There is no moon.

[Mystery Man]

Hmmm, I just read somewhere that without the moon the earth would be rotating much faster than it is now. The days would only be something like 6-8 hours long.

Opposite actually, the closer it would be the faster earth would spin.

 

[mhensley]

I think having one celestial body would set a tone for monotheistic religions.

Or it would just make the stars much more prominent and lead to religions chock full of gods.

 

[Desdichado]

If the earth had no moon it wouldn't spin on an axis, its spin would wobble and tumble if it even spun at all. For instance our axis wobbles ever so slightly now because the moon has been pulling away from us steadily over the eons and will continue to do so. The spin will also grow slower as the moon pulls away.

It would still spin, just as the other planets in the solar system that have insignificant (as a ratio of their mass) or no moons spin. In fact, it would likely spin much faster, the only planets that spin slowly are those that are in resonance with the sun itself. The resonance with the moon is what moderates our days.

I'm not sure what the effect of much shorter days and nights would be, but that could be interesting...

As for the moon pulling gradually farther away from the earth, I've never heard that before.

 

[Mark Hope]

I also guess it depends how scientific your world is. I have a homebrew where the world (although nominally the same as ours in appearance and function) does not follow the same laws of physics. It was kinda inspired by that line in the old Spelljammer box that said something like "These are not the laws of Einstein and Newton - these are the laws of Mordenkainen and Elminster" or something like that. So electricity was due to and governed by the behaviour of paraelemental lightning. Atoms, molecules and the like do not even exist - it is the interaction of various magical and spiritual forces that determine what reality is like. The world itself is alive and dreaming.

In that sort of scenario, the moon can have any sort of function you like. Maybe it governs certain types of magic (as per Dragonlance) or maybe it is a deity that watches over the night. An absence of moon might mean that some magic runs wild when it is dark, or certain elements of reality behave differently to how they do during the daytime. Maybe there is a moon, but it wander off from time to time, doing who knows what, and its return is a time of great joy, or great fear, or just really great cheese.

On a more scientific note, our own moon has probably shielded us from a few hefty impacts from asteroids and the like, so maybe a world without a moon has suffered a higher than usual amount of meteoric smackery. Enormous craters might litter the landscape, remnants of a more violent time in the planet's history. Maybe these impacts yielded a certain type of metal that was a much-used natural resource in antiquity and the largest craters are home to the ruined cities of an ancient civilisation that simply mined itself out of business. Its monstrous undead survivors (because there are always monstrous undead survivors, aren't there?) seek to cause fresh impacts in order to generate more of the precious element smackonium. Or something.

 

[fusangite]

I would like to put forward a very different view from the other posters: While the physics of the D&D world, as described in the core rules are incoherent, if one is searching for coherence within them, one should not look to the science of today. It is clear that in the D&D world, the laws discovered by Newton, Darwin, Mendel, Einstein, et al are generally not in effect. In my view the physics of the D&D world are much closer to those of Aristotle. In support of this I offer three examples:
(a) the D&D world has 4 elements not 100+; these four elements are earth, air, fire and water.
(b) according to the magic system and the appraise skill, objects have an absolute real monetary value that is unaffected by money supply or the needs and wants of purchaser and vendor
(c) falling damage is non-exponential
Furthermore, if modern science were true, why would magic work? Why would there be so many creatures occupying the same ecological niche? etc.

I would therefore recommend that if you want to look at what a world without a moon would be like, you base it on the medieval science we believed in before the scientific revolution.

So, if Aristotelian physics are true,
(a) life did not evolve; it was created by a divinity
(b) the earth is surrounded by a set of perfect crystaline spheres along which planets rotate
(c) the sphere that separates the earth from the outer spheres is the one occupied by the moon
(d) there are four elements in the sub-lunar sphere which are, from lightest to heaviest, earth, water, air and fire -- heavy things move down; light things move up
(e) the area between the earth's surface is occupied, at the lowest level, by watery and airy things (like rain clouds), next by airy things (non-rainy clouds), next by airy and fiery things (clouds that produce lightning) and finally by fiery things (meteors)

Unless you want to move the sun closer to the earth's surface or make it less hot, the sublunar sphere will become larger, resulting in more dramatic and exciting weather and more visible meteors. You would also probably have to change to a six-day instead of a seven-day week as the seven-day week is based on an association between the seven spheres between the earth and the sphere of fixed stars.

 

[mhensley]

Opposite actually, the closer it would be the faster earth would spin.

I just went back to find the where I found this interesting tidbit-

http://curious.astro.cornell.edu/question.php?number=104

The Earth's rotation is being slowed through tidal interactions with the Moon. If the Moon had never existed the Earth would be spinning much faster. In fact, our day would probably only be about 6 hours long! The fast rotation rate would lead to faster winds and stronger storms. The fast rotation speed would also have implications for plant photosynthesis, and animal hunting and sleeping cycles.

He also goes on to say that the earth would probably be hit by more asteroids. Pretty interesting stuff.

 

[Mystery Man]

As for the moon pulling gradually farther away from the earth, I've never heard that before.

Yup. One hundred years from now, the day will be 2 milliseconds longer than it is now! It's kind of a slow process.

 

[Castellan]

If the earth had no moon it wouldn't spin on an axis, its spin would wobble and tumble if it even spun at all. For instance our axis wobbles ever so slightly now because the moon has been pulling away from us steadily over the eons and will continue to do so. The spin will also grow slower as the moon pulls away.

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.

As mentioned earlier, the Earth would also be spinning faster. As the Moon drags against the Earth's oceans -- the effect which creates tides -- the friction of the oceans against the Earth's crust causes the Earth to slow its spinning. The energy has to go somewhere, though, and it goes into the Earth-Moon system, causing the Moon to move farther away from the Earth. Some day, in the very distant future, the Earth will be "tidally locked" with the Moon as it already is with us. In other words, only one side of the Earth will face the Moon, and folks living on the other side of the planet will take vacations to "see the Moon." The Moon will move a lot farther away from the Earth, too.

Mind you, that energy-transfer is not what keeps the Earth stable. The gravitational interaction between the Earth and Moon does that. However, that's correlation, and not causation.

...the only planets that spin slowly are those that are in resonance with the sun itself.

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...

As for the moon pulling gradually farther away from the earth, I've never heard that before.

Gotta love the conservation of energy.

 

[JustKim]

Or it would just make the stars much more prominent and lead to religions chock full of gods.

That's possible, though it would be difficult for the layman to identify and track stars, so it's not as fulfilling to give them such significance.

If you don't want to play with the pantheon, removing the sun/moon duo might shake up other conventions such as the gods condoning single-partner marriages between men and women. With many moons polygamy might become the norm, and with no moons the concept of marriage could be abolished. People might have a tendancy to live for themselves if the heavens are represented by a lone sun.

 

[Desdichado]

Well, I'll be. That's actually relatively accurate; the transfer of rotational energy from the earth gradually slowing down does elevate the Moon to a higher orbit. I had forgotten that until I looked it up after seeing your post.

Although that's an interesting campaign idea right there -- a really ancient world, with very, very long days and nights, and a moon that stays in exactly the same place in the sky at all times. A world with such prolonged days/nights (a single cycle taking about month) would have a hard time getting conventional vegetation to grow well, and would likely have temperature extremes due to the prolonged heating or cooling of the sun.