# If the moon suddenly vanished, would there be tsunami-like tidal waves? **URL:** \ **Category:** Factual Questions\ **Created:** [April 25, 2007, 2:16pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580 "2007-04-25T14:16:44Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 25, 2007, 2:16pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/1 "2007-04-25T14:16:44Z")
If the moon were to abruptly _vanish_ - not be whisked away or broken into pieces, just undergo Spontaneous Massive Existence Failure - what would be the short-term effect on Earth? Of course the lunar component of ocean tides would stop happening, but those tidal forces are currently keeping a big pile of water heaped up on both sides of the planet - if that force were to suddenly _let go_, that would make some pretty substantial waves when it reached the shore, wouldn’t it? ---
**Author:** ![Potato\_Pancakes](https://avatars.discourse-cdn.com/v4/letter/p/65b543/32.png) [@Potato\_Pancakes](https://boards.straightdope.com/u/Potato_Pancakes)\ **Post date:** [April 25, 2007, 4:59pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/2 "2007-04-25T16:59:45Z")
It sure seems like it would, but SMEF is hard to study. I’m waiting for someone to bring the science to this thread. ---
**Author:** ![Ins\_Outs\_What-have-yous](https://avatars.discourse-cdn.com/v4/letter/i/8797f3/32.png) [@Ins\_Outs\_What-have-yous](https://boards.straightdope.com/u/Ins_Outs_What-have-yous)\ **Post date:** [April 25, 2007, 5:13pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/3 "2007-04-25T17:13:52Z")
Cecil column: [I plan to destroy the Moon. What effect would this have on the earth?](http://www.straightdope.com/columns/001027.html) ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [April 25, 2007, 5:36pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/4 "2007-04-25T17:36:48Z")
[I’m going to blow up the Moon. What are you doing this summer?](http://boards.straightdope.com/sdmb/showthread.php?t=372855) 🙂 ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [April 25, 2007, 11:31pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/5 "2007-04-25T23:31:07Z")
The large amounts of water held up by the moon and other factors would slosh back across the ocean - I can’t guess how big they would actually be, but they probably wouldn’t be as big as the mega-tsunamis you hear about, but they would be similar to the Kelvin waves that occur during El Nino and NAO phenomena. It would be interesting to see a “tidal wave” that was actually caused by the moon. See more here: [Megatsunami - Wikipedia](http://en.wikipedia.org/wiki/Megatsunami) ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 25, 2007, 11:32pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/6 "2007-04-25T23:32:56Z")
Thanks, but both of those links are about blowing it up - leaving its mass and debris in the local area. I want to know what happens if the moon suffers SMEF. The moon makes water heap up in the middle of the tidal swell - I think I’m right in assuming that the effect of moonSMEF would be approximately the same as dumping that volume of water (the bit that is heaped up above the level it would be without the moon) instantaneously into the middle of the ocean, only trouble is, _how much water are we talking about?_ ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 25, 2007, 11:34pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/7 "2007-04-25T23:34:33Z")
[QUOTE=wevets] No. Tsunami is a better name than tidal wave because “tidal” implies that the large waves are caused by the tides, which are in turn largely influenced by the moon. Tsunami translates as “harbor wave,” chosen because of the wave’s devastating impact on harbors and shorelines. Tsunamis/Tidal waves are not caused by the moon. See more here: [Answers - The Most Trusted Place for Answering Life's Questions](http://www.answers.com/topic/tsunami) [/QUOTE] So you didn’t read the OP? ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [April 25, 2007, 11:36pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/8 "2007-04-25T23:36:12Z")
[QUOTE=Mangetout] So you didn’t read the OP? [/QUOTE] Oops. Sorry! :smack: ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [April 25, 2007, 11:39pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/9 "2007-04-25T23:39:41Z")
I thought maybe I could edit my response to actually address your question before anyone read it, but apparently I just wasn’t fast enough. My apologies. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 25, 2007, 11:42pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/10 "2007-04-25T23:42:47Z")
Heh. No problem. ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [April 25, 2007, 11:44pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/11 "2007-04-25T23:44:22Z")
Wouldn’t a wave caused by the disappearance of the Moon be an untidal wave? Now my real question: How empty is the vacuum around the moon? Wouldn’t its sudden disappearance leave some void? A popping sound? a woosh of wind rushing in to fill the void? ---
**Author:** ![Translucent\_Daydream](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/translucent_daydream/32/16170_2.png) [@Translucent\_Daydream](https://boards.straightdope.com/u/Translucent_Daydream)\ **Post date:** [April 25, 2007, 11:48pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/12 "2007-04-25T23:48:05Z")
As I understand it, the Earth will wobble around in its orbit, destroying seasons as we know them and making the world a pretty crappy place to hang out. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [April 25, 2007, 11:52pm UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/13 "2007-04-25T23:52:09Z")
The Earth and Moon orbit their common centre of gravity like a big asymmetric dumbbell. Since the Earth is so much more massy, the common centre of gravity is deep within the Earth which is why we regard the Moon as orbiting around it. If you were to vanish the Moon, as well as the tidal humps suddenly slumping, the Earth’s orbit is going to change a bit. No idea how significant that effect would be, and it might depend on the relative positions of the Earth, Moon and Sun when the Moon vanishes. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [April 26, 2007, 12:10am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/14 "2007-04-26T00:10:22Z")
[QUOTE=matt] If you were to vanish the Moon, as well as the tidal humps suddenly slumping, the Earth’s orbit is going to change a bit. No idea how significant that effect would be, and it might depend on the relative positions of the Earth, Moon and Sun when the Moon vanishes. [/QUOTE] Rough back of the envelope calculations … it will change Earth’s orbital velocity by +/- 0.05%. This is several orders of magnitude less than the normal variation due to the eccentricity of Earth’s orbit. Very minor climate changes, if any. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [April 26, 2007, 12:20am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/15 "2007-04-26T00:20:39Z")
I seem to recall that Tsunamis are a sort of shock wave that piles up as a displacement wave when it reaches the shallows. I am not a moon exterminator, but I suspect that there would be a huge displacement wave, with the wave flowing across the oceans. It would probably be a lot like a storm surge, minus the hurricane. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 26, 2007, 12:24am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/16 "2007-04-26T00:24:56Z")
It is a displacement wave, yes - instead of seafloor moving to displace water from below, we’ve got water falling in from above, but a fair old bit of it and spread across a large area. I think the wavelength would be very large and with tsunamis, it’s the wavelength that makes them devastating - as I understand it, because it’s not a _wall_ of water heading for the land, it’s a _plateau of water_. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 26, 2007, 12:51am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/17 "2007-04-26T00:51:33Z")
The tsunamis (or equivalent, whatever you want to call them) would certainly be devastating, but I’ve a hunch that SMEF of anything as large as the Moon (or perhaps even of anything at all) would have other effects, far more devastating. For one thing, you’d get monopolar gravitational waves (forbidden, so far as we know, in our actual physical Universe), and who knows what effect those would have? I’m not entirely certain that the effects would even fall off with distance. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [April 26, 2007, 1:25am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/18 "2007-04-26T01:25:33Z")
[QUOTE=matt] The Earth and Moon orbit their common centre of gravity like a big asymmetric dumbbell. Since the Earth is so much more massy, the common centre of gravity is deep within the Earth which is why we regard the Moon as orbiting around it. [/QUOTE] I suspect the actual location of the common center of mass has little to do with the perceptions of an observer on either body. If you were part of a moon-based civilization, you’d probably think that, despite its mass, the earth orbited the moon. Indeed, we hear tell that residents of the earth until recently commonly believed that the sun orbited them. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [April 26, 2007, 2:15am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/19 "2007-04-26T02:15:09Z")
[QUOTE=Mangetout] … those tidal forces are currently keeping a big pile of water heaped up on both sides of the planet - if that force were to suddenly _let go_, that would make some pretty substantial waves when it reached the shore, wouldn’t it? [/QUOTE] I’m not sure that’s correct. I understand that planets are naturally oblate spheroids, and there would be a bulge of water around the equator, moon or not. Tides are caused by the moon pulling the bulge to one side, and without a moon it would settle in the middle. Am I right, Chronos? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 26, 2007, 2:28am UTC](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580/20 "2007-04-26T02:28:54Z")
> [@](#): > > and without a moon it would settle in the middle. And it’s the process of settling that we’re referring to as the mega-tsunamis. The rest of what you have written is correct, but I’m not sure how it’s relevant. The bulge from the Earth’s rotation is a different phenomenon than the tidal bulge. [Next page](https://boards.straightdope.com/t/if-the-moon-suddenly-vanished-would-there-be-tsunami-like-tidal-waves/401580.md?page=2)