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Hollow Moon Planetology: Blue Moon - Atmosphere & Gravityby Sharon Dornhoff
The interior of the Nearside is blanketed in a breathable atmosphere similar to Mystara's, although slightly higher in carbon dioxide than what PCs are used to. This difference is too small to endanger life or affect a character's performance, but adventurers from Mystara will find themselves breathing hard for the first few days of their visit. This excess of CO2 results from Matera's sparse vegetation -- in such a dim-lit world, the oxygen-generating process of photosynthesis is less efficient -- and may seem quite stuffy at first ... unless, of course, the PCs' ship is already suffering from a fouled atmosphere, in which case the lunar air seems fresh by comparison! Sulphur gases are also quite common in the Hollow Moon, thanks to the large number of volcanoes; a faint odour of rotten eggs prevails over most geologically-active regions (which includes just about anywhere that's not the maria), although it's not strong enough to inconvenience lupins' scent-recognition or other olfactory abilities, let alone pose a danger to anyone.
Another reason PCs will gasp for air, upon first entering the Hollow Moon, is that the atmosphere within Matera varies in its density from place to place. Whereas gravity from Matera causes the Mystaran seas to periodically rise and fall as tides, as the visible moon orbits the planet, Mystara's gravity always pulls its strongest on the same region of Matera: the "midlands", which are located on the equator at the very centre of the Nearside. The farther away one moves from the midlands -- and hence, from the part of the Hollow Moon closest to Mystara's attraction -- the thinner the available air becomes. The altitude limit of the inner moon's breathable atmosphere also varies, from nearly twice the height of Mystara's in the midlands (37,500' high), to less than half of Mystara's in the "rimlands" (9000'), along the Nearside/Farside border. On the Farside rim's various peninsulas of outjutting land -- which include the Redlands and the rugged vicinity of Mare Orientale -- the internal atmosphere is thin as the air on a high Mystaran mountaintop; in the midlands, the air is thick enough to retain extra heat and provide a near-subtropical warmth, despite the Hollow Moon's rather chilly (40 degrees F) mean air temperature.
In spite of the fact that PCs may FEEL as if they are suffocating, between the extra carbon dioxide and the rarefied air they'll encounter near Mare Orientale, there's no less oxygen in the Materan atmosphere than in Mystara's. After a few (Mystaran) days -- specifically, 60 hours minus three times a character's Constitution score -- visitors will become acclimatised to the conditions, and cease to find the CO2 levels uncomfortable. The rimlands' rarefied air will still leave them gasping if they've spent long periods in the central Nearside, but that's no different from moving from a lower to a higher altitude, on Mystara.
Within the Hollow Moon, Earth-normal gravity prevails, drawing creatures downward towards the lunar "shell", much as does the Hollow World's internal gravity. In Matera, the gravitational pull of Mystara has a lesser but significant effect, as well; although it doesn't alter the fact that things fall to the ground, even in the rimlands, it creates a "perpetual tide" in the midlands that affects both air and seas. This gives the central Nearside both a thicker atmosphere, and deeper waters, than would otherwise exist within that region, were the gravity equal in all directions in the HM setting. Within the rimlands, where Mystara's gravity pulls perpendicular to Matera's own, plum bobs hang at a full four-degree angle toward the central Nearside; a definite sense of "tilting" can be plainly felt by characters in these regions, giving the impression that one is ascending a slight slope as one moves in the farward direction on level ground. (This drives AD&D dwarves and gnomes bonkers, when they try to estimate the slope of an underground passage. ;-D) Rimland architecture must take this skewed gravity into account, so the lunar rim-dwellers build one side of their homes' foundations a bit taller than the other to compensate, not unlike hillside-residents on Mystara. This "tilting" levels out as you move closer to the midlands, where Mystara's gravitational pull is parallel to Matera's.
Much like Mystara's exterior, the inner atmosphere of the Hollow Moon is bounded by a Skyshield. Its altitude, however, varies with the actual depth of the lunar atmosphere, from nearly twice (150,000') to less than half (36,000') the elevation of Mystara's Skyshield. Vortigern's Vortex and tubular breaches do not occur, in the Hollow Moon; however, the seasonal events called "circle tides" offer aerial vessels the brief opportunity to lift off and cross over the internal Skyshield*. The inner moon's Cordillera and Rook Mountains, with their thick underlying bedrock and steep 5-mile-plus ascents, actually poke their highest peaks right up through the Farside's lower-than-normal Skyshield! Piloting a ship from Mare Orientale to the inhabited points beyond, therefore, entails navigating along low-altitude valleys within these two concentric mountain ranges, rather than sailing above them. Fortunately, safe passes through the double ring of mountains are easily found.
[* - The outer Matera, having no atmosphere to retain, also has no Skyshield to inhibit ships' passage. If you're using Bruce Heard's idea of giving all of Mystaraspace an Alphatian-created, breathable atmosphere as of 1016 AC, then the Immortals -- leery of intruders stumbling upon their extradimensional city of Pandius, on the outer moon's Farside -- will set up a new lunar Skyshield at that time... but it serves to keep air OUT, not in!]
Beyond the Hollow Moon's Skyshield, the interior of Matera is similar to outer space -- it lacks air, warmth, or protection from UV radiation (which, as previously mentioned, is able to pass through the crystal Firmament in abundance). Unlike the deadly-cold temperatures encountered in open space, Matera's inner Void is only a few degrees below freezing; within the moon's enclosed confines, trapped volcanic heat and crystal-refracted sunlight keep conditions moderate by comparison. The hazard of ultraviolet, on the other hand, is all the more severe, for the dimness of the Hollow Moon's environment may mislead explorers into foregoing all precautions against it! (Note that even in campaigns which treat outer space as being filled with breathable air, the interior of Matera should STILL be an airless Void. Given that -- unlike the Hollow World setting -- virtually every PC who visits Matera will arrive in a Void-worthy ship, it's a necessary device for campaigns to ensure such craft can't easily take short-cuts through the inner moon's sunless core, bypassing huge stretches of territory and the dangers and adventures they offer.) The whole of the internal Void is subject to zero-G.
Although there are no stars in the Hollow Moon, the open region beyond the Skyshield is home to a scattering of tiny, flashing points of light, which occasionally move about. To natives of the moon, these "Shimmerlights" are believed to be many things, from distant fireflies to Immortal omens to benign spirits of the dead; in reality, most are flecks of phosphorescent quartz or reflective mica floating in the gravity-free void. They are most visible during the darkest portion of the lunar "day", as this is when light shines upward through the Nearside maria and can be reflected back down to the setting's watching inhabitants.
While the peninsulas of land that protrude into the Farside have an atmosphere that's about half as thick as Mystara's, even this meagre air tapers away, by the time one moves about 200 miles past the Nearside/Farside border. The central Farside, itself, is totally devoid of an atmosphere. Hence, although it's entirely possible to circumnavigate the Firmament by walking around the rim of the moon's crystalline hemisphere, only a voidship or other method of space-travel can carry anyone to its very centre.