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Hollow Moon Planetology: Blue Moon - Timekeepingby Sharon Dornhoff
Timekeeping and the Lunar "Day"
Whereas the Hollow World setting (and the original "hollow world" of fantasy, Pellucidar) gives visitors a sense of perpetual timelessness -- its inhabitants wouldn't know what to do with a clock, if they had one! -- the inner world of Matera, like the outer Mystara, has a day/night cycle. Light levels rise and fall, depending on how great a fraction of the outer Moon's translucent Farside is facing the sun; to viewers within the moon, the patch of brighter, greenish light that represents the HM "sun" moves across the Firmament. Even during fulldark, when sunlight shines on the outer Nearside and the Firmament goes black, faint shimmers of light gleam up through the crystalbarrens, tracing the unbroken course of the sun's rays across the Materan surface. Lunar "days" are a lot longer than Mystaran ones -- indeed, I'll be using "lunar day" and "month" pretty interchangeably, from here on in -- but they still give HM cultures a strict awareness of time's passage that only the most astronomically-inclined Hollow Worlders, such as the Azcans, even come close to having. (Having to keep track of when the next volcano's gonna blow is a great motivator, for making sure you know what day it is!)
To keep things simple, each "lunar day" in the HM is 28 Mystaran days (i.e. 672 hours, or 40,320 minutes) long*. For slightly less than half of that time, the patch of greenish brightness which constitutes the "sun" is visible to Nearside observers, tracing its path along the equator of the Farside. From "true" dawn to "true" dusk -- 6 am to 6 pm on our ol' 24-hour clockface, as described under "Lighting Conditions" -- the progress this light-patch makes in its journey can be used to measure the passage of time. During most of the remainder of each day/month, although the patch of green light is not visible, the terminator between light and darkness can still be distinguished: it's where the indigo glow of the illuminated portion of the Firmament gives way to unlit blackness. Only during the 56 hours of fulldark, when the sun is directly "beneath" the Nearside and no light can reach the Farside, does the Firmament offer no clues at all as to how much time has gone by, in the course of the lunar day.
(* - Technically, this should actually be a little bit less, since sidereal months (e.g. months from the moon's point of view) are shorter than the months seen from planetside ... but timekeeping works a lot more neatly this way, it makes it easy for the lunar and Mystaran calendars to be synchronised, and darned if I'm going to crunch all those numbers, again!)
The mere passage of light across the Firmament, in itself, wouldn't necessarily tell a Materan all that much about time's passage... at least, not enough to "tell time" with any great precision. Remember that the "daylight hours", from a HM point of view, actually last for a whole two weeks! That means that the green light-patch seen beyond the Farside's crystal moves only about 1 degree every 12 hours ... far too little to distinguish any difference with the naked eye. Even sundials won't work much better -- not even if the viewer's vision is light-sensitive enough to detect the faint shadows of the Hollow Moon setting -- because the sun doesn't move enough, from one hour to the next, to allow for anything more than a good guess as to which 6-hour interval you're living through.
That's where Mare Orientale comes back into the picture. Or rather, the pattern of cracks the original Orientale cometary impact created, when it smashed its way through the crystalline bedrock of Matera. This pattern of embedded fracture-lines*, which run three-quarters of the way across the Farside and become clearly-visible as sunlight shines through the Firmament, offers an irregular 'gridwork' of lines, forks, and jagged angles, against which the "sun's" progress can be measured and anticipated. Whenever the light-patch passes directly behind an individual crack, refraction resulting from the discontinuity in the crystal causes the fracture's entire length to shine green; as the angle of incidence of the sun's rays changes, particular lesser cracks become starkly-visible, while others vanish into the background. Even in the dim hours of pre-dawn, the indigo gleam of the western Firmament reveals such transient cracks in a display which changes from hour to hour. Only during the slim window of time when the "sun" has set beneath Mare Foecunditatis, and the terminator has moved past 3 pm on our made-up clockface -- from 9 pm until 1 am, when the western Nearside rim again begins to brighten -- does the crystalline fracture-pattern cease to be thrown into sharp relief, by the sun.
(* - Check out photos of Europa, the ice-covered moon of Jupiter, to see what such enormous fractures might look like.)
Materans have been watching the sun's traverse, and studying the fracture-lines, for generations; in fact, they name cracks in the Firmament, in much the same way that people of Mystara or Earth name the stars! Likewise, major confluences of these cracks are given names based on what they resemble -- the Hawkbat, the Ship, the Horse (a fantastic beast from ancient Vedali folklore, universally held to be extinct ;-D) -- just as outer-world cultures imagine pictures in the constellations. Moreover, primitive HM societies use the fracture-lines as their only means of accurate timekeeping, on less than a month-by-month basis: they'll arrange to rendezvous with others the next time a certain crack goes bright or falls dark, for example; or they might tell travellers that the next village lies within "the span of the Ship's shining", meaning that it'll take as long to walk there as it does for the sun to cross that specific array of cracks. Materan tribal cultures don't have any units of time that correspond to the Mystaran hour or day; sleeping-patterns vary so much, from one race to the next, that concepts like "sleeps" aren't nearly as useful among the diverse species of the Hollow Moon as they are, for the mostly-human inhabitants of Mystara's interior.
Apart from the fixed duration of a month, and the 18-month lunar "year", the most primitive societies of the Hollow Moon have only one reliable time-unit: the "peeent". This word -- which DMs should render as a non-verbal, cicada-like buzzing; it's onomatopoeic -- refers to the duration of a certain, ubiquitous lunar insect's mating call, which can be heard buzzing away practically everywhere except on the open crystalbarrens. A "peeent" is 3 seconds long.
The gnomes of the Taurus Mounts have perfected the art of clock-making, and such chronometers are readily available in the Cacklogallinian trade-cities. Lunar clocks use a time-unit based on how long any given equatorial fracture-line in the Firmament remains bright, when it's illuminated from behind. Thus, this unit -- the "glitra" -- is 56 minutes long ... the time it takes the solar disk to cross a given point in the skies of outer Matera. There are 720 glitrae in each month. A typical routine for inhabitants of the trade-cities is to sleep for 9 glitrae and then be awake for 18, giving them a full "daily" schedule of 25.2 hours*; as in the Hollow World, activity patterns are staggered so that about one-third of the populace is sleeping at any given time. Gnomish clocks have a nine-glitrae face, on which they also monitor peeents and the months of the Storm Cycle.
(* - This is close to what IRL humans revert to, when they've lived in caves as an experiment about human circadian rhythms, or are otherwise cut off from the 24-hour light/dark cycle found in nature.)
The lunar day/month is further subdivided into four "quatri", of 180 glitrae each (i.e. a Mystaran week). Because this is only twenty 9-glitrae periods, urban Materans who are asleep during the first time-block of one quartus will sleep during the second time-block of the next one, and the third, of the quartus after that. This staggering of activity-patterns across quarti means that every city-dweller must carry out his or her fair share of working during fulldark ... a time when many of the less dark-adapted residents -- particularly the albheldri -- would rather be abed. It also places the burden of preparation for Fire Times on the backs of different people each time, which is part of why the lunar system of timekeeping was designed to work in this way.