The Perpetual Full Moon


On Friday, while thinking about how the length of day and night varies with the seasons and latitude, I realized that, while the phases of the moon work as I describe in this post, they do not always correspond with sunrise and sunset in the manner I assumed.

It’s a terrible blunder. The phase of the moon is, of course, independent on sunrise and sunset, which varies according to how high the sun is in the sky and the latitude of the observer. In tying the phases of the moon to sunrise and sunset, it treats it as a local event when it’s not.

At latitudes that aren’t all that far north or south, the phases of the moon roughly seems to corresponds with sunrise and sunset, but the closer you get to the poles, the less this holds true. This means that “close enough” spreadsheet I did for my work in progress isn’t.

At this point I don’t have a good solution besides tinkering with Stellarium. I’m toying with some quick and dirty ideas, but this blunder has me extra cautious.

I’m leaving the post below as-is, error and all. Just be aware that the phases of the moon are not tied to sunrise and sunset, and my “close enough” spreadsheet isn’t “close enough” at all.

Oh, and one more thing: I also didn’t mention the variation between the length of the day and latitude with the seasons, though that should be obvious. I also didn’t factor in the length of twilight, which extends usable light somewhat beyond sunset and before sunrise. But that’s all for another post.

Original Post

Moonlit nights are wonderful things for writers, especially we who create medieval or quasi-medieval fantasy worlds. Does a character need a nocturnal adventure? Why, a full moon is in order; just the thing to provide light to see. It’s seldom obscured by clouds, unless we want a few drifting across the moon for a nice, on-demand, horror effect. Overcast nights? Seldom happens. A last quarter moon that rises late? Um … can we go back to the full moon now? Well, no, we can’t, not if we had one a week before. And yet, when the moon shows up in literature, usually it’s always full.

This is at least an admission that, before the ever present lighting that marks our place and era, nights were dark. Being around artificial lighting, we forget just how dark nights can be. I remember an otherwise good fantasy story by a well known author with a description of the sort of night you would only get on the outskirts of a modern city. Even where I grew up, where we didn’t have security lights, we could see glowing areas on the horizon, all from the street lights of distant towns. But in a medieval setting, there are no street lights, and the most common light at night was the moon, distantly followed by the stars, or none at all.

To get an idea of what a dark night night is like, here’s a link to the Bortle Scale. The Bortle Scale is a guide for astronomers to gauge just how dark the night sky is at their location. Of course, our medieval or fantasy adventurers aren’t going to be looking up at the sky and ticking off a list of the Messier objects, but clouds are going to look dark, not lit from below by city lights, because city lights don’t exist. When the clouds move in, our characters are dealing with a darkness few of us have experienced.

I remember one major storm during my childhood that knocked out electricity for miles around. The sky was overcast, and if there was a moon, we couldn’t see it. It was dark, the proverbial so dark you couldn’t see your hand in front of your face; so dark you saw phospenes, that faint light you can see in the dark with your eyes closed. For a child having grown up with at least some light always around, it was quite unnerving. Yet this is the sort of night characters would experience in a medieval world. So would rural Americans up through the 1950s or later. The reason some secret societies met during the full moon wasn’t for mystic purposes; it was so their members could see to come to the meetings.

Our medieval characters may have torches of the flammable variety, but they don’t compare with the convenience, safety, and brightness of a flashlight. When it gets dark, they aren’t going to be able to see very well, if at all.

That doesn’t necessarily put an end to night-time adventures, but it does give them a serious crimp. If there’s an overcast sky and no light at all, a character is really going to need a strong reason to venture out.

This brings us back to the perpetually full moon. Do our characters need to see at night? Behold: A full moon in the sky. Do they need to see on successive nights? Behold: a moon in the sky. If we give some thought, we’ll realize that after the full moon, part of the moon will be darkened and so there’s less light. But the moon is also going to rise later after sunset.

Nit-picky? Not really. On average, there’s 29.53 days between full moons, which means each night the moon rises about 49 minutes later than the night before. The moon rises at sunset during a full moon; the next night about 49 minutes later; the next night over an hour and a half after sunset; and so on. By the time we have a last quarter moon, about a week later, the moon is rising about halfway between sunrise and sunset. In another week, it’s a new moon, rising with the sun; and in yet another week it’s a first quarter moon, rising about halfway between sunrise and sunset.

This, along with the phase of the moon, affects how much light our characters have at night. If it’s been a week since a character sees a full moon, the moon’s not going to be up if he’s bumbling around an hour or so after sunset. If he waits until the moon is right overhead, he’s waited until sunrise, and won’t need the moon to see by at all.

If we have a story without a tight time frame, we can fudge things and have a convent moon in a conveniently clear sky. In the book tentatively scheduled for January release, there’s a time gap of several months between night scenes, and, wouldn’t you know it, they all start with a moon at or near full. But each full moon puts a clock into motion. When a character sees the thin crescent of the waning moon in the sky ahead of the sun, she realizes about two weeks have passed since a major event. Months later, after an event set at night, the clock starts ticking again. The next night, the moon rises later; the next, even later. After several nights, the character, now among buildings, has to creep along best as she can, with only the stars above for a dim light.

In my current work in progress, which has an entire month of recurrent night scenes, the clock is set at Easter. Since Easter occurs after a full moon, the number of days after the full moon is the starting point. Each night, the moon rises later and later, and that affects what the characters are able to see.

Difficult? Not really. I set it up in a spreadsheet and incremented moon rise time in respect to sunset by 24/29.53 hours per day. That’s close enough for writing, as it gives some idea how much light the character has on certain nights and when, and whether the moon is visible and how much of the moon the character sees.

Another complication is keeping track of the terrain. In my work in progress, a significant feature is a tall mountain range running roughly from north to south. Where the characters are in relation to these mountains is going to affect what they see. I could have calculated how much of the sky was blocked by assigning an average height to the mountains and how far the character was away, but there was no reason to be that precise. Knowing he was close to the mountains was sufficient to describe what the character saw when he looked up and gazed at the night sky.

I did fudge things; so far, there is only one overcast night described in the story, and only one day of rainy weather. So when the character looks up at the sky, he might see a few clouds or perfectly clear viewing conditions.

As to what the character sees in the sky … well, that’s a subject of another post.