Hollow Moon Planetology: Blue Moon - Seas & Tides
by Sharon DornhoffSeas 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)