1. The Moons
To start with, what we know, the in-setting author of the World of Greyhawk Guide tells us that "the Great Moon (Luna) waxes and wanes in fixed cycles of 28 days each, upon which our months are based; while the Handmaiden (Celene, the small aquamarine satellite) follows a path which shows Her in full beauty but four times each year, thus showing us the time for our Festivals. When both Mistress and Handmaiden are full, things of great portent are likely to occur." However, the Glossography tells us that Luna actually has 13 cycles in each year (because of the four week-long Festivals that occur between the 12 months) and therefore the months do not correspond to the cycles of the moon (i.e. the full moon occurs on the 11th day of the first three months of the year, the 4th day of the next three months, the 25th day of the next three, and the 18th day of the last three months). Furthermore, it also details cycles for Celene that are irregular - there are 22 days between full and 3/4, 20 days between 3/4 and new, 21 days between new and 1/4, and 28 days between 1/4 and full.
The only significant detail that the Gord novels add regarding the moons is the concept of the "whole" moon - when Luna is at 3/4 and Celene at 1/4 on the same night. Under the cycles described in the Glossography, this phenomenon occurs once each year, on the 4th night of the month of Harvester.
I know this is the Official calendar and it's easy enough to just use it and not think too much about it, but every once in a while despite myself I do think about it too much, and when I do I don't like it. I don't like the months not lining up with the cycle of Luna, and I don't like Celene having an irregular orbital period (and I also don't like the proposed "fixed" regular Celene cycle that means the whole moon as described in the Gord novels will never occur). What I've done to resolve those issues in my mind is to depart fairly drastically from Official Greyhawk and shorten the length of the year - in my Greyhawk calendars the four Festival weeks don't fall in-between the twelve Lunar months, but instead overlap them, so that Needfest occurs on the 26th of Sunsebb through the 4th of Fireseek, Growfest on the 26th of Coldeven through the 4th of Planting, Richfest on the 26th of Wealsun through the 4th of Reaping, and Brewfest on the 26th of Harvester through the 4th of Patchwall. Thus, the year has 336 days instead of 364. Luna is full the first night of each month, which means that Luna is always full when Celene is full (and the Dark Time/Dim Nights mentioned in the Glossography no longer exist), and the "whole moon" occurs four nights a year - on the 8th night of Coldeven, Wealsun, Harvester, and Sunsebb.
At first blush this seems like a major departure, which is why I hesitated over it at first, but the more I've thought about it the more comfortable I become, especially when I recall that the 364-day-long year isn't graven in stone. In fact, in the 1980 folio version of the World of Greyhawk the year is 360 days long (with each Festival only lasting six days instead of seven). The change in the 1983 set was presumably made to make the calendar and lunar phases work out more regularly. So I'm just doing the same thing as TSR already did, and for the same reason even. I'm just doing all the way what they only did partially. Does it matter if Oerth's year is 29.25 days shorter the Earth's? Not really. Every 12th year an Oerth person is effectively one year younger biologically than an Earth person the same "age" - a small enough difference to be pretty easily overlooked, at least for me. Perhaps Oerth orbits its sun more closely than we do ours, which could help explain the long summers of the Flanaess, but might also mean the inhabitants are absorbing more solar rays leading to slighty lower average lifespans (or some other handwavey malarkey...).
2. The Sun and Planets
On this subject, the Guide tells us that "the sun travels once around Oerth in 364 days," and also mentions "five wandering stars," presumably a reference to planets. In the post-Gygax era at TSR this reference was taken literally and the entire "official" cosmology (as detailed in, for instance, TSR's Spelljammer line from the 1990s) is Oerth-centric with the sun and other planets as, effectively, additional moons (I haven't read enough of that material to know whether the outer planets have epicycles to explain retrograde motion, but am guessing they probably do). However, sharp-eyed readers of Saga of Old City (written by Gary Gygax and published by TSR in 1985) will notice an entirely different cosmology described by the druid character Curley Greenleaf:
Or, in other words, a standard heliocentric solar system. Later in that same chapter Curley further explains that a large jacinth from that cache of gemstones "must represent the planet of Rao, greatest of the celestial spheres in the family to which Oerth belonged." This tells us that at least one of the planets is named after a Flannish greater deity. In later email correspondence, Gary reportedly told Gene Weigel that the Oerth system was intended to have ten planets total - the same nine as ours (Pluto was, of course, still categorized as a full planet at that time) plus "one beyond." Combined with the reference to the "five wandering stars" in the Guide suggests the last four planets aren't visible to the naked eye or known to laymen such as the "savant-sage," but would be familiar to experts such as the druids and worshipers of Celestian (who don't just watch the sky but actually travel in space)."Here," Curley Greenleaf said, placing down a huge sphere of uncut yellow corundum, "is the great globe of our sun. This emerald orb here is Oerth, I think; that opal represents Luna, and the star-sapphire of smaller size stands for the blue disk of Celene," he continued, placing each piece in its correct relative position. "These various stones are the spheres which accompany our world in its circuit of the sun…. These round diamonds are stars, and the little black opals the various moons and other celestial bodies whirling and spinning their pathways through the system," he concluded, not bothering to specifically place each of the smaller pieces. "What such imitations were used for is lost to us now, lads, but they represent a fortune to us all!"
Taking all of this together tells us pretty much about the planets - that they generally correspond to the planets of our solar system, that they're named after greater gods, and that they are symbolically represented by various gemstones. That leads me to something like the following:
Sun/Pelor Yellow Corundum
- Luna Opal
- Celene Star Sapphire
Other moons, comets, etc. Black Opals
(It is not coincidental that the gemstones representing the planets and stars, but not the moons or sun, also correspond to the Orders of Celestian)
This isn't something that's going to be immediately relevant or useful in most games, but in the right circumstances it can add color and at least an illusion of depth - the representation of the planets as gemstones and their connection with the greater gods allows for symbols and patterns when describing temples and treasures, creating rituals, and so forth. Perhaps a party of adventurers can discover some means of traveling to the red planet Ulaa and having adventures there, in the same manner that Erac's Cousin visited Mars (Barsoom) in the original, pre-publication, version of the Greyhawk Campaign.
3. The Stars
The Guide offers almost nothing about the stars, mentioning only that there are "12 Lairs of the Zodiac" through which the sun passes "in an appointed round which never varies." There's not much more in the Gord novels, only a mention (in Artifact of Evil) of a constellation called the Eldest Griffon whose extended wing points north and can be used as a navigational aid.
However, even that little bit of data gives us something to work with. We know that the sky of Oerth has constellations of stars, that the sun passes through twelve of them and forms a Zodiac, and that there's one called the Eldest Griffon (implying a separate Youngest Griffon) that functions the same way as our own Ursa minor (aka Little Dipper) - as a guide pointing to the North Star. Between all of this and what we know about the solar system above (and keeping in mind Gary Gygax's revelation in Polyhedron #21 that Oerth and Earth (and Aerth, Yarth, Uerth, and possibly others) are parallel worlds differentiated by the level of magic present on each), it doesn't seem unreasonable to assume that the constellations in the sky over Oerth are, by and large, the same as those over Earth, just given different names. [This is also consistent with the adventures Rob Kuntz ran in the pre-publication version of the Greyhawk Campaign set in another solar system, indentified by Rob as Fomalhaut.]
Classical astronomy included 48 named constellations (and modern astronomy has 88), and I haven't been ambitious enough to attempt to rename all of them (and some of them, already named after mythical creatures present in D&D, perhaps don't need to be named - Centaur, Dragon, Hydra, Pegasus, etc.), but I have proposed a version of the Zodiac constellations, as follows:
The Efreet 22 Coldeven - 21 Planting
The Gorgon 22 Planting - 21 Flocktime
The Ettin 22 Flocktime - 21 Wealsun
The Kraken 22 Wealsun - 21 Reaping
The Sphinx 22 Reaping - 21 Goodmonth
The Dryad 22 Goodmonth - 21 Harvester
Istus 22 Harvester - 21 Patchwall
The Phoenix 22 Patchwall - 21 Ready'reat
The Shedu 22 Ready'reat - 21 Sunsebb
The Satyr 22 Sunsebb - 21 Fireseek
The Djinn 22 Fireseek - 21 Readying
The Triton 22 Readying - 21 Coldeven
As with the details about the planets, this isn't material that is likely to be immediately useful in an at-the-table gaming context, but it does add some extra color. More pertinently, though, because the Oerth Zodiac is intentionally close to the Earth Zodiac, it allows for the inclusion of astrology-based symbols and references without having to either expect players to familiarize themselves with an entirely new fictional astrological system or justify a distinction between in-game and out-of-game knowledge. At very least, having the Oerth Zodiac line up with ours makes The Abduction of Good King Despot more easily usable as a World of Greyhawk adventure, without having to perform mental backflips to explain its astrology-based theme and symbols.
In most games, none of this is going to come up or matter. But if it does, I'm happy that I've already got it figured out :)
[Note: most of the above was workshopped and brainstormed in a recent discussion thread at the Doomsday Message Boards operated by my friend (and fellow Gygax/Greyhawk/AD&D aficionado) Scott Gregg. I don't want to take undue credit for any ideas, suggestions, or research contributed by any of the other participants in that discussion.]