A Discussion on the Moons of Gor

Moonwatching

Questions about the moons of Gor are not the most pressing concern in the world, but they do come up from time to time, and if you are such as I am – a Scribe with a strong hint of Builder and a fondness for debating how many Priest-Kings can dance on the point of a pin – then the topic is worth a little discussion.

What the books say

First of all, how much does the author tell us about the moons? Precious little. To my shame, I have yet to read all the Gor books – but in all the 20+ books I have read, I can remember reading only the following facts:

• Gor has three moons
• They are of varying sizes
• One of them – not the largest – is named “the Prison Moon” for no reason that Tarl Cabot knows
• They are always mentioned together.

The last point is interesting. None of our narrators ever mentions, say, that one of the moons was rising while another was setting and the third was high in the sky. Nor do they remark that one of the moons is full while another is gibbous or half-full, which would be the case were they radically separated in the sky. To be sure, the mere absence of any such reference does not imply that such is never the case. Gor and its moons could be a simple and natural multi-satellite system such as Mars or the giant planets – and these are interesting and diverse enough in their own right. More of this shortly.

The power of the Priest-Kings

On the other hand, we know that the Priest-Kings brought Gor into Sol system some time in the ancient past and that therefore they are able to move planets over interstellar distances. Indeed, this does not say the half of it. Supposedly the Priest-Kings originated in another galaxy, and not just any galaxy but a particularly distant one – “one of the fantastically remote Blue Galaxies” – which implies that they either have, or had then, the ability to move Gor either many times faster than light, if some kind of “space warp”, “wormhole” or “hyperspace” double-talk is to be allowed, or at an extremely high fraction of light-speed, such that subjective time for the travellers would be greatly reduced. For we know that the Priest-Kings have longevity to such an extent that human lifespan, even with the Stabilisation Serums, is that of a mere mayfly – but such galaxies are hundreds or thousands of millions of light-years distant, too far for even Priest-Kings to live through the journey unless they were aided by massive time dilation. They would need relativity’s help.

One thing, then, that the moons of Gor probably are not – the planet’s original satellites in their original orbits. We know that the Priest-Kings can move planet-sized bodies, or could (it’s not unfeasible that they could have lost their technology, or no longer have the resources for such an effort) and we can presume that the current configuration of the Gorean satellites was arranged artificially by the Priest-Kings after they arrived in Sol system. It’s also perfectly possible that the moons of Gor are indigenous to Sol system, retrieved from the asteroid belt while the Priest-Kings were settling Gor into its present orbit. Perhaps Gor originally had satellites, and these were left behind when the planet was moved, and the Priest-Kings plugged in some new ones to replace them. The books are silent on the matter. They also, to reiterate, do not explicitly state that the moons always appear together in the sky, but this essay assumes that such is the case.

Moonwalking

The point is that it is a highly unnatural configuration for a planet’s moons to orbit it as a single unit. Nothing of the kind exists in our solar system. Earth, of course, has but the one moon. It is most unusual in that it is large compared to the primary – a full 2% of the planet’s volume, which is utterly unmatched unless you consider the Pluto/Charon system as a planet and moon rather than a double planet; some call it one way, some the other – and, by a startling coincidence, almost exactly the same apparent diameter as the Sun. All other planets, save Mercury and Venus, have multiple satellites, and each satellite has an orbit of its own and so revolves around its parent in a “month” all of its own.

The Martian moons

Mars is interesting, for its two diminutive satellites orbit close to the planet in orbits not far distant from the Martian day. Phobos, orbiting low (less than 4000 miles from the surface of the planet) moves so fast from West to East that it overtakes the planet’s rotation and so rises in the West and sets in the East approximately twice per day. Deimos’s 30-hour orbit causes it to remain in the Martian sky so long that it goes through all its phases twice between rise and set. We would find this strange and exotic (but the small size of Deimos and Phobos, only in the tens of miles, would render them less than spectacular) but they are still, so far as it goes, normal and natural orbits compared to the weird moons of Gor.

Some physics

For those who do not know, an object’s orbital period about its primary is dictated by the radius of its orbit. The nearer to the parent, the faster the orbit. The rules were worked out by Johannes Kepler. To understand his law of planetary motion, you might like to draw a diagram. Draw two circles around a common centre, representing the orbits of two planets. Mark an arc on one circle to show how far Planet A travels in some unit time. Draw the radii from the centre to the ends of the arc, and shade in the enclosed sector. Now if you repeat this with the other circle to show how far Planet B moves in the same time, the shaded sectors must have the same area.

Planets actually move in elliptical orbits, not circular, but this is not a problem. Kepler’s Law shows us that when a planet is closer to the Sun, it moves faster than when it is further away; specifically, that if you mark out the sectors, as above, they will be equal in area for equal time-periods. I really wish I could oblige you with a diagram.

So as the cries of “Get back to the point, Socrates!” grow ever louder… The upshot of this is that if our Gorean moons are in different orbits then they will have differing orbital periods. And if the Priest-Kings go so far as to force the moons to go either too fast or too slow for their respective orbits, they will also need to push them continually towards Gor or away, or else the moons will either plummet into the planet or fly off into space.

Of course the Priest-Kings have, or had, such power, but if I were in their position I would arrange matters so as not to need continual interference. There are several ways to achieve this, happily.

One solution: The Kemplerer Rosette
If the moons were of comparable size then a Kemplerer Rosette (also Klemperer Rosette) would be a possible solution. I’m indebted to another science-fiction author, Larry Niven, for bringing the concept to my attention in his 1971 novel Ringworld. The Ringworld was not itself such a rosette, but one was used as a minor plot element early in the story (and it says much for the scientific acumen of the man that he can toss in such a concept as a “minor plot element”). A Kemplerer Rosette consists of a number of bodies spaced equally around a circle. Their gravitational attractions on each other exactly balance and they can orbit either yet another body or a vacant point. I am assuming that three moons in a Kemplerer Rosette could orbit Gor at a large enough distance that the Gorean gravity would not itself disrupt the Rosette, but I don’t have the math to work it out. Considering the imaginary circle as an imaginary wheel, with the axle pointing along the plane of Gor’s orbit, our Goreans would see the three moons tumbling eternally one over the other, sometimes seeing the Rosette face-on, sometimes edge-on.

We are told, though, that one of the moons is larger than the other two. This doesn’t rule out the Kemplerer Rosette, though I think it would have to be asymmetrical, but it does admit of some other solutions.

Moons of a moon

If the largest moon is large enough and distant enough, then the other moons could orbit it just as Gor orbits the Sun. As long as they are near enough to the largest Gorean moon (LGM) that it, and not Gor, is by far the largest gravitational influence on them, then this arrangement will be stable. Moreover, their orbital plane about LGM need not coincide with LGM’s around Gor (nor Gor’s around the Sun), and supposing that this is so, sometimes the lesser moons (smallest Gorean moon a.k.a. SGM, and the Prison Moon a.k.a. PM) will appear above LGM, sometimes below, sometimes in front of it, sometimes behind, sometimes casting shadows on it or on each other, and sometimes disappearing into LGM’s shadow. (It is here that we discuss the difference between eclipse and occultation. When SGM or PM disappear from view behind LGM or each other, they are occulted. When they disappear into shadow, they are eclipsed. The two phenomena need not happen at the same time.)

Trojans

A special case of this arises if the lesser satellites orbit at the right distance from LGM. Orbital periods depend not only on distance from the primary, but also on the strength of its gravity. (The more you are being pulled towards a massive body, the faster you must go to remain in orbit.) Clearly it is possible for SGM and PM to be at such a distance from LGM that the time it takes them to orbit LGM is the same as the time it takes LGM to orbit Gor. Technically, they are then at the Lagrange Points of LGM, or informally, the Trojan Points (after two small clusters of asteriods known as the Trojans that are in just such a relationship with Jupiter). They will then be in the same position with respect to LGM all the time. How close in the sky they will appear to LGM depends on some data we don’t have; the mass of LGM and its distance from Gor.

Conclusions

To reiterate, the books do not provide enough detail for us to know which of these configurations is the right one, if any – though any of them (rosette, mini planetary system, Trojan) would hardly have arisen by accident. The reader can take his pick of the possible spectacular views in Gor’s night sky: Tumbling endlessly one over the other, dancing around in a whirl of occultations and eclipses, or locked eternally side by side and face-on to Gor. Unless it pleases the author to enlighten us, we do not know what he meant.

A consolation for the Earthbound

But we Earthlings, denied such celestial wonders, can take comfort in this at least: Unless the Priest-Kings took great care to get the apparent diameter of at least one of the Gorean moons exactly right, to us alone is reserved the magic moment when the dark Moon exactly covers the photosphere, night falls at midday, and we see the glory of the pale ghost-flame of the Sun’s corona in the splendour of a total eclipse.