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Hollow Moon Planetology: Blue Moon - Seas & Tides

by Sharon Dornhoff

Seas of the Hollow Moon

There are four saltwater seas in the HM setting, none of which -- given the small size of the moon as a whole -- can really be considered an "ocean", by Mystaran standards. To Materans, however, the Great Spindrift Sea certainly LOOKS like a full-fledged ocean -- complete with tides, deepwater currents, and a wide range of temperatures: from arctic conditions on the fringe of the crystalbarrens, to subtropical ones in Sinus Medii at the very centre of the Nearside. Such a drastic variation in climate, over such a small area, is made possible by the extra-dense atmosphere of the midlands, which traps both heat and humidity over Sinus Medii at the centre of the sea; and by the refrigerating conditions of the open crystalbarrens, along the ocean's outer rim.

The Great Spindrift Sea varies in depth from a hull-scraping couple of fathoms out along the edge of the ice-flats, to approximately the same as Mystara's continental shelves (as the abyssal depths of the Hollow World already provide a nice, twilight sanctuary for dark-loving deepwater species, there's no need for deep seas in the Hollow Moon!). Like all the lunar seas except Humorum, its bottom is of exposed, chilly crystal -- prevented from icing over by water pressure -- and is quite flat and smooth, beneath a superficial scattering of sand-flats and basalt islets. Apart from the ice-flats, its coastline consists of estuarine marshes -- "the Rilles" -- in the region southeast of Mare Vaporum; this gives way to a rockier, jagged shore with numerous capes and sea-caves, along Sinus Medii and southward to Mare Nubium. To the north, the Apennines have built up an impressive underwater massif of basalt, by their volcanic activity, all along their southern slopes; this region, unlike the better part of the Great Spindrift, is never illuminated from below, and so remains pitch-black even when sunlight shines up from beneath the seabed. The rest of the ocean is treated to the sun's direct rays for several Mystaran days each month, and can thus support a vibrant ecology of bottom-dwelling algae, marine life, and Matera's unique light-gathering, siliceous corals.

The second largest body of water, and second deepest, is that which encompasses both Maria Tranquillitatis and Foecunditatis. Like the Great Spindrift, they have crystalline bottoms and are quite bountiful in their sea life -- especially Foecunditatis, which is the first of the eastern seas to catch the warming light of "true" dawn every month, and which lies directly upon the equator where algae can thrive in such direct rays. The two seas are joined by a strait that passes between the smouldering volcano Mt. Taruntius and the sandbar-encircled Cape De Vinci* ... a perilous arrangement, and one which is often treacherous for shipping. The bustling, mercantile cities of the Cacklogallinians -- their streets, dockyards and markets, permanently illuminated for the benefit of night-blind human and avian eyes -- can be seen from many points within the Hollow Moon, on the coasts and promontories of these two great eastern seas. The isthmus which sets them apart from freshwater Mare Nectaris is low and hilly with only a few volcanoes, and the sandy shores of the northeast are gentle; but their other coasts are generally rocky and steep, making safe harbours few and far between.

[* - Gosh, these moon-map names are fun! ;-D]

Foecunditatis is so heavily-grown with coral, that its seabed is actually much more shallow than its sister-sea's -- this, despite the fact that all HM crystal bedrock is at constant depth -- and atolls and reefs have added onto the many small islands already found there; thanks to the glowing corals' transmission of light-rays, life on the Foecunditatis seabed can take in sunlight from both Firmament and sea floor, and is every bit as verdant as that which grows on a Mystaran continental shelf! There are far fewer islands and crust-flats on the coral-sparse, sandy bottom of Tranquillitatis, and its ill-lit depths are a mystery to the city-dwellers. They also don't know that the northern edge of Tranquillitatis -- a high, unbroken ridge connecting the Archerusia Promontory to the opposite shoreline -- is a deliberate artificial creation of the Immortals: one that keeps the two seas from flooding the Mare Serenitatis crystalbarren. It's also one of the few pieces of geography that the Immortals actually changed, while making the Hollow Moon habitable -- unlike the Hollow World, where Ka went allosaur-wild, moving mountains around! ;-) -- and is the reason why Mare Tranquillitatis (which has an entire sea on its inner surface) looks much, much darker than Mare Serenitatis, to Mystaran astronomers.

The other eastern sea is Mare Crisium, which is noted for its extreme clarity and shallowness. Its waters are so clear, due to a comparative lack of silicates and other minerals, and its depths so slight -- about 40', which is easily within skin-divers' range -- that the light which shines up from the seabed at fulldark reaches the surface almost undiminished. Conversely, the glow of the Firmament reaches the Crisium floor so easily, a tiny fraction of it even penetrates the crystal and reaches the outer moon! (One of the "Adler's" crew actually observed this eerie phenomenon on Matera's exterior, although Mystaran astronomers can't see it ... it doesn't make it past the planet's atmospheric haze.) Only a paucity of minerals keeps corals and algae from taking over the depths, under such circumstances ... that, and the careful husbandry of an amphibious nation of shark-kin, who claim Crisium and the surrounding shorelines for their own. Bright, clean, and lagoon-like, Mare Crisium would be the kind of paradise that Mystara's Ierendi islanders would love ... if only those crystal-clear waters (55 degrees F) weren't so darn chilly.

The remaining saltwater sea, Mare Humorum, is Matera's deepest body of water, and is every bit as murky and dim as Crisium is clear and bright. Surrounded by high mountains, it's far too deep for divers without magical means of pressure-adaptation to reach the sea floor. Not only that, but there's a big canyon-like fissure, within the crystal seabed, that opened up as a result of stress-fracturing when Mare Orientale's entryway was created. That trench, Venturer Deep, is 3150' long and the Immortals-only-know HOW deep: the Humorum shark-kin -- an entirely marine culture that's a LOT more aggressive, than their cousins from Crisium on the opposite end of the Nearside -- have never been able to find its bottom ... and they can dive as deep as a mile and a half! Mare Humorum is also beset with aquatic monsters of all sorts (the weirder, the better ... make stuff up! ;-D), treacherous currents, and very unproductive, low-oxygen waters -- adversities which the stoic shark-kin take deep pride in their ability to endure, but other races won't find at all inviting.

Tides in the Hollow Moon

While Mystara's tidal cycles (and the Hollow World's, for that matter) are caused by Matera's gravity, it's Patera -- Matera's invisible sister-moon -- which accounts for the rise and fall of the seas, within the Hollow Moon. Because the Nearside is always facing Mystara, the planet's gravity always pulls on Matera's landscape and seas in exactly the same direction; it keeps the Midland Ocean in the midlands, and makes the atmosphere considerably thicker there, but it doesn't move HM water or air from one place to another.

Patera, on the other hand, has a powerful tidal influence on Materan seas and weather ... an influence which the Immortal Seshay-Selene deliberately amplifies, in order to keep the Hollow Moon's climate, geological activity, and hydrology dynamic and vital. If physics, alone, were allowed to run its course, Matera would be a frozen, dead world, hollow or otherwise; it's by Immortal intervention, and the re-directed gravitational force of Patera -- a tiny moon, but one with a high-gravity core we already KNOW is magical -- that the HM setting remains livable to anyone but the desert ghosts (who'd probably think a frozen, dead moon is preferable to a live, wet one :-D).

Patera, as CoM states, has an orbital period of three and one-half Mystaran days, and it traces a circumpolar path in its journey around the planet. To this, I'm adding just one new detail*, never before observed by Mystaran astronomers (easy enough, with an invisible moon!) -- every time Patera orbits the planet, it also "follows" the bigger moon Matera longitudinally around Mystara, moving two-and-a-half degrees eastward for every orbit it completes. In any given 28-day month, Matera orbits 360 degrees around Mystara and experiences a single lunar "day"; in that same period, Patera orbits Mystara eight times, and also moves 20 degrees around the planet's longitude. Although every orbit carries it over the North and South Poles of Mystara, it takes 18 months, and 96 circumpolar orbits, for Patera to return to its starting point at the same vector of movement it was on previously.

[* - This partly contradicts the CoM information on celestial phenomena as seen from Patera's surface (e.g. the sun not only "wobbles", but it should also vary dramatically in its position relative to Mystara). However, CoM also states that the Pateran calendar is so complex, it takes a year for someone to learn it -- translation: "We had better things to do at TSR than figure out something so arbitrary and/or silly!" :-) -- so adding in this new factor isn't likely to have a significant effect on anyone's campaign. It even makes Patera's "year" more colourful, as its orbital orientation at a 90-degree angle to the sun, which happens a number of times each year, would produce a similar effect to the arctic "midnight sun" IRL.]

For the Hollow Moon, this means that Patera's tidal effects are strongest whenever the invisible moon crosses Matera's orbital plane within a few degrees of its larger sister-moon, and mildest when it's at a 90-degree angle or nearly that (when the orbiting invisible moon heads northward at a 180-degree angle, of course, it'll then come close to Matera on its southward pass, and vice versa) to Matera's location. If Patera comes close when it's moving from the South Pole to the North Pole, it'll pull the seas and atmosphere south-then-north; if it's heading in the other direction, it'll pull them north-then-south. Twice in an 18-month orbital cycle -- the (seasonless) HM setting's "year" -- Patera moves through a complete orbit at a 90-degree angle to Matera, and generates what are called "circle tides": a three-and-a-half-day tidal and weather phenomenon that's unique to the Hollow Moon, in which all tides, winds, and cloud formations circulate around the Nearside in the same spiralling pattern, moving counterclockwise on the first yearly occasion and clockwise nine months later.

Believe it or not, I actually fiddled around with the math until I'd worked out the whole course of the 18-month Materan year*; and the same pattern consistently showed up, as to when the two moons come near enough for the Hollow Moon's weather, tides, and geological activity to be affected. I'm planning to chart out the whole thing -- tides, weather, and even natural disasters (!) -- in an upcoming post; but for now, I'll just say that over the course of an 18-month "year", tides tend to go from south to north in the lunar "night-time" (a 14-day period!) and north to south in the "daytime" (ditto) for the first three months; they slack off considerably for the next three, with a circle tide taking place at skybright in the 5th month; and then the seas move south to north by day and north to south by night, for the next three months. The following nine months do exactly the same thing, but in reversed order, with a circle tide at skybright on the 14th month of the year.

[* - I have no life. :-P]

Why did I go to so much trouble, just to work out the Materan tides ... something that most DMs, and players, couldn't care less about? Frankly, if it was JUST tides, I wouldn't have. But as it implies above, the Hollow Moon's tides aren't the only thing, that depend on Patera's gravity to keep them moving! In an environment as tiny and confined as the HM setting, weather-patterns don't have the complexity of what you'd probably see on a huge world, with globe-spanning oceans and multiple continents. Materan storms don't build up over time, like Earth's do; they are spontaneously generated, VERY quickly and abruptly, by the same gravitational shifts that move the tides around. Storms, blizzards and floods are extremely common, in the volatile Hollow Moon, yet there's a regular pattern to their occurrences which diminish the danger they pose to its inhabitants. Even the hundreds of volcanoes which cover the moon's inner landscape are only likely to erupt when the moons are in close proximity, which occurs on a frequent but PREDICTABLE basis! In short, the environment of the HM setting -- for all its extremes of climate, geological upheavals, and perpetual state of disquiet -- can't really take the natives by surprise, with its outbursts of violence ... just moon-ignorant adventurers from Mystara, who wouldn't recognise a Storm Time if it bit them!

(Oh... and except for the eclipses. Materans don't like to talk -- or even think, much (shudder...!) -- about them. ;-D)