The Visible Moon: Matera

 This moon, well known to anyone living on the outer surface of Mystara, 
is a silvery, lifeless, crater-marked satellite. Like the moon of our 
Earth, it waxes and wanes in a predictable pattern, controlling the 
tides and lycanthropy, but it isn't very interesting to Mystaran 
adventurers - unless, of course, they know of the gateway to the 
Immortal City of Pandius located in one of Matera's large craters. As of 
AC 1013, the moon has remained unexplored.
 Matera has a diameter of 2,160 miles and orbits 238,600 miles above 
Mystara's surface.
 DMs wishing to liven up Matera up a bit may place unknown, secretive, 
and very reclusive underground creatures on this moon. Or explorers 
looking past the uninspiring exterior of this "lifeless chunk of rock" 
might find evidence of long-lost civilizations here. How about the 
wreckage of Voidships that undertook the perilous voyage from distand 
worlds only to crash on an inhospitable moon within sight of a 
magnificent world?

The Second Moon: Patera (Myoshima)

 Mystara has a second moon, unknown to all but its inhabitants and a 
very few other mortal creatures. The Immortals call this moon Patera; 
its inhabitants call it Myoshima. The moon can't normally be seen by 
anyone outside its Skyshield, due to its core's unusual light-bending 
properties. It is small, with a circumference roughly equal to 3,000 
miles, and a diameter of 950 miles. The central core of Patera is 
extremely dense and magical, allowing a gravity comparable to that of 
Mystara. It orbits a scant 34,200 miles above Mystara's surface.
 This moon completes a full revolution around Mystara in three days and 
twelve hours (or two revolutions per week). Patera follows an exact polar 
orbit above the Known World, so that the moon passes above almost every 
point on the globe. Patera does not have a rotation of its own. One 
hemisphere (called Nearside) always faces Mystara, and viewers on the 
opposite side of Patera (Farside) never see the world they orbit. The 
pattern of day and night cycles on Patera is thus very complex because of 
Mystara's axial tilt, which provides Patera's seasons. The sun appears to 
wobble back and forth across the sky over a 3 1/2-day period as it also 
appears to travel around the Patera globe along a great cycle every 336 
days, Mystara's year. (Myoshiman calendars take a year of study to be 
understood, and its inhabitants have no fixed cycle of wakefulness or 
sleep.) Nearside usually continually receives a small amount of light 
reflected from Mystara.
 Full daylight on Patera is not as bright as on Mystara, being more like 
twilight. The sky changes color during a "day", ranging from fiery tones 
at noon to tamer red and purple hues at dusk or dawn. This happens 
because Patera has a light-reflecting shield at the immediate edges of 
its atmosphere. This shield bends light rays except at the extreme ranges 
of the visible spectrum - ultraviolet and infrared are unaffected by the 
shield. Light rays hitting Patera's atmosphere are deflected back to the 
surface or off into space. In effect, this causes the planet to be nearly 
invisible from the outside and allows little light to filter in (only 
Mystara, Matera, and the sun can be seen from Patera's surface). Patera's 
core generates the light shield's effects.
 Patera is mostly covered by steaming jungles and earthquake-prone 
volcanic mountains that surround three freshwater seas. Rain clouds cover 
a third of Patera at all times, and precipitation is abundant. The two 
polar areas of the moon offer at worst a temperate climate.
 The vast majority of the sentient population is made of various breeds 
of rakasta, humanoid felines with human intelligence. Unlike the Mystaran 
species, these all have infravision good to 60'. It is known that they 
have an architectural sense similar to the inhabitants of Ochalea - 
exotic, tiered, slope-roofed structures - and that they breed riding 
sabre-tooth tigers which, through the use of magical /flying collars/ 
and /airmasks/ for their riders, fly through the airless void between 
Patera and Mystara.
 Patera is divided into three major political blocks. The largest and 
potentially most aggressive is the Empire of Myoshima itself, a nation of 
feudal provinces controlled by daimyos, with a dingle emperor who rules 
them all.
 Next is the nation of Rajahstan, made up of twelve allied realms. Each 
realm is a sovereign state ruled by holy gurus (who handle law, 
education, religion, and internal politics) and maharajahs (who handle 
the economy, military, and foreign politics). Together these form the 
Spiritual Council to run Rajahstani affairs as a whole.
 The third block consists of many loosely allied petty kingdoms and 
principalities. Among the more prominent territories are Kompor-Thap (a 
valley of a thousand temples), Selimpore (a mercantile matriarchy), 
Malacayog (a nation of headhunters), and Surabayang (fierce island 
pirates). These territories are politically aligned with placid 
Rajahstan against imperial Myoshima - when they are not fighting each 
other.
 DMs may wish to map out the various Rakastan dominions - or nations 
ruled by even stranger creatures. Myoshima could provide an excellent 
way station for adventurers just venturing out into the Void from 
Mystara's atmosphere. There are plenty of opportunities for adventure in 
the rakastan cities or in Patera's wilderness. If the PCs get bored with 
Myoshima's surface, they can get involved in skirmishes between Rakastan 
Voidships and Heldannic Warbirds or try to negotiate trade agreements 
between inhabitants of Mystara and Patera.
 There are at least two Myoshiman languages: "common" and "poetry".
 Very little is known of Myoshima; it was visited twice in recent 
history by Alphatian adventurer Haldemar of Haaken, and the Heldannic 
Knights have apparently had unfriendly contact with the Myoshimans, but 
Patera has not been visited by diplomats or scholars from Mystara since 
Prince Haldemar, in AC 965.

Sources: The Voyage of the Princess Ark, episode 7 (Dragon #160)
         Champions of Mystara boxed set
         Poor Wizard's Almanac, PWA II, PWA III

Sources missing: Polyhedron #120 (Provides some details on how one RPGA 
         member detailed Myoshima, using material from Oriental 
         Adventures (both rules and modules).)