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Hollow Moon Quick Reference Guide

by John Calvin from Threshold Magazine issue 15

The following quick reference guide lists important terms and concepts relating to the Hollow Moon setting.

Lunar Geology

Lunar Crust/Crystal Bedrock

The lunar shell is composed of a bluish, magical, nearly indestructible crystal. The crystal is cold to the touch and prolonged exposure will lead to hypothermia and death, preventing most life from living directly upon it.


The hemisphere of the moon that faces toward Mystara, the Nearside of Matera (both inner and outer) is covered in a layer of encrusted basalt, out of which volcanoes, mountains, hills, and plains are formed. It is only on the Nearside of Matera that life can be supported1.


The outer region of the encrusted basalt “bowl” that forms Nearside’s interior, the Rimlands abutt the crystal firmament along Matera’s equator.


The center region of the encrusted basalt “bowl” that forms Nearside’s interior.


The portions of the Nearside that remain exposed crystal bedrock are called the Crystalbarrens. Most of the oceans in the Hollow Moon cover such areas, but some remain completely exposed, including Mare Procellarum (the largest such area).

Farside/Crystal Firmament

The hemisphere of the moon that faces away from Mystara, is called the Farside, or the Crystal Firmament. Fully 80% of the Firmament (both inner and outer world) is exposed crystal bedrock, with very few encrusted basalt islands to support life.

Crystal Fractures

Fractures in the Crystal Firmament caused by a prehistoric comet crashed onto the outer surface of Matera, which are used by Materans as constellations to mark the passage of time throughout the lunar day.

Lunar Directions

Cardinal Directions

The terms east and west are reversed inside of the Hollow Moon, since like in the Hollow World, the inhabitants are on the inside of the moon’s shell. Likewise the directions north and south hold little meaning for Materans, who are more concerned about how close or how far one is from the center of the Nearside.


This describes the direction one would take to travel toward the center of of the midlands. Also sometimes referred to as Spinward - as the Great Spindrift Sea lies at the center of this region.


This is the direction one would travel when heading toward the rimlands and the Firmament.


This is the direction toward the location of the rising sun in the Materan west, over Mare Procellarum. Travelling farward to the west is the same as travelling dawnward.


This is the direction toward the location of the setting sun in the Materan east, over the rimlands past Mare Fecunditatis. Travelling farward to the east is the same as travelling duskward.

Lunar Light


Sunlight entering the Hollow Moon is filtered by the crystal firmament and is not the white light common on the surface of Mystara, but rather a muted bluish green color at its brightest. During skygright, sunlight shines down upon the nearside from the crystal firmament, while during fulldark, it passes up through the crystalbarrens and seas of the Nearside.


The Hollow Moon’s many volcanoes provide localized lighting, as each cycles through constant and predictable stages of activity. While the crystal firmament filters out most light in the red-orange-yellow spectrum, volcanoes produce this color of light internally, and so are one of the few locations where such colors can be seen naturally.


Much of the flora and fauna of the Hollow Moon have been altered by Ordana to produce bioluminescent light. This is especially true in sea life, but land based flora and fauna may also sport luminescent patterns across their bodies which may aid them in finding prey and mates.

Artificial Light

Artificial light may be produced to varying degrees in many of the cities in the Hollow Moon, often used to light streets or to aid with reading. Cooking fires are also common, but generally kept small and hidden when necessary, as artificial light can draw unwanted attention from predators adapted to the dark.

Lunar Time

Lunar Day

A lunar day, from one fulldark to the next, lasts 28 Mystaran days. During the first 14 days light increases, finally reaching its height at skybright. During the last 14 days, light begins to diminish until the sun sets at dusk and eventually returns to fulldark.


Skybright is midway between dawn and dusk and is the brightest period of the lunar day producing a bluish green lighting equivalent in brightness to twilight on Mystara. This is the time of a New Moon.


Fulldark is midway between dusk and the new dawn and is the darkest period of the lunar day, though not completely lightless. Light shines upward through the crystalbarrens and most of the Hollow Moon’s seas. This is the time of a Full Moon.

Lunar Year

The Hollow Moon year is 18 months long, each of which corresponds to a single lunar day, and is governed by Patera’s orbit around Mystara and its proximity with Matera. Patera’s orbital period controls the tidal cycle inside of Matera, which shifts from north to south during lunar night and south to north during lunar day.

Lunar Seasons

Storm Time

Temperatures fluctuate throughout the lunar day creating weather patterns in conjunction with the tides. These storms range from mild to severe, but occur on a predictable schedule. Storms that form during dark periods tend to bring snow, while those that form during light periods bring rain, sleet, or ice.

Fire Time

Several times per year, Patera passes close enough to Matera to induce periods of intense geothermal activity causing volcanic eruptions throughout the Hollow Moon which are referred to as Fire Times by Materans. Individual volcanoes erupt according to a predictable schedule ranging anywhere between 1 and 100 Materan years.

Circle tides

These are three-and-a-half day tidal phenomenons in which all tides, winds, and cloud formations circulate around Nearside in the same spiralling pattern. During the month of Itiukkin the tides spin counterclockwise, and during Itinissatu they spin clockwise.

Doom Times

One event in the Hollow Moon can never predicted, and scares most Materans to death - a lunar eclipse. On those rare occasions when Mystara fully eclipses the moon, volcanic activity (and the utter destruction caused by it) reaches its zenith.

1For more detailed information about Hollow Moon geography see, Hollow Moon Planetology: Blue Moon - Lunar Geography by Sharon Dornhoff at The Vaults of Pandius