The Red Moon, as we all know, changes colors, from red to black and back again, embedding the essence of edgy Gothiness into Glorantha at a cosmological level, and the phases of the Red Moon mimic those of our own Moon. The black areas of the Red Moon form a curved terminator* line as they advance and retreat.
Now, the phases of our own Moon are from the Earth's shadow falling between the Moon and the Sun. The roundness of this shadow is a bit of evidence for the roundness of the Earth known for millennia. Glorantha is not round like a ball, and the Earth cube is vastly larger than the Red Moon, so this seems rather unlikely to be the explanation.
In addition, our Moon merely reflects like a mirror, but we all know the Goddess is the mirror and the mask and glows in her own right, with her own light. Light in Glorantha is associated with the eyes in a way that suggests a hybrid intromission/extromission theory of light is true in Gloranthan optics: light emits from glowing objects but also very faintly from mortal and animal eyes, which must be actively suppressed by active sources of Darkness. Contrasting this, light in Glorantha presumably has a finite speed, because that allows Mastakos to be faster than it.
So while I have several theories about the physical manifestation of the fundamental truth of the Goddess as represented by Her weekly Lesser Waning, these must be modulated by uncertainty over the nature of light in Glorantha. I have given these theories a brief title to summarize them.
1. "The Goddess is tossing Her hair."
The phases of the Red Moon are materially caused by a physical veil of the same real size as the portions of the Red Moon they cover, which blocks the Goddess's light, but allows some to faintly shine around the edges, which is more visible under the Glowline.
2. "A thirsty star will drink a river of light."
The apparent size of this shadow is spurious- a small object moves across the face of the Red Moon and absorbs the Goddess's light, bending it towards themselves and eating or storing it. This appears to be a large shadow, with light at the edge of the sphere of influence of the object leaking through. This object is presumably related to the Red Goddess just like the other stars that cluster near her in the Middle Air.
3. "The Red Moon is always full, no matter what your eyes tell you."
The Red Moon actually changes color and emission spectra to produce its phases, and this includes everything on the surface. Including visiting adventurers. The leaking bits of light near the edges are due to some kind of light scattering phenomenon that undoubtedly has a place of pride in the Lunar College of Magic's quals.
4. "Effy, you simpleton, it's all epicycles."
The divine mystery the Goddess has set before us is simply beyond our ability to analyze in this matter. Her decisions, Her actions, Her sartorial choices, they are all things we cannot understand without becoming Her, at which point they cannot be communicated to anyone who is not already Her.
Or something like that.
It is worth noting that apparently the Red Moon's phases are digital, and the Goddess presents the same face all day and only changes it at night. I am unsure how literally to take this, but it certainly implies some things!
So after all this... what thoughts do people have about this tenebrous presence hanging over Glorantha, unnamed and only incidentally commented upon?
*The Dying Moon phase provides a lovely folkloric explanation for using this real astronomical term.
