting the light from different suns. This could be possible, but then you would have to move the world firmly out of Earth’s astronomical rules and in the direction of a completely different set of them, which isn’t something that most authors care to do. If your moons are the same phase, then you can still have the neat different things like moons of other colors than white, which are possible in our own solar system, without sacrificing suspension of disbelief.

3. Don’t assume that the moon rises at the same time every night.

It doesn’t. The full moon often rises close to sunset in our own world, for example, and the new moon near midnight. This needn’t happen in your own world, particularly if you have some reason for making the moon rise at different times, or you want multiple moonrises. Once you have a schedule, however, stick with it. One particular amateur fantasy I read had the moon rising at midnight every single time, regardless of season or phase. This seemed to be more for the convenience of the heroes, who wanted to tell time easily, than anything else.

This is another case of remembering that parts of the fantasy world exist outside your heroes’ orbit. If you would shrink at the thought of bending laws and magical rules for them, just remember not to do it with astronomy.

4. Keep track of the weather.

This may seem fairly obvious, but without your keeping track of it, it’s amazingly easy to state in one paragraph that it’s cloudy and spitting rain, and then a few lines down that your heroes can somehow see the moons and stars. (I know. I’ve done it myself). If it’s rainy, sleeting, or snowy, your heroes’ first concern should be getting under shelter, not stargazing. And they won’t be observing any amazing astronomical phenomena that night, either.

If you have a comet, a lunar eclipse, or something else of great significance, have it occur on a clear night. And if it’s something that will repeat, like a comet that’s visible in the sky over a period of several days or months, then remember that your heroes, though they may often look for it, won’t be able to see it all the time (unless they happen to live in a desert).

5. Solstices and equinoxes aren’t just to mark celebrations; they also mark the highest, lowest, and equal points of the sun’s light.

No long days in winter, and no short days in summer. Of course, your characters might live in your world’s southern hemisphere, but as long as you’re still following rules roughly equal to Earth’s, that doesn’t excuse them from still having a procession of seasons and variations of sunlight. Closer to the equator, the change of the seasons is less noticeable, and the weather more constant, but when it’s a solstice, day and night still shouldn’t be of equal length.

This is something to keep in mind when you’re choosing a season in which to set a book. Often I read stories that could have worked just as well in a different season, or ones where I think the author chose the season primarily for picturesque effect. Try to make the season an integral part of your story, and consider what it really means for it to be there. Setting forth in winter may give you all that pretty snow swirling around, but it will also mean colder weather, extremely short and pale days, and long cold nights.

6. Keep time zones in mind.

A character who lives not far from your world’s North Pole will not see the sun rise at the same time as a character who lives in the middle of your world’s southern hemisphere. In the case of a fantasy where the plot relies on speed, it might be tempting to have two groups of characters in two wildly different places racing against the clock, but if they really need to reach the same point at the same time, one will have to have a deadline that’s a few hours ahead of or behind the other one. If they both need to perform spells at sunrise, then the group who’s behind the other will have a little longer than the first group will.

Of course, this doesn’t need to be so if your world isn’t an exact copy of Earth, and has a different pattern of orbit for the sun, a different size, or no twenty-four-hour day. Again, though, this is something that needs to be established early on and kept track of. Steven Brust, whose world Draegara has thirty hours in a day and 289 days in a year, keeps mentioning it so that his readers won’t forget it and start thinking this is Earth. If you create a different system, don’t expect your readers to intuitively understand it, or excuse everything that seems to be a mistake because of it.

7. Don’t use familiar constellations unless your world really is Earth.

Tolkien could get away with doing this because Middle-earth is supposedly our own world at some point in the distant past. Alternative histories or alternate futures can do the same thing. But other times fantasists simply adopt constellations such as Ursa Major, names such as Polaris, or legends such as that of Andromeda, even though their worlds are quite definitely not Earth.

This seems to be simple need to fill in the background without taking the time to create a wholly separate background properly. Your world can still have constellations, but try to come up with unique names, formations, and legends to explain yours. If you use the familiar ones, your readers will often start looking for connections with Earth that you might not want to suggest, especially if your world is supposed to be in a completely separate universe.

Alternatively, your world might have very few stars visible in the sky, for whatever reason, or constellations might have many different names in many different places, without a familiar and well-established system. Or perhaps your people simply don’t pay much attention to the stars. If you don’t feel like creating a constellation system, this last might be the best course. Better to do that than to carefully create a world that can stand on its own, and then slap the Little Dipper in its sky.

8. Consider how the heavenly bodies influence your calendar.

If your calendar is strictly lunar, it would make sense to celebrate the phases of the moon, but no sense to still have twelve months when there are thirteen turns of the moon in a year. If your calendar is strictly solar, it would make sense to count time from some memorable event of the sun—perhaps days run from sunrise to sunrise—but no sense to decree a month over when the moon turned from full to full. If you want a calendar that counts time based on one way and yet has celebrations of a different kind, or even another method of counting time, mixed into it, try to justify this by your world’s history. It might even be fun to have multiple ways of counting time, and show how the two different peoples who use them get confused when they come into conflict.

Also remember that counting of time won’t be as precise as it is in our modern world (unless you have a highly mathematical society, such as the Mayans in our own world were). Because the pace of life is also probably slower, your people don’t have to worry as much about it. “Next full moon” will probably be a more practical time for many people than “the sixth hour after sunrise on the second day of the month,” particularly if your people don’t count time by hours or have months following a lunar calendar.

Well, that got longer than I thought. It’s irritating to see facts go ignored for the sake of making something pretty, though, or even to have authors contradicting themselves. It, again, punctures the fantasy world.