one small step for mefi - the moon's spin and orbit

January 25, 2006 11:19 PM

Is it cosmic coincidence that the moon spins once on its axis in exactly the same time it takes to orbit the earth (hence the dark side we never see from earth) or is there some reason why this should be more likely?
posted by humuhumu to Science & Nature (23 answers total)

Maybe I'm wrong, but the way I understand it is the moon doesn't spin on its axis. We see only one side because its position is fixed.
posted by zardoz at 11:23 PM on January 25, 2006

Best answer: It's not a coincidence; it's a steady-state behavior of the Earth-Moon sytsem, which takes millions of years to reach. The mechanism is called "tidal locking." [Wikipedia] The Earth generates tides inside the Moon, and the movement of all the mass inside the Moon due to the tides causes friction which gradually slows down the Moon's rotation, until the Moon's rotation matches its orbital period.
posted by ldenneau at 11:31 PM on January 25, 2006

Fascinating. I didn't know that.
posted by wsg at 11:35 PM on January 25, 2006

Most of the moons of the solar system whose rotation periods we know are similarly locked.
posted by gubo at 11:36 PM on January 25, 2006

Response by poster: Amazing: both the speed and perfection of answer and the phenomenon itself.
posted by humuhumu at 11:40 PM on January 25, 2006

Question the second - is tidal locking responsible for the slowing of the Earth's rotation, or are we too far out for there to be significant tidal forces? The WP article isn't very clear on this point.
posted by Ryvar at 12:22 AM on January 26, 2006

Best answer: Ryvar: yes. The slowing of the Earth's rotation is due to tidal lock with the Moon and not the Sun. Left to themselves the Earth and Moon would be locked in a ~40-day orbit facing each other. This article has a good explanation. The Sun has an effect, but it is very small compared to the Moon's.
posted by ldenneau at 12:41 AM on January 26, 2006

Awesome. Thanks, Idenneau.
posted by Ryvar at 1:24 AM on January 26, 2006

Best answer: I read somewhere that if the moon had turned just a bit more before becoming locked, that instead of the 'man on the moon' image, we'd see a large crater that takes up most of the surface of the moon. This would make the moon look like a giant eyeball looking down upon the world.

Thinking about how that would effect society and religion is mind boggling.
posted by Phynix at 1:57 AM on January 26, 2006

In chapter 11 of the Wells classic The Time Machine (read it), the protagonist ends up in a twilighty deep future, when the earth is tidally locked with the sun.
posted by Plutor at 5:39 AM on January 26, 2006

The Sun has an effect, but it is very small compared to the Moon's.

In that vixen, Mercury is tidally locked to the sun, in a 2/3 resonance. This has the effect of the center of the Caloris Basin never seeing sunset -- instead, the sun just slowly rotates across the sky, about 60 degrees above the horizon.
posted by eriko at 5:53 AM on January 26, 2006

If there was no moon, and it was only the earth interacting with sun, would the earth's spin eventually slow down to a full year so that only one side of the earth is ever facing the sun? (Similar to the way the moon's spin has slowed down to a month so that we only ever see one side of the moon -- if I'm understanding the discussion so far properly)

Will this still happen eventually to the earth, even with the moon affecting us?

If my question doesn't make sense, I guess it means that I haven't understood the answer ldenneau gave.
posted by omair at 7:33 AM on January 26, 2006

You did understand the answer Idenneau gave. The short answer is yes, given enough time, but I leave it to the astrophiles to say whether "enough time" is within the timescale of the sun's projected life.
posted by solotoro at 7:49 AM on January 26, 2006

Eriko--Up until the sixties (?? I think--pretty recently), it was thought that Mercury was tidally locked so that one year = one day.
posted by adamrice at 8:38 AM on January 26, 2006

Phynix, that's amazing. We could have had our very own Eye of Mordor...yikes.
posted by youarenothere at 8:50 AM on January 26, 2006

Maybe I'm wrong, but the way I understand it is the moon doesn't spin on its axis. We see only one side because its position is fixed.

zardoz: walk around a person once, always facing that person, and you'll see that you've turned one revolution.
posted by weapons-grade pandemonium at 9:36 AM on January 26, 2006

A perhaps more interesting question to ask is whether the length of the average human female menstrual cycle (28 days) is related to either or both of the time taken for the moon to rotate once and complete an entire orbit (27.32 days, the sidereal month) and the time taken for the moon to completely transition between two full moons (29.53 days, the synodic month) by anything other than coincidence. Sexual selection as a result of culture? The recent analysis of the gene cluster responsible for determining a large portion of skin colour in humans seems to indicate that the colour shift happened too rapidly for it to have resulted only from unassisted natural selection. Therefore some measure of sexual selection during pair mating seems to have been responsible. Skin colour became fetishised, and mating was selected for specific characteristics. I wonder did something like this operate for ancient hominid females?

The moon/menses thing is of course an ancient observation - month, moon and menses all stem from the Formal Latin mensis (moon).
posted by meehawl at 9:54 AM on January 26, 2006

Adamrice -- it was 1965 when radar observation proved the 3/2 resonance, caused by Mercury's orbit, which is much less circular than the Earth's -- Mercury's distance from the sun varies from about 48 to 70 million kilometers, a more eccentric orbit than any other planet in our system other than Pluto.

We thought it was a 1-1 tide lock, because when we could best observe Mercury, we saw the same face. Of course, we were -- but that was true of not only a 1-1 resonance, but of many others.

Even better than the never-setting Caloris Basin is that, at times, because of the eccentric orbit, the orbital velocity speed of Mercury exceeds the rotational velocity. This means that the Sun will actually travel retrograde while Mecury is close enough to perihelion -- from about 4 days before to 4 days after.

In the right spot, at the right time, you could watch the sun rise, climb to about 45 degrees above the horizion, hang for a bit, then turn right around and set in the same spot.
posted by eriko at 10:18 AM on January 26, 2006

In the right spot, at the right time, you could watch the sun rise, climb to about 45 degrees above the horizion, hang for a bit, then turn right around and set in the same spot.

I've had days like that.

Fantastic thread.
posted by DrJohnEvans at 11:10 AM on January 26, 2006

i love metafilter.
posted by poweredbybeard at 11:37 AM on January 26, 2006

Put it another way: A day on the moon is the same as a year on the moon.

I didn't know why though. Cool.
posted by o2b at 11:50 AM on January 26, 2006

omair: While I don't recall whether it deals with the tidal locking from the Sun specifically, there's a pretty cool book called What if the Moon didn't Exist? which walks through different scenarios for Earth's development and/or destruction based on different factors -- if the Moon had never formed, if the Sun had been larger, etc. -- along with a few more fantastical ones like nearby supernovae and black holes. Might be worth checking out.
posted by ubernostrum at 12:43 PM on January 26, 2006

I read somewhere that if the moon had turned just a bit more before becoming locked, that instead of the 'man on the moon' image, we'd see a large crater that takes up most of the surface of the moon. This would make the moon look like a giant eyeball looking down upon the world.

Here's a Quicktime movie of the rotating Moon. The "target" is the Mare Orientale -- it isn't that big, but it is huge (the size of Texas). There's a good semi-interactive map of the Far Side here (and don't forget the alas, nearly useless Moon.Google.com).

One of the more interesting implications is that we as a society might have been more aware of the danger of asteroid impacts.
posted by dhartung at 8:12 PM on January 27, 2006