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Hollow Moon Planetology: Blue Moon - Lighting Conditionsby Sharon Dornhoff
Unlike Mystara's Hollow World, the interior of the moon Matera has no direct source of sunlight to illuminate it. Apart from artificial light sources, bioluminescence (very common, in HM life forms), and the hot glow of volcanoes, the only light by which to see within the Hollow Moon is that tiny fraction of sunlight that can penetrate the 30-mile-thick crystalline bedrock, of which the Farside, and all but a few Nearside maria, are composed. Because the lunar "day" is 28 Mystaran days long, this means Matera's inner world passes through a cycle of illumination every month, going from twilight conditions at the new moon (when the sun shines directly down upon the semitransparent Farside), to the near-blackness of a planetary moonless night (during the Mystaran full moon), and then back again to twilight. This gradual variation in brightness is easily portrayed, using the PHBs rules on sighting ranges under various light levels. For combat, the "Moonlight/Medium fog" penalties should be applied to non-infravision-users under any light level except fulldark, during which the harsher "Starlight/Dense fog" penalties to attacks, AC and saves apply (caves and other underground sites, naturally, are pitch-black even to Matera's nocturnal races).
Probably the most convenient way to keep track of this variation in light levels, for a DM, is to picture a 24-hour clock face with noon (1200 hours) at the top, and midnight (2400 hours) at the bottom. The upper half of the clock-face represents the lunar Farside, while the lower half is the Nearside of Matera. The clock's hour hand points to the direction in which the sun lies, at any given instant. One full rotation of the hour hand, through a complete "24 hours", actually represents a full month of game time; thus, each "hour" on the clock-face is actually 28 hours (1/24th of a month) of game time.
When the hour hand points to midnight, the sun is shining directly at the Nearside, with little light able to penetrate its maria. This part of a lunar day/Mystaran month is called "fulldark" by Materans, as it's as fully-dark as the Hollow Moon ever gets. (The fact that it's also the night of the full moon, from a Mystaran's point of view, is a complete -- but convenient for the DM's memory! -- coincidence. ;-D) During this period, the Hollow Moon is as dim as a moonless night on the surface of Mystara, and the Firmament -- i.e. the crystalline dome of the Farside -- is much too dark to see. Fulldark lasts from 11 pm to 1 am (2300 to 0100) on the imaginary clock face... a period of 56 hours.
Between 1 am and 3 am (0100-0300) on the clock face, the Hollow Moon's "sunrise" occurs, as the terminator between light and dark peeks over the Nearside/Farside border at the 6 am (0600) mark. A HM sunrise consists of a subtle brightening of the Firmament above, and several miles beyond, Stygia and the Oceanus Procellarum. This glow isn't enough for human or other diurnal eyes to see much by; but the crystal itself can at least be distinguished overhead, as the sun's rays give it colour. Over the next 56 hours, light levels within the Hollow Moon are equal to those of a full-moon-lit night on Mystara.
By the time the sun has reached the 3 am (0300) mark, the western Firmament of the Hollow Moon can be seen to glow from the Nearside rim to the 9 am (0900) point -- half way to the very top of the Farside's dome. From 3 am to 6 am (0300-0600) on the 24-hour clock face, light within the HM is roughly one-third as bright as twilight on Mystara. To illustrate this, DMs should cut all the "Twilight" sighting-ranges from the PHB by 2/3, except for the "Detail" range which is halved. Up until this moment, the sun is, in fact, still beneath the Nearside's "horizon"; but its indirect illumination has crept much farther up the dome of the Farside, reaching the "high noon" point by 6 am (0600) and "true" dawn. These conditions persist for 3 "hours" on the clock-face, or 84 Mystaran hours.
As the sun crosses the 6 am (0600) mark, direct incoming sunbeams hit the Farside, raising light levels abruptly, to 2/3 as bright as a Mystaran twilight. Actual light-rays fall upon the lands of the Nearside; objects cast distinct shadows, for the first time in a lunar "day". The sun can be distinguished as a vague patch of greater brightness that crosses the Firmament, moving along the equator. This "morning" on the clock face, along with the lunar "afternoon", is the longest-lasting of the various lighting stages which the Hollow Moon goes through: 140 hours by Mystaran reckoning, or 5 "hours" on our made-up clock. Sighting distances are 2/3 those in the "Twilight" bracket of the PHB table, except for "Detail" range, which is undiminished from its usual 10 yards.
High noon -- or rather, 11 am to 1 pm (1100 to 1300 hours) on our clock-face -- is when the sun can bathe the entire Farside in direct radiance, providing light levels equal to normal Mystaran twilight for the Hollow Moon. This brief (56 game-time hours) period of strong sunlight is called "skybright" by lunar cultures, and is the brightest solar illumination the HM setting ever receives.
Through the lunar "afternoon" and "evening", the light levels fall, in an inverse order to that by which they rose: 2/3 of twilight from 1 pm to 6 pm (1300 to 1800), in the lunar "afternoon"; 1/3 of twilight from 6 pm to 9 pm (1800 to 2100) following the lunar "true" dusk, when the sun sinks below the eastern Nearside rim and Mare Foecunditatis; full moon conditions, from 9 pm to 11 pm (2100 to 2300); and finally a return to "fulldark's" moonless-night dimness, at 11 pm (2300 hours). Remember that every "hour" on the lunar 24-hour clock face is equal to 28 Mystaran hours ... or 168 turns of game-time.
Although the crystal of which the Farside is comprised is remarkably translucent, it isn't 100% clear. Passage through the crystalline bedrock screens out some wavelengths of light sooner than others, much like the passage of sunlight through seawater. Hence, while the light that hits the surface of Matera is white sunlight of all colours, only those wavelengths at the blue/violet end of the spectrum actually make it to the moon's interior; reds, oranges, and yellows tend to be reflected out into space instead, giving the Farside its distinctive "golden" radiance to observers in orbit. Much like in the deep sea, the meagre sunlight of the Hollow Moon shines in deep blues, indigos, and violets. Greens are also able to pass through, but only when they strike the crystal head-on, between "true" dawn and dusk; thus, when the sun breaches the rimlands' horizon and traverses the Firmament, it is visible as a brighter, greenish glimmer amidst the overall indigo glow of the crystal dome.
As any scuba-diver will be able to tell you, the effect of screening out all reddish or yellowish colours from sunlight is to render those colours indistinguishable, on all objects that look red, orange or yellow under white illumination. Odds are that the first thing that will strike PCs as bizarre, apart from the landscape, in the Hollow Moon is how their clothing, vessel, and various possessions magically "change colour": reds and oranges turn black; yellows become blues or (at skybright only) greens; whites or pale greys take on a bluish tint. Flags, banners and coats of arms that were designed on Mystara may become unrecognisable, or impossible to tell apart, due to this colour-shift effect. Spell components may be difficult to sort out, as when the red sand for a Colour Spray spell looks no different from another spell's soot. Vermeil-tinted Inheritors from the Red Steel setting may find themselves looking like AD&D drow! Natives of Matera are used to this condition, of course, and so design all their flags, cultural symbols, and garments accordingly, using only colours visible under the Hollow Moon's natural light.
Magical light, as from a Light spell, and the light of torches and other fires is not subject to this screening-effect; under these types of artificial illumination, things will appear in all seven of the "normal" colours of the Mystaran spectrum. (Red and orange objects also appear reddish by the light of the Hollow Moon's many volcanoes, although only at uncomfortably-close quarters.) Most races who have spent more than a couple of centuries in the HM setting have lost their languages' old words for "red", "orange", or "yellow". Instead they refer to such things as "fire-coloured", in the event they should see them under artificial or volcanic light.
Ultraviolet light travels through the Farside's crystal easily, without the bedrock's posing any significant impediment; thus, while much Hollow Moon UV is blocked by the Nearside's Skyshield, enough reaches the ground to prevent rickets and other metabolic disorders among the inhabitants. (Ironically, it's possible to get a sunburn in the Hollow Moon!) Only in the midlands, where Mystara's stationary gravity has concentrated a particularly-thick atmosphere, is the UV sparse enough for a lack of it to pose a problem for long-term visitors from Mystara. Infrared also passes through the Farside's crystal, with an extraordinary ease. This is one of the reasons -- along with volcanic activity -- why Matera isn't a frozen, dead rock like Luna.