Moonless Earth

In this thread, **vanilla **asks what would happen if Earth were to suddenly slow down and during the ensuing conversation, the Moon and its effects on the earth are discussed a bi, including what would happen if it were to suddenly shift its orbit to any appreciable degree.

That got me to wondering about what would have happened if the moon had never formed in our orbit but I did not want to hijack the conversation, hence this thread.

How would the planet be different? Assuming we still managed to evolve on its surface without it there to shield us, how changed would our lives be? Would we have tides at all?

(Mods, sorry if this belongs in GD. I was unsure… feel free to move it if it belongs there.)

Check out What If the Moon Didn’t Exist?: Voyages to Earths That Might Have Been, by Neil F. Comins.

And this other moon thread.

Humans would have never evolved. if the Earth had no moon it would oscillate wildly on its axis. Basically what this means is it would go from summer to winter in a single day. If not more extreme than that if I recall.

Discovery channel has a real good show out there: “If the Earth had no Moon.” It’s a really intresting show. I recomend anyone watch it who gets the chance.

As Bill Bryson says in A Short History of Nearly Everything: “Not many of us think of the Moon as a companion planet, but that is, in effect, what it is…Without the Moon’s steadying influence, the Earth would wobble like a dying top, with goodness knows what consequences for climate and weather…About 4.4 billion years ago, a Mars-sized object slammed into Earth, blowing out enough material to create the Moon from the debris. This was obviously a very good thing for us - but especially so as it happened such a long time ago. If it had happened in 1896 or last Wednesday, clearly we wouldn’t be nearly so pleased about it.”

Exapno Mapcase beat me to the punch in mentioning What If the Moon Didn’t Exist? To sum up the physical effects Neil Comins describes for a moonless Earth:
[li] Tides would be one-third as high as they are today, since the Sun would be the only body exerting a large enough gravitational pull.[/li][li] Earth’s rotation would still have slowed since the beginning, as the Sun-induced tides would act as a drag on the Earth’s rotation just like Moon-induced tides do. However, given the relatively weak influence of the Sun tides, Earth would currently have a year of 1,095 eight-hour days, instead of 365 24-hour days.[/li][li] Winds would be much stronger, and more zonal in style of circulation (think of the bands of clouds on Jupiter and Saturn that stay static with resepct to latitude). With stronger winds come higher and rougher ocean waves. Between the wind and the waves, high rates of erosion would keep mountain ranges from getting as high as they are at present.[/li][/ul]

Comins doesn’t mention it, but I have seen discussion elsewhere* that without the Moon as a stabilizing force, Earth’s obliquity (the tilt of its rotational axis with respect to a vertical line) would not have remained between 23 and 25 degrees, but rather may have fluctuated drastically between near-vertical to 85 degrees. That sort of variability would have its own impacts of course on the degree of seasonality and long-term climate trends.

Comins goes on to speculate what impacts this setting would have had on the evolution of life - slower path to more complex macroscopic life forms, life forms that are not very tall owing to the windiness, less dependence on hearing as a sense (since the roar of the wind would tend to mask sounds), different life cycle owing to the very short days, etc. This part of the discussion is interesting as a thought experiment, but since the mechanisms of life are nowhere near as “clear” as those of planetary physics, I would not take it as a sure thing. :wink:

  • Laskar, J., Joutel, F., and Robutel, P., 2003, Stabilization of the Earth’s obliquity by the Moon: Nature v. 361, no. 6413, p. 615-617.

Does Bryson explain whether Mercury, Mars, and Venus wobble like dying tops, and if they don’t - and I don’t think they do - why not?

Yeah, I have to say I’m surprised by these mentions of the moon being ‘a steadying influence’ and that the earth would wobble in its orbit if it had never been there. Everything I’m familiar with puts the influence of such a large companion as the moon in the category of a destabilizer, an irritant – if sometimes a very useful irritant. It drives the strongest tides, I would think it would have a similar effect on the winds.

And of course, old Isaac (Asimov) had his theory that because the tidal action of the earth concentrated heavy elements like thorium and uranium in the earth’s crust as it was forming, without the moon advanced and diversified life would never have evolved on our planet. But personally I think that’s a stretch.

Mercury is tidally locked with the Sun, with it’s orbit and rotation in a 3:2 ratio. I read in a paper about Venus that its thick atmosphere causes its rotation to be nearly zero, due to heating from the Sun, so the Sun may also be stabilizing Venus. Don’t know about Mars.

Here’s a link to a PDF of the paper about Venus, Long Term Evolution of the Spin of Venus. The first sentence of the Abstract reads “Due to planetary perturbations, there exists a large chaotic zone for the spin of the terrestrial planets (Laskar and Robutel, 1993, Nature 361, 608–612).” That’s the same reference sunfish gave, so maybe he can see if the chaotic zone reaches to Mars.

Oops. Same authors, same publication, same topic, but not the same reference.

And of course, many sci-fi writers like to speculate that without that huge ball hanging in the sky at night, constantly changing, humans would have been less motivated to astronomy, thus delaying or preventing all kinds of cultural advancements.

The axial tilt (obliquity) of Mars is believed to vary chaotically.

Keep in mind that the axis changes its angle on timescales of tens to hundreds of thousands of years.

This is the best plot a quick Google search could find, but you can compare to a plot of the milder changes in the Earth’s obliquity.

This effect makes itself particularly felt for those people living on planets in sextary (if there is such a word) star systems.

We’re not talking about the Earth “wobbling” about in its orbit in this context, though - the orbit itself remains (relatively) stable (the degree to which it varies is called eccentricity). It’s the angle of the rotational axis, i.e., obliquity, that is apparently stabilized by the Moon’s presence. Yes, the Moon has the greatest impact on solid earth and ocean tides by virtue of its closeness to Earth, but it’s also very happily occupying a stable orbit around us that’s only about 5 degrees out of the plane of the ecliptic, IIRC. I can’t remember where now, but I have seen references to the Earth and Moon as more of a binary planetary system than as a “planet with satellite,” precisely because of the relative sizes of Earth and Moon (compared to Mars, Phobos and Deimos, say), with the gravitational balance point existing at a point in the Earth’s upper mantle.

Re the chaotic zone for planetary spin: the abstract of the Laskar and Robutal (1993) paper that ZenBeam cites reads:

I don’t have access to the full text just now, unfortunately, so I can’t elaborate on the details as the authors see them, but the answer to your question, ZenBeam, is that Mars is included in the chaotic zone. (On preview, I see that Podkayne’s already answered that. Ah well. And FWIW, I’m a she. :slight_smile: )

[slight hijack]

Back to Earth for a moment - the obliquity cycle, in which the earth’s axial tilt varies between ~22 and 24.5 degrees, currently lasts about 41,000 years. (It’s one of the Milankovitch cycles thought to be regulating climate; see this link for a nice explanation of the man and his theory.) It is this type of motion that the Moon is thought to have stabilized, by reducing the range over which the axial tilt can vary (just a few degrees rather than nearly 90).

The “wobble like a top” motion that I think Exapno Mapcase is referring to is called precession. On Earth, this is also manifested as a Milankovitch cycle, lasting about 23,000 years at present. It’s caused by the combined gravitational pulls of the Moon and Sun on the Earth’s equatorial bulge, to reduce the angle of the Earth’s axial tilt with respect to the ecliptic and to the plane of the Moon’s orbit. The net effect is that the direction of the Earth’s axial tilt with respect to perihelion and aphelion (Earth’s minimum and maximum distances to the Sun) changes through time, which in turn influences climate.

As to why Mercury, Venus and Mars don’t have similar wobbles - Mercury and Venus have no equatorial bulge (i.e., oblateness = 0) and no satellites the size of the Moon to exert torque on their rotation, and although Mars’s bulge (oblateness = 0.006) is larger than Earth’s (oblateness = 0.0034), Mars’s satellites must be too small to do the job.**

On a much smaller scale, there is a few meters’ variability in Earth’s rotational axis on a daily basis, called Chandler wobble. One possible explanation for Chandler wobble is that it is produced by changes in pressure at the bottom of the ocean caused by temperature, salinity and wind-driven ocean circulation changes.* While the effect is measurable, I doubt most people have ever taken notice of it. :slight_smile:

[/slight hijack]

  • Gross, R.S., 2000, The excitation of the Chandler wobble: Geophysical Research Letters, v. 27, no. 15, p. 2329-2332.
    ** Oblateness data from Kaufmann, W.J., 1998, Universe, W.H. Freeman.