There is no moon.

[Mystery Man]

The Earth would spin on an axis, just as it does, now. However, precession would be outrageous. The "poles" would -- over time -- drift all over the place. It is theorized that this stabilizing effect of the Moon is one reason why life was able to take hold on Earth.

Well if it drifts all over the place it's not much of an axis now is it?!?

From the Encyclopedia Brittanica.... edit: I removed the links to figures and tables.

Principal characteristics of the Earth-Moon system


The movement of the Earth-Moon system is illustrated in Figure 1, and the Table lists some salient characteristics. The two bodies orbit each other about their centre of mass—called the barycentre—a point inside the Earth about 4,700 kilometres from its centre. The distance between the centres of mass of the Earth and the Moon varies rather widely due to the combined gravity of the Earth, the Sun, and the planets. For example, during the period 1969–2000, apogee (the greatest distance) varies from 404,063 to 406,711 kilometres, while perigee (the least distance) varies from 356,517 to 370,354 kilometres. Tidal interactions have braked the Moon's spin so that presently the same side always faces the Earth. As discovered by Gian D. Cassini in 1692, the Moon's spin axis precesses with its orbital plane; i.e., its orientation changes slowly over time, tracing out a circular path. The orbital geometry gives rise to the Moon's phases and to the phenomena of eclipses. When the Moon's orbital line of nodes (the points at which the Moon crosses the ecliptic plane) coincides with the Earth-Sun direction and the Moon is near a node, a solar or lunar eclipse can occur. These events happen in groups, sometimes called the Saros cycle, about every 18.6 years—the period of precession of the lunar orbital plane.

In accord with Kepler's laws, the eccentricity of the Moon's orbit results in faster and slower motion along the orbit. Combined with the Moon's constant spin rate, this gives rise to an apparent libration, enabling more than half of the lunar surface to become visible. In addition to this apparent motion, the Moon actually does rock slightly to and fro in both longitude and latitude, and the observer's vantage point moves with the rotation of the Earth. As a result, more than 59 percent of the lunar surface can be seen at one time or another from the Earth. The orbital eccentricity also affects eclipses of the Sun. If a solar eclipse occurs when the Moon is near perigee, observers along the Moon's umbral (dark, inner) shadow path see a total eclipse. If the Moon is near apogee, it does not quite cover the Sun; the resulting eclipse is annular, and observers can see a thin ring of the solar disk.

The Moon and the Earth presently orbit the barycentre in 27.322 days, the sidereal month. Because the whole system is moving around the Sun once per year, the angle of illumination changes about one degree per day, so that the time from one full Moon to the next is 29.531 days, the synodic month. These periods are slowly changing with time owing to tidal interactions. Tidal friction slows the Earth's rotation, but the angular momentum of the Earth-Moon system remains constant. Consequently, the Moon is slowly receding from the Earth, with the result that the month and the day are both getting longer. Extending this relationship back into the past, both periods must have been significantly shorter hundreds of millions of years ago, and this hypothesis is confirmed by measuring the diurnal and tide-related growth rings of fossil corals.


Because the Moon's spin axis is inclined only 1 1/2° from the normal to the ecliptic (see Figure 1), the Moon has no seasons. Sunlight is always nearly horizontal at the lunar poles, resulting in permanently cold and dark environments.

 

[Desdichado]

This is true of Mercury, but Venus and Mars don't follow that pattern. I'll have to do some looking to see if there are any currently viable explanations for that...

Well, true Venus isn't in resonance with the sun per se. Most astronomers seem to believe that it's long rotation is a "resonance effect" -- i.e., a result of Venus gradually "locking into" resonance.

Mars is even more interesting. Although it's a still not quite a mainstream theory today, some folks have proposed that it's rotation is the result of the massive collision in the Hellas Planitia.

Of course, the creation of the Moon is theorized to have occured when a Mars-sized object struck the earth, which could have had profound impacts on the rotation of our own planet as well.

 

[Liolel]

You'll have to figure out new conditions for werecreatures to change shape.

I don't think you need to find new conditions for were creatures to change. What you need to do is throw them out the window. They will lose all the flavour and such that makes them intresting and no matter what new condition you come up with the creatures no longer a werewolf.

 

[Psion]

I don't think you need to find new conditions for were creatures to change. What you need to do is throw them out the window. They will lose all the flavour and such that makes them intresting and no matter what new condition you come up with the creatures no longer a werewolf.

And there is no other possibilities for flavor? Such as in eastern mythology where creatures who seek to upset the natural order take human shape?

 

[Umbran]

Tidal friction slows the Earth's rotation, but the angular momentum of the Earth-Moon system remains constant.

I should note that "tidal friction" is not friction between the ocean and the crust. Remember that the entire crust of the Earth is rather like the skin or an apple - compared to the mass of the planet as a whole, the oceans are mostly negligible.

While the ocean tides are the things most obvious to us humans on a daily basis, the tidal action of the moon does act on the whole of planet Earth, deforming it somewhat. If you deform the planet, some of the bits inside it rub together, and that's the major tidal friction.

Now, here we get to something important - that intrenal friction may be required for life. There are only two things that keep the planet's core molten in the long-term: Tidal friction and decay of radioactive isotopes. If you don't have a molten core, your planet is tectonically dead - no vulcanism, no plate tectonics. If it's tectonically dead, you don't get new landmass built. You don't get replenishment of the atmosphere and recycling of carbon. Eventually, you get a planet that looks a lot like Mars today - no liquid water, thin unsupportive atmosphere.

 

[fusangite]

I'm getting a sense from this thread that everyone else runs Newtonian heliocentric universes. Am I really that exceptional?

One thing I omitted from my post: one can use the Copernican modifications to Aristotle's system and run a heliocentric (rather than geocentric) system that still maintains the spheres and other Aristotelian features.

 

[Achan hiArusa]

Poles Wandering

Actually the poles wouldn't just wander, they would be contained in about 10 degrees of latitude over time. Mars will get to the point where the equatorial regions are one ringed pole. Also the length of day would change over time as the wobble changed.

Yes, the moon is slowly receeding from the earth and as it does, the earth's day will length considerably. Hartmann's Moons & Planets and de Pater & Lissauer's Planetary Systems both contain discussions of this phenomenon.

 

[Ferret]

I was learning on this a while ago, I thought the earth spun a lot faster when the moon was closer. Makes sense the other way round though. Would a wobbling solid core counteract the lack of a moon?

Does anyone have figues on the distance per day that the moon is leaving the earth?

 

[Umbran]

I'm getting a sense from this thread that everyone else runs Newtonian heliocentric universes. Am I really that exceptional?

Don't get that sense. The thread self-selects for discussion of Newtonian heliocentric arrangements, so it isn't a solid indicator. And the fact that we discuss it doesn't mean we run it that way.

Honestly, even though I'm a physicist by trade, I generally don't specify the astronomical arrangement for my fantasy worlds. I describe what the PCs see, but they aren't in a position to know the exact layout. I only specify it when I feel it is relevant.

 

[John Morrow]

Now, here we get to something important - that intrenal friction may be required for life. There are only two things that keep the planet's core molten in the long-term: Tidal friction and decay of radioactive isotopes. If you don't have a molten core, your planet is tectonically dead - no vulcanism, no plate tectonics. If it's tectonically dead, you don't get new landmass built. You don't get replenishment of the atmosphere and recycling of carbon. Eventually, you get a planet that looks a lot like Mars today - no liquid water, thin unsupportive atmosphere.

Well, Mars and Venus are pretty geographically spectacular, despite Mars being pretty dead geologically and Venus having no Moon. And neither have much of a magnetic field, either. Cracking due to a shrinking crust as well as asteroid impacts can give you some pretty good land masses, as can dead volcanoes, even if your core is solid. Of course Venus rotates backward and I don't think they are sure how it's surface was formed yet.

That said, the most important byproduct of the Earth's molten core, in my opinion, is that it generates a magnetic field that protects the planet from solar radiation. The Moon's biggest contribution to the Earth's molten core was probably the violent impact that formed the Moon in the first place, leaving most of the heavy material beind in the Earth and ejecting more of the light stuff out to become the Moon, not to mention the residual energy from the impact.

FYI, two large moons are not stable. Computer simulations to show how the Moon might have formed generated two fairly large moons about a third of the time. When they let the simulations run, the system isn't stable. They either run into each other or one crashes into the Earth and the other flies off into space because of how their gravity interferes with one another.