# Effects on the Earth if the moon was at 1/10th current distance?

According to Wikipedia and this answer, the Moon is orbiting the Earth at a distance around 20 times greater than than the point where it would break up. As far as I can tell, that means in a fictional world, we could have a moon the size of our Moon, only ten times closer to the Earth (as in $\frac1{10}$ its current distance, or around 38,000km), with absolutely no fear of collisions or the moon breaking up.

Such a satellite would be quite a sight, lighting up the night sky (when its orbit matched up—I guess it'll be moving faster now) and just generally being the biggest thing up there (as far as appearances go).

What I'm wondering, though, is if there would be any geographical changes to the Earth as a result of this closer relationship with the moon (if you're thinking about this question, I checked; it didn't cover what I'm asking here). Would the Moon pull mountains from the depths of the oceans, or would its tides sweep away our coastlines? Would there perhaps be shifts in weather/climate (not sure if this can be considered geography, but I'd still like to know)?

Just as a hint, according to this answer, "tides are proportional to $\frac{\text{mass}}{\text{distance}^3}$", so a moon 10x closer would have 1000x the tidal forces. I assume this wouldn't lead to waves one thousand times higher than normal, but I can't believe that everything would be the same.

• When you say ten times closer...I feel like that measurement doesn't make sense... – James Oct 6 '15 at 14:40
• @James Oh, right, I see what you mean. Well, then, how would I say that? At a distance 1/10 to what it is currently? – DaaaahWhoosh Oct 6 '15 at 14:42
• Honestly I am not sure, physics isn't really my deal, conceptually I like to think I understand it but the math is beyond me. – James Oct 6 '15 at 14:44
• 1/10 the current distance would be the proper phrasing. – Michael Richardson Oct 6 '15 at 14:46
• Geostationary orbit is at about 36,000 km radius (30,000 km above ground). The smaller the orbital radius, the shorter the orbital period. (Examples: the ISS takes about 90 minutes to orbit the Earth at 400 km; a satellite in GEO at 30000 km takes 24 hours; the Moon at a shade under 400,000 km takes several weeks.) You are dropping the Moon just outside GEO altitude. It follows that one consequence would be that the Moon now orbits the Earth in a little over a day. Assuming an Earth-prograde orbit, apparent movement of the Moon as seen from a given location on Earth would thus be much smaller. – a CVn Oct 6 '15 at 20:11

It would not raise mountains directly. However, the planet (and moon!) would likely be much more volcanically active. More, and larger, volcanos would result in a much more dynamic landscape.

Tides would also be much higher. Shorelines would be mostly cliffs and rocks and mudflats and would also result in a much more dynamic landscape. I would think that sandy beaches would be rather rare.

Any kind of docks would need to be quite a ways up rivers to be useful. Smaller islands would have to have floating platforms well offshore to do any business with passing ships. Small boats would then row or paddle the goods ashore.

Ships would absolutely avoid the seas around islands like those of Indonesia. The differences in ocean depth would change it between a solid landmass and a scattering of islands on a daily basis. Such islands may actually be uninhabitable, as they may be worn down almost as soon as volcanos would raise new ones, not allowing time for life beyond birds to really set up shop.

Well, the prevailing theory is that the Moon did form about 10 times closer than it currently is, after a giant collision between proto-Earth and a Mars-sized body.

The Earth back then probably had 5-10 hour days, and the Moon was about 24,000 km away on formation. It has been moving apart due to tidal interactions, which have also slowed down the rotation of the Earth.

Notable effects would be:

• crustal tidal heating on Earth from the Moon, so lots of earthquakes and volcanic activity
• 100-300 meter tides as soon as the crust was cold enough to allow the water Ocean to re-condense. This amplitude is based on my recollection of the claims made in "Rare Earth," by Peter Ward, so at least someone thought this was credible enough to publish. Regardless, I may need to go back and do some math to verify/correct this, but no time at the moment.

Note that the effects would be strong enough to cause to moon to recede at a noticeable pace, so it would probably drift away significantly in the first million years:

• I'm assuming lakes like Superior would have a very noticeable tide as well? – bowlturner Oct 6 '15 at 18:19
• @bowlturner With 300 meter tides, Lake Superior would get flooded by the ocean twice a day! – Serban Tanasa Oct 6 '15 at 19:16
• The closer the object is to geosynch, the less the tidal drag. Tidal drag would be zero at geosynch. So not sure it would be strong at that distance. Also, since moon is inside geosynch at new orbit, the moon would move inward, not outward. – BowlOfRed Oct 6 '15 at 21:54
• @bowlturner, unlikely. (Ocean) tides are caused by water movement. You can pull really hard on a lake and the water just can't move that far. What would really help is a basin where the water sloshes with a frequency aligned to the moon. At the very long period it would have in the new orbit, that seems difficult. – BowlOfRed Oct 6 '15 at 21:56
• @BowlOfRed, I think direction of drift has to do with relative direction of spin of Moon compared to planet... – Serban Tanasa Oct 6 '15 at 22:20

It will affect Earths magnetic field, people without sun creme with 1.000.000 factor would "have a very bad day" because of the heavy particles would reach the atmosphere, and the tidal waves would make quite an surf experience.

Geomagnetic changes would trigger a lot of rapid geological effects boosting volcanic activity, and Earths central mass would be "shifted" due the gravitational pull so the effect would be disastrous and equal to an extinction of complex life on the planet if not a complete annihilation of the planets biosphere and the planet itself since at the distance the moon would sooner or later be pulled out of its already unstable orbit toward earth (guess) and well - nobody would live to tell the story what was after that.

Don't know what about the atmospheric effects, but there would be some changes to the atmosphere as well.

• Do you have any sources for the magnetic field changes? They seem speculative. Sun creme is used for UV radiation which is unaffected by our magnetic field. Why would a closer moon cause changes the earth's dynamo? – BowlOfRed Oct 6 '15 at 23:27
• both celestial bodies are "locked" within their orbital rotation – change the distance from 2 magnets, their magnetic field would change, same effect, different scale. Moon's magnetic field is quite weak, but we speak of a event with of such magnitude that everyone would be affected. Earths atmosphere is very much dependent on the earths magnetic field protection, since that field would be disrupted, protection from heavy particles would be weaker/different/changed – but yes, sun creme won't help you with anything with more energy then the uv spectrum – Inoperable Oct 6 '15 at 23:34
• It rather a simple deduction then proven fact, but I already read about moon's em affecting earth already – aside from that gravitational force exerted by earth on moon should be equal to the centripetal force, shrinking the orbit (radius) would very much affect the magnetic fields of them both. i previously oversimplified the magnet to magnet comparsion since there are additionally gravitation, centripetal force and the electromagnetic effects all bund together (correct me if I am wrong) – Inoperable Oct 6 '15 at 23:48