World-Building #1

Suns, Moons, and Rainbows


There's Alien, and Then There's Silly

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I have the distinct feeling that a good many writers never look at the sky in any meaningful way. It's just there. They don't think about what they're seeing or how it works. Maybe they live in cities where it's too smoggy to see the moon and too dry for rainbows. This article is so you never have to be among them.

"The Planet was So Strange
That the Sun Rose in the West."

The definition of the word "west" is "the general direction in which the sun sets." To be precise, where it sets on the days of the equinoxes. If you're standing in Minnesota and the sun is rising over California, the location of "east" has changed. The direction of sunrise determines the name of directions. There is no absolute direction beyond that: it's the basis of direction. So this just doesn't happen.

While we're looking at alien suns, let's consider all that astronomical business with red suns and white suns, green suns and blue suns. These are only astronomical classes, and in their own solar systems these supposed colors are not visible. Our own sun is classified as "yellow." Yet if you look (carefully and briefly) at it in the heights of the sky, it shines white. The light it sends down is white. It only goes gold or pink or blood red when it's so low down that the angle through the atmosphere is messing with the light.

The same applies to all the other colors of suns. They all burn at such astronomical temperatures that the light coming off them, their color as seen from a planet in their solar system, is white. So despite many a complex scene of blue and red suns casting violet light like mixed stage spots -- sorry, no, it doesn't work like that. Hey, I was disappointed, too. One astrophysical friend has pointed out that red stars are seen to be lacking in the yellow-green spectrum, so there may be a color shift under those stars. I doubt it, though. He's talking about spectrographic analysis, not standing in its light.

Having brought up multiple stars, let's note that they can result in nights being shorter than if there were only one. If the suns, as seen from the planet, are close together and it goes around them both, it's like one big star. If they have any separation to speak of, one will rise, then the other. Repeating that at sunset, you will get a longer day with a shorter night, always.

If the planet dances the highly hypothetical and probably impossible figure-8 orbit that crosses between two stars, then there will be no night at all during the time it is between them. Note that for this to happen, the stars must be so far apart that the orbit would be so long that this "no nights" period would go on for our equivalent of years. It could cause some very odd life cycles native to the planet, which could be a lot of fun.

"The Full Moon Rose at Midnight"

Moons, however many you have, are dependent for their light on the sun. I know you know that, but there's some people I'm not so sure about. They seem to treat the moon as a kind of gigantic firefly, blinking on and off very slowly. To sum up the most common mishandlings of the appearance of the moon that I run into in terrestrial fiction, or on planets with a Lunar moon:

Goof #1) It's always a full moon. Unless the writer needs darkness. Then there's no moon. But then it will be a full moon again as soon as convenient. Like two nights later.

It takes about 29.5 days on this planet for the moon to turn from full to full. Fourteen days away from full, it is the dark of the moon. In between, it is more or less a crescent or a trimmed-off circle.

Whatever planet you are on, however many days it takes a moon to go through its cycle, it will be a crescent more than it's full. The full moon is usually considered to be the three nights around and including the true full moon. Ditto the dark of the moon.

Track your story on your own planetary calendar. If there are some nights the moon must be full, it will have to be less than full on the days around that night. Opposite that day, you will have tiny slivers that aren't even up at night. Which brings us to ...

Goof #2) The moon rises at sunset. After all, the moon is there to light up the night when the sun goes away. So it sets at dawn, too.

The moon is not there for a night-light. The moon is busy with its own orbit and doesn't care if we trip over things in the dark. Most of the month the moon rises during daylight hours or sets before dark. Honest. Go out every day for a month and you can observe this yourself. However, you need to live someplace with clear air to see the moon up in the daytime sky in the morning or late afternoon. It's beautiful! The sunlit areas are white, the shadows all sky blue, like it was appearing out of blue water.

Otherwise, go out every night for a month and look for that moon that's supposed to be up playing streetlight. Most of the time, it isn't. Now you know what every Neolithic farmer and Paleolithic hunter did: most nights are dark to darker, for more or less hours.

To put it simply, only the full moon rises around sunset (assuming you're at an equinox)(yes, this all gets complicated when you go to ultra-accuracy, but I'm giving you the rules of thumb). At the dark of the moon, the moon is actually rising at sunrise and hiding in the sun's glare while turning its lit face entirely away from us. The new moon, the thinnest sliver, rises just after the sun. The first quarter moon rises at noon. The last quarter moon rises at midnight. (Again, this is simplified because the moon's orbit isn't linked to the rotation of the earth the way you might think.)

The rising time and the phase of the moon are interlinked and cannot be changed, no matter how far away the galaxy is. When our moon, any Lunar moon, rises at true (not clock) midnight, it will show itself in last quarter. The rising time determines the phase, if you like. So if someone says, "On my planet, the full moon does rise at midnight," then on this planet water can run uphill, too, and I don't want to be there. The relative positions of moon, planet, and sun determine, at one and the same time, the phase and the rising time.

If you are not at equinox, the full moon will be rising in late afternoon at summer solstice and in the early night at winter solstice, assuming you're so near the poles as to make a difference in day length. The temperate zones are near enough, which is why you folks have daylight savings time. The tropics have relatively little change between solstices, and don't bother with DST.

If you are on Earth or someplace like it, grab an almanac to plot the time change. Your best is The Old Farmer's Almanac. If you are on another world, where maybe the month is longer or shorter, just divide the time evenly so that the moon doesn't make any jagged jumps in rising time.

Goof #3) Multiple moons will always be in matching phases, and all rise together. If they rise at different times, their phases will match. Always full, of course.

This is spreading mistakes #1 and #2 over multiple victims.

If a planet has multiple moons, they must be in different orbits. The orbits will be different sizes and the moons will travel at different speeds. The idea that they will travel at such speeds that they will always be in the same place on their orbits, as seen from groundside, is so difficult to achieve with artificial satellites that doing it with natural ones can be listed as implausible if not impossible.

Each moon will have a different length month. You are going to have to chart these. As well, if they have different orbital tilts, they may rise in somewhat different spots on the horizon and have different paths across the sky. You can use this to give your people more points of direction reference.

Non-Lunar Moons: the Martian Equations

In case you were wondering about that apparently redundant adjective, Lunar moon, not all natural planetary satellites act like Luna. Specifically, I ran into a lovely article in my 1959 Americana on the solar system than confirms what I wrote above in "The Lunar Problem," but gives us more leeway in "Interesting Motions of Martian Satellites."

To cook it down:

The Martian day is 24 hours, 37 minutes.

The revolution period of Phobos is only 7 hours 42 minutes, whipping along only 3,700 miles (6000 km) above the Martian surface. As a result, it acts more like our artificial satellites. Because of the short period, it does rise in the west -- moons, unlike suns, can rise anywhere, including in the north or south, as suits their orbits. It is visible above the horizon only 4 hours and 18 minutes at a time. However, only 11 hours will pass between risings. As well, because of its low orbit, it is only visible below 68º N & S on the surface. Closer to the poles, Phobos can't be seen.

Deimos has a period of 30 hours, which is so close to the rotation of Mars that once it rises (in the east) it is up for 66 hours. That's over two and a half Martian days. While crawling across the sky, it goes twice through its cycle of phases between new and full and back again. (Yes, my brain is staggering, too, trying to picture this.) This could give you a natural time-keeper, as in, "Let's meet again at the second third quarter after the next rising."

So you don't have to have a Lunar moon on your planet: you can have a Deimosian or Phobosian moon of much longer or shorter period. They're the more common sort of moon for terrestrial planets. Luna is more like Terra's partner in a binary planet arrangement.

Do note that Phobos is a doomed moon: by traveling so close to its primary, it is below a permanent orbit and is slowly headed down. Estimates say in a mere 50 million years more it will break up or crash. So Phobosian moons are relatively temporary phenomena.

"At Noon, the Storm Broke and a
Beautiful Rainbow Spread from One Horizon to Another."

If it's a fantasy story, this says that definitely magic is afoot. This would have to be sorcerous light or illusion. Natural rainbows can't spread from one horizon to another and they only come out in the early morning and late afternoon.

At least the writer had a storm in the vicinity. Other common rainbow mistakes are:

Goof #1) The viewer gazes into the rising or setting sun while admiring the rainbow.

For a rainbow to happen, the light source must be behind the viewer. If your friend sees a rainbow and directs you to stand in the middle of the arc your friend perceives, you will see the sun or moon directly above or behind your friend. You cannot see the light source that creates the rainbow and see the rainbow, too.

This is because a rainbow is only a trick of the light not an independent or concrete thing. If you stand with your back to the light and a layer of moisture is in the air in front of you, the water droplets refract the light, creating the spectrum that spreads out to look like an arch. BTW, the shortest lightwaves, the red, are normally on the outside.

Goof #2) It happens in a dry place.

There must be something refracting light into spectra to get a rainbow. We normally find rainbows before or after rain, when water droplets are in the air, but when the clouds are placed to let sunshine through. We also see them in the mist rising from the base of a waterfall, when the sun is right. Theoretically, if you had high, wind-blown crystals of transparent quartz, shaped just so, you could get a rainbow effect, too. It's so theoretical, though, you can say it doesn't happen. Certainly, I never saw rainbows the whole time I lived in the southwest desert clime. Even in Oregon and Virginia and New England, they were very rare, to the point I never caught one.

Here in Hawai'i, on the other hand, they're extremely common, which is one reason I've been able to study them avidly.

Goof #3) There's more than one, in different places.

Triple rainbows happen. Stand outside Ala Moana Center at the intersection of Kapiolani Blvd and Keeamoku at about four in the afternoon and gaze up toward Manoa Valley. The Manoa Mist (an almost daily occurence in all but the driest seasons) provides the rain and the lowering sun is placed just right. Double rainbows aren't uncommon from there, though triples are rare.

But a multiple rainbow is one true rainbow and its reflections in heavy water haze. First, there's the normal rainbow. Inside or outside it is its mirror image, the colors reversed so the violet is on the upper arch. Then comes the reflection of the reflection, reversed colors reversed back to normal. So the original rainbow has to be very bright, made by strong light. Each reflection is weaker, the third normally pretty faint.

Always remember: rainbows are not objects, but illusions obedient to certain laws of observation. Aerial rainbows, when multiple, will be one inside the other, not scattered around or with their ends overlapping.

Goof #4) The rainbow stretches from horizon to horizon across the center of the sky.

Because the rainbow is a light trick depending on the observation point, the light, and the moisture, both ends of the rainbow will be in sight without turning your head (unless you have an unusually limited field of vision). The reason you never see the ends touch down is that either the water is not there, the light angle is wrong, or there's something in the way. In theory, and a few times that I've observed, you can see the rainbow all the way down to, say, the ocean or harbor surface. Sorry, no pot of gold. Menehunes only leave taro. (Naturally, rainbows always stay in front of you while they're visible, so you never can get close to the ends.)

In fact, there's a limit to how high up the sky the top of the arch can reach. The best I've seen in many years of rainbow-watching is half-way up the sky, and usually they are more like a third of the distance. This seems to be dependent on the height of the sun at the time: the lower the sun, the higher the arch.

This also means that they don't happen from mid-morning till mid-afternoon: the sun is too high.


STORMY WEATHER: Siting Jungles and Deserts


or, There's Alien, and Then There's Silly

STORMY WEATHER: Siting Jungles and Deserts


Siting Mountains, Rivers, Valleys, and All That

THE ETERNAL CITY: Do-It-Yourself Political Geography

Caves, Caverns, and the World of Eternal Night.

Some Basic Minerology for Planet Building