#scifi Many planets have satellites, generally called moons when they are visible in the planet’s sky. These moons, like most other aspects of a planet, are subject to physical laws. Their apparent color, size, phase, periodicity and the strength of the tides they cause are all linked together.

To start with, let’s assume a planet with Earth-like gravity (which means Earth-like mass and size if it’s rocky) and a moon large enough that self-gravity has pulled it into a sphere. What will that moon (or moons) look like?

First, color. A moon shines by reflected sunlight. We’ve already discussed the fact that the light from just about any star will look white if there is no other color to compare it with, and the same will be true of moonlight. The color of the moon’s surface may have a slight effect, but only if it contrasts sharply with another moon in the sky at the same time. But what kind of contrast could we expect?

Water and free oxygen are very unlikely on the surface of a moon-sized object circling a planet that can be lived on by human beings, so the red iron oxides and blue water of Earth are out. Copper-containing rocks may be blue or green, but given the elemental abundance of copper in the universe it is unlikely that they would be present in such quantities as to color the entire surface. Sulfur is certainly a possibility–it gives Io its pizza-pie appearance—but it is so widespread on the surface of Io because intense volcanism, fueled by the enormous tidal pull of Jupiter, is continuously resurfacing that moon. Moons could certainly be brighter than ours (which is actually about the color of tar) but it is unlikely that they would look any color but white to our eyes.

How about size? Halve the diameter of our moon, while keeping it at the same distance, and it will look half the size in the sky. But putting it twice as far away, while keeping the diameter constant, will have exactly the same effect on apparent size. It will also, however, lengthen the orbital period—approximately the time between successive new moons. Doubling the distance will increase the period by a factor of 2.8.

Any moon will cause tides, and the tidal force will be proportional to the apparent size of the moon and its density. Thus our sun and moon look very nearly the same size in the sky, but the moon has over twice the tidal effect of the sun. Why? Because the sun, being largely compressed hydrogen, is less than half as dense as the moon. A moon the apparent size of ours but with more iron (denser) would produce stronger tides; an ice moon (virtually impossible at our distance from the sun) would produce weaker ones.

Moons have phases because they are lit by the sun on only one side. The phase thus depends totally on the angular distance between the moon and  the sun. In particular, a full moon rises at sunset and sets at sunrise. It will be relatively high in the midnight sky in winter; lower in summer. (This is really marked in interior Alaska, where the fact that the moon’s orbit is not quite parallel to that of the earth gives us times when the full moon, but not the sun, is circumpolar, and other times, in summer, when the full moon stays below the horizon.)

A half moon is always 90 degrees from the sun. A waxing half moon will rise around noon and set around midnight with the round side always toward the sun; the waning half will not rise until midnight. Crescents are always fairly near the sun in the sky. A waxing crescent sets a little after sunset; a waning crescent rises a little before the sun. In either case the horns point away from the sun.

Multiple moons in different parts of the sky will have different phases.

An example? Here’s one, from a trilogy I’m working on:

She opened her eyes to see the room, with a gibbous moon just above the horizon and a smaller half moon much higher in the blue morning sky.  Sunlight from the opposite window lay on the wall, and when she took a step forward she saw that a shabby-looking city lay below her, between the building and the prairie beyond.

Suns and moons all over, but they are in proper relationship to each other.