In order to understand the geography of the Nine Worlds, it is crucial to start with Yggdrasil, the World Tree. Conceptions of the World Tree have been found in tribal societies from Siberia to Polynesia; they differ in some aspects, but generally come with some kind of upper world in the top branches of the tree, some kind of ancestral world of the Dead at the roots, and various other worlds in the middle. It has been theorized that they are different trees on the same model, or that they are the same tree existing in many different dimensions, with different worlds in each, which seems more right to me somehow.
We refer to the dimension of the Tree explored by the ancient Norse/Germanic/Saxon peoples as Yggdrasil. Ygg is a byname of Odin, and Yggdrasil means "Odin's steed", a kenning. This doesn't mean that Odin owns or controls the Tree - he doesn't - but he was hung on it once as part of a pain ordeal, and the reference is to him "riding" the tree in this way. The Tree itself is sentient, and has been known to send messages to people (and through them), but the messages are usually long, slow, and cryptic. The Tree is not a humanlike being, and its way of knowing and communicating is much longer and slower than ours.
Yggdrasil exists in a void of nothingness called Ginnungagap. Nine worlds spin around the tree - Asgard, Ljossalfheim, Vanaheim, Jotunheim, Midgard, Muspellheim, Svartalfheim/Nidavellir, Niflheim, and Helheim. When I term them "worlds", it must be recalled that they are exactly that. The ancient Norse, not understanding the concept of a round world, or for that matter anything larger than their own small flat piece of Earth, told of them in the only ways that they could conceive of - as countries or continents on a flat plane. They spoke of moving between worlds as one would move from Germany to France (by crossing a river) or from Denmark to Britain (by crossing an ocean).
Here's where things get strange. The geography and physics of the space around the Tree is not like our extremely physical world. Each of the Nine Worlds is like a small pocket world, with its own time, seasons, year, etc. They don't line up with each other. Between each of the worlds is a world-barrier of some sort. When you pass through it from one world to the next, the time of day and year will be different. In Jotunheim, it's spring and the middle of a morning thunderstorm; in Vanaheim it's early autumn and a clear, crisp evening is falling over the golden leaves. The feeling as you pass from one world to another is subtle but definite. It is more apparent to your astral body than your physical one, and the physical effects of world walking are discussed in the chapters on how to do it.
I've noticed that with a few specific exceptions, the world-barriers tend to be on water - large rivers or ocean. It may be that water holds a world-barrier better, or that it tends to collect around the "edges" of worlds. The idea of three-dimensional worlds that still somehow "butt up" against each other is pretty strange to us, but this is just one of those things that we have to take with the hope that eventually it will be better explained. In the meantime, it's a matter of using what we know, even if we can't understand it.
In talking to different spirit-workers, there seem to be two different conceptions of how the worlds are arranged around the Tree. The first conception sees the worlds as arranged in flat layers, with Midgard in the center of the middle layer. The tree grows up through its middle, and it is surrounded by Muspellheim, Vanaheim, Niflheim, and Jotunheim. Ljossalfheim is suspended above it, with Asgard above that. Svartalfheim is suspended below it, with Helheim below that. The tree pass through the middle of five worlds like a central axis. Some theorists have conceived this arrangement as positively molecular in its neatness. Spirit-workers who actually visit the Nine Worlds admit that it is far less neatly arranged once you are actually there.
The other conception is that the worlds are arranged in a spiral around the Tree, in the order listed at the beginning of this chapter. While one world might be "under" another on the spiral, it can be reached by going off the "far edge" and downward; thus both Jotunheim and Niflheim can technically be to the north of Midgard. In this conception, Midgard is simply one world in a line, rather than the central point.
I should mention now that when we are talking about the relationship of worlds to each other, the concepts of "north", "south", "east" and "west" are not the same as they are in our single physical world. It's more as if one divided the space occupied by the Tree into four quarters and then arbitrarily decided that they corresponded to those four directions -i.e. if we declare this direction randomly south, then the point at ninety degrees to it must be west, etc. We use these terms because they are what our ancestors understood, and they work as well as any for these complicated concepts.
Denizens: Yggdrasil is generally pictured as an ash tree, but this may be human interpretation. On the very top of the tree sits an eagle, Hraesvelg ("Corpse-Eater"), who is actually a wind-giant locked in eagle form. He is very old, of the first generation of Ymir's kin. Winds - or rather, energy currents - blow from his wings, and are controlled by the wind-deer. At the bottom of the tree, crawling back and forth between Helheim and Yggdrasil's exposed root in Niflheim, is the great dragon Nidhogg. She gnaws on Yggdrasil's root, forcing it ever into new growth. Ratatosk, a squirrel-wight whose name means "Teeth That Find", runs up and down the bark of the tree carrying messages (mostly insults) between Hraesvelg and Nidhogg. All three of them do not generally talk to most visitors and are not very approachable.
Four deer run through the upper branches. They are the keepers of the power of the Four winds, and are named Duneyr (Rest), Durathor (Slumber), Dvalin and Dain. Dvalin was once a Duergar, the son of the great Duergar leader Dvalin the Old. Dain was once an Alfar lord. Duneyr and Durathor were both mortals from Midgard. All four agreed to take on the forms of immortal deer and control the winds that blow from world to world.
This does not mean that they necessarily control the atmospheres of the various worlds. However, there are "winds" - currents of energy, really - that "blow" from world to world, and can sweep things and beings with them. Magically skilled folk can learn to "ride" these currents, but attempting to control them for one's own purpose will bring one or more of the Wind Deer down upon you. While they do not engage in combat as such, their control of the winds means that they can sweep you quite literally off of Yggdrasil and into Ginnungagap.
The Guardians of the Four Directions are four Duergar known as Austri, Vestri, Sudri, and Nordri. Actually, it is unknown as to whether they are or ever were actually Duergar, or (more likely) whether they are divine entities who simply take on Duergar form. They are quite capable, and often do, take on other forms as well - elemental spirit-forms, birds, horses, dragons, etc. The Duergar of Nidavellir worship them as gods, and consider them to be divine entities who give the race of Duergar the honor of taking their forms. The four of them can be called on to help out if you have lost your way, as they know all the paths between worlds (and many within worlds) like the backs of their hands. They like to be invoked and poured for at gatherings and workings, but other wise do not require much in the way of offerings.
Niflheim and Muspellheim drifted in and out of each other's orbits. Finally, they drew close enough together that melted ice from Niflheim flowed out over the gap and formed the river Elivagar, whose every particle held a shard of ice-poison. This ice piled up into an iceberg, but it slowly melted as Muspellheim grew closer. As it melted, it revealed the body of a sleeping giant, Ymir. Ymir was an enormous mountain-sized frost-etin, without much of a brain; he mostly lay there semi-paralyzed and mewed.
Also thawing out of the ice was a great cow, Audumhla. Ymir suckled on her udders and found nourishment; she licked at the salty ice for her own food and uncovered yet another creature, a much smaller creature named Buri, who would be the ancestor of the Aesir. Meanwhile, as Ymir slept, his sweat gave rise to a male and female etin, and his two legs rubbed together and gave birth to yet another male giant. Meanwhile, Buri grew up in the company of various giants. (There seems to be some discrepancy about how many giants were actually created; it may be that there were already some about before Ymir, or that more than just three were created. Like all creation stories, the reality of it was lost in the mists of the previous generations.) After many generations of giant-breeding, Buri had a son named Bor by an unnamed giantess, and Bor eventually sired three sons by another giantess named Bestla. These sons were Odin, Vili, and Ve, and they began all the trouble by taking it into their heads to kill the enormous, helpless Ymir.
Up until this point, the two worlds of Muspellheim and Niflheim were separated; the great void that lay between them was called Ginnungagap. It was apparently possible, with difficulty, to pass from one to the other across the Gap. Some giants had already done so, living under the leadership of Surt the Black, and were busily evolving into fire-etins. The sons of Bor apparently felt that more useful land was needed, so they killed Ymir and used his corpse as a variety of magical anchors to create new worlds. Some pieces floated off and created worlds by themselves - Svartalfheim, populated by duergar who also sprang from Ymir's body, is one example, Jotunheim is another - and some, like Asgard and Alfheim, the sons of Bor created themselves, for Bor's people and the otherworldly Alfar.
The problem was that Ymir's blood gushed out and caused a huge flood, washing away most of the unwarned denizens of the two worlds. It was so great that it actually put out the fires of Muspellheim for a time, and plunged the world into darkness. The sons of Bor worked fast to triage the damage; Ymir's body was placed across the gap to form a world-bridge, but it fell apart. They grabbed for whatever pieces they could get, and anchored them to the Tree. Meanwhile, Bergelmir the leader of the frost-etins shoved his wife and whatever of his people he could find into a lur, a hollowed-out tree trunk that could serve either as a boat or a coffin. They washed up on the piece of Ymir's back that would become Jotunheim.
The sons of Bor continued their world-building, creating skies and solid ground, anchoring worlds to the Tree, and making stars out of sparks from half-drowned but still-sputtering Muspellheim. Ymir's broken bones made mountains, his teeth rocks and boulders, and when the final flood of his blood went down, the remaining fluid made the rivers, lakes, and oceans of the various worlds. Ymir's brains were flung into the sky, where they became clouds. The sons of Bor then called into the world by some arcane process the Four Guardians of the directions, whom they charged to guard the four corners of the World Tree.
There are conflicting stories about the creation of humans. One says that after having finished their world-building, the sons of Bor were walking along the beach and came upon two washed-up logs. These they breathed life into - Odin giving them breath, Vili giving them movement, and Ve giving them identity - and the race of humans was born from these first two human, Ask and Embla (Ash and Elm). On the other hand, there is a conflicting account that states that Ask and Embla were actually called into this world before the great flood, and that they were saved by the giant Bergelmir, who stuck them high in a dead tree that he hoped would survive the flood. With this other account, there is no knowledge of who actually called them into the world; both the sons of Bor and the giants take credit for it. At any rate, Midgard was created as a place for them to live.
Time And Seasons: Yggdrasil itself is suspended in space, in the black nothingness of Ginnungagap. It has no seasons per se; the Sun and Moon-chariots pass from world to world around its trunk, but outside them, there is no discernable seasonal change. As the worlds rotate around Yggdrasil, however, each of them comes close to our world and then draws apart. Midgard is always close, for some reason, which is why it is referred to as our "sister world". (For more information on this complicated concept, see the Midgard chapter.) It is possible to visit any of the worlds at the time of (our) year that they are furthest away, but it is simply easiest to do it when they are close. Sometimes, during that period, they come so close that things "leak" over. For example, "interference" from the Elves on Beltane has a long and legendary history.
Yggdrasil itself is a source of light, as is the land of Muspellheim, and these were originally the only source of light. However, when the last worlds were built, the Aesir decided that there needed to be a more regularized and brighter light source, and they recruited (some tales imply a nonconsensual draft) certain etins to eternally ride the skies, bringing greater and lesser light to all things in turn. The Sky-Etins are not a specific variety of jotun; rather, they have created their own tribe and culture out of necessity. They are a close-knit clan, and while they are interested in the problems of those below - and who wouldn't be, with all that perspective and vision during every day? - they will not brook interference with their schedules, because they know that granting it to one would mean granting it to all.
Before the Aesir set up their day-and-night technology, the three ancient worlds were lit by the pale greenish light of the tree, and the glowing fires of Muspellheim. This gave a permanent twilight to Niflheim and the underworld. After the flood, Jotunheim was formed, and as it was closer to Muspellheim it got more light, a dull orange glow seeping through the world-barrier. When the Aesir set up Asgard, they put together a system whereby the Sun and Moon roll through the sky of each world on a set path.
The gods of the Sun and Moon are Sunna and Mani, respectively. They were originally the children of a giant named Mundilfari, who was known as the Turner of Time. They were snatched up by the Aesir to forever ride the sky in turn. Their track takes them through all the Nine Worlds; while Mani is over one world, Sunna will be over another one. This strange schedule reminds us that these are actually separate worlds and not just regions of a single world; although they may share an artificial sun and moon, they do not exist in the same space together. Sunna can be over Midgard for fourteen hours, and then pass through eight other worlds with varying day-lengths spent in each one, and then be back in Midgard ten hours later. This is due to the time dilation that occurs as she passes through the world-barriers. One circuit for her may be several days' worth of ride, yet each world is only in darkness for the length of its night. It's one of those things that make your head hurt if you think about it too closely.
Sunna is tall, golden, and beautiful, and has been called All-Bright, Everglow, and Fair Wheel. Her personality is impulsive and fiery, almost childlike in her innocence and enthusiasm. She is married to a fire-giant, one of the sons of Surt, whose name is Glow. Since her job never stops, he generally visits her in the chariot. (One assumes that she gets the day off occasionally.) He cares for their young daughter, who is being groomed to take over Sunna's dangerous job should something happen to her (such as Ragnarok).
Sunna charges across the sky every day with wild abandon, never swerving from her path in the sheer love of chariot-driving. Her Sun Chariot is drawn by two golden horses, Allsvinn (All-Swift), and Arvaker (Early-Waker), who pull the sun behind them. Its heat would be too much for them, except that Odin created a talisman called the Isarnkol, which hangs above their shoulders on the double yoke and constantly spreads cool mists, protecting them from heatstroke. Ahead of Sunna, as a herald, travels Daeg, or Day, a sky-etin who rides the red-bay horse Skinfaxi. The horse's mane sheds light as it goes.
Mani is calmer and more thoughtful than his sister. He has a compassionate heart, and is struck by seeing those in need, especially children who are being abused, and he is often prayed to in order to protect specific children. He is known to have rescued two children, Bil and Hjuki, snatching them up from the earth and sending them to live in Asgard. (Their names seem to be the foundation of the "Jack and Jill" rhyme.) He is also a god of calendars, mathematics, and other rational thought that requires counting and numbers. He plays a flute as he walks, and is especially revered by the Dark Alfar and the Duergar. Those who work with Mani say that his moon-cart is pulled not by horses, but by large dogs, and sometimes he simply walks, being a patron of travelers and walkers in general.
Mani is preceded and heralded by Nott, a Jotun goddess who rides a black horse named Hrimfaxi. The dew drips off of Hrimfaxi's bit as he passes over the worlds. Nott's father Norfe was the first architect of Jotunheim, and designed many of the great halls, including the halls of Thrym and Utgard-Loki. Nott herself is a very old giantess, one of the oldest from before the flood, which she survived by being in the realm of the Dead at the time. She has been married three times (that we know of) and had numerous affairs. Her first husband was named Nagifari and to him she bore a son named Aud; her second was a cliff-giant named Annarr who sired on her the earthy etin-bride Jord, eventual mistress of Odin and mother of Thor and Meile. Her third and current husband, Delling ("Dawn") gave her a son who later become Dag, the god of Day and Sunna's herald. She also had an affair with the old Vanir-god Frodi, and bore him Njord the sailor-god before leaving him with the infant boy. Nott is not known for being maternal; she tends to be a wanderer, leaving her various children to be raised by their fathers.
To make sure that the Sun and Moon chariots ran on time, the Aesir made a deal with two fierce Jotun werewolves, Skoll and Hati, sons of Fenris by an Iron Wood giantess. They gave them the power of flight, which they can use only to chase the sun-chariot or the moon-chariot. The truth is that neither of them spend their entire time chasing chariots; they are simply on call if something goes wrong and the moon or sun are not on time. Sunna is almost always on time; if anything, she tends to be early in her enthusiasm. She treats Skoll, the wolf who is in charge of chasing her, like a fun, competitive game, and enjoys racing him. Mani, on the other hand, is much more interested in what is going on down below, and as such has a regrettable tendency to stop and watch, or even interfere. Hati, the greater of the two wolf-brothers (also referred to as Managarm), will eventually show up and chase him back onto his scheduled path. Mani despises the wolf, and hates being shepherded in this way. He is also well aware that should Ragnarok start, the two wolves will attack and kill them. (Sunna seems to have a "let 'em try to catch me!" attitude about that.)
The amount of time that Sunna and Mani spend in each world varies wildly from world to world. Asgard gets the most light, with some days as long as summer-solstice days at our Arctic Circle, without the accompanying long nights during the rest of the year. This is one reason why it is the "Shining Realm". Ljossalfheim also gets a great deal of light, and Vanaheim somewhat less but still more than Midgard.