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Author Topic: Why does the moon always show us the same face.  (Read 23592 times)

Offline MoBiJo

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Why does the moon always show us the same face.
« on: 15/11/2006 02:58:33 »
When I moved from Europe to New Zealand I noticed that the moons face is upside down. Not suprising of course, but it raised another question:

Why does the moon actually always show us the same face, why do we never see the so called "dark side" of the moon, which of course also gets sunlight, so it's not dark.
Anyway, it means that the moon has to make exactly one rotation around its own axis in the time it rotates around the earth. Conincidence or physically explainable?


 

Offline neilep

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Re: Why does the moon always show us the same face.
« Reply #1 on: 15/11/2006 03:08:28 »
Hi MoBiJo, and welcome to the forum.

YAYYYYYYYYYYYY !!

Well, I wish i knew the answer...but someone will post you some words with more meaning than mine !!..

It IS quite an amazing coincidence isn't it....it's the same extraordinarily amazing coincidence that the moon is something like 400 times smaller...well..here's Wikipedias quote.....

............ " The Earth's distance from the Sun is about 400 times the Moon's distance from the Earth. The Sun's diameter is about 400 times the diameter of the Moon...........That is kind of bizarre isn't it ?

It certainly makes for eclipse joy !!

I suspect the answer to your original question will be found in how the moon came to be in existence...I gather the most favoured explanation is that it was formed when a Mars sized object collided with the earth early in the earths formation.............


I'll leave the rest up to an expert to answer for you.

WELCOME !!
 

Offline Karen W.

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Re: Why does the moon always show us the same face.
« Reply #2 on: 15/11/2006 05:38:03 »
 Good Question I would never have thought about it Like that!
 

Offline daveshorts

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Re: Why does the moon always show us the same face.
« Reply #3 on: 15/11/2006 09:14:46 »
The size thing is as far as I know coincidence, however the showing the same face is not.

It is all to do with the tides. If you think think of all the energy that goes into sloshing water around the earth, where is it coming from. As the earth spins near the earth different bits of water are attracted to the moon at different times, so because the earth is spinnign faster than the mon is orbiting the energy is actually coming from the spinning of the earth, hence the earth is slowing down and days are getting longer - 200 million years ago there were 400 days in a year.

Now the earth is 80 times heavier than the moon, so the tides would be 80 times stronger (they actually act on the rock of the moon) and the moon being smaller is easier to slow down, so in a rough calculation I think the moon will slow down 10000 times faster than the earth, so much so it has pretty much stopped now - it does swing back and forth slightly like a pendulum now but shows the same face to the earth all the time - which means rotating once a month)
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #4 on: 15/11/2006 10:03:43 »
With regards to the moon being upside down viewed from New ZeaLand people often remark on Whirpools etc rotating in the opposite way down under, but of course they don't they still rotate in the same way relative to the Earths rotation but they just look different because you are upside down
 

Offline science_guy

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Re: Why does the moon always show us the same face.
« Reply #5 on: 15/11/2006 15:51:28 »
The moon at it's present stage is almost "tidally locked".  This occurs when the gravity pulling on the closer side of the moon is stronger than the gravity pulling on the far side of the moon.  Because the moon no longer makes complete spins, we can no longer see the "dark" side of the moon.  Although the Moon is almost tidally locked, it is not quite close enough to be completely still in what angle it faces the earth.  hence the Pendulum effect.
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #6 on: 15/11/2006 17:52:32 »
The moon is still rotating quite happily relative to the Sun and the remote galaxies it just so happens that its orbital period around the Earth and its rate of rotation are the same
 

Offline lightarrow

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Re: Why does the moon always show us the same face.
« Reply #7 on: 16/11/2006 08:12:09 »
The moon is still rotating quite happily relative to the Sun and the remote galaxies it just so happens that its orbital period around the Earth and its rate of rotation are the same
Yes, but why this? daveshorts has given an explanation of it.
 

Offline Ophiolite

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Re: Why does the moon always show us the same face.
« Reply #8 on: 17/11/2006 14:42:07 »
Although the moon is now tidally locked with the Earth, as has been pointed out, the tidal interaction of the two bodies continues. The net effect is to reduce the rotational speed of the Earth. Since angular momentum must be conserved the moon acquires the angular momentum lost by the Earth. This moves it into a more distant orbit. It is retreating at a rate of around 1cm per annum. (I think I have that right.)
So the coincidence by which we can observe solar eclipses is exactly that - a time relaated chance. In several million years we shall glimpse only annular solar eclipses.
 

Offline Heliotrope

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Re: Why does the moon always show us the same face.
« Reply #9 on: 17/11/2006 21:14:04 »
With regards to the moon being upside down viewed from New ZeaLand people often remark on Whirpools etc rotating in the opposite way down under, but of course they don't they still rotate in the same way relative to the Earths rotation but they just look different because you are upside down

LOL !
:D

Actually there were some rather careful experiments done some years ago into the corriolis effect.
The experiments used a very large tray of water that was isolated from vibration and allowed to stabilise for several days before the experimental run.
I'm trying to remember the name of the chap who ran the experiments.
Anyway, the effect was, as expected, found.
And indeed the water did rotate in different directions.
The strength of the effect however was extremely small indeed.
So I'm afraid Michael Palin and his bowl of water is rather more romanticism than experiment.
You have absolutely no chance of observing the effect in a hotel bathroom.
I know, I tried for long enough.
:D
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #10 on: 17/11/2006 21:32:47 »
I have visited Australia several times and have noticed that the water spirals down the waste pipe the the same way relative to the earths rotation as it doe's in Britain.
The water does not rotate in a different direction it is just that you are viewing it from a different perspective
 

Offline chris

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Re: Why does the moon always show us the same face.
« Reply #11 on: 21/11/2006 01:01:18 »
No, this is not true.

Water swirls around wash basins, baths and toilets predominantly due to local effects - the off-centre taps for instance - which introduce a spin to the water entering the receptacle. Faced with little resistance to its motion the water continues to turn for some time. Pulling out the plug exaggerates the visible motion.

BUT - conducted correctly you can see the effect of the turning earth on the direction in which water goes down the plughole. This coriolis effect is very weak and requires special apparatus to demonstrate it, but it has been done, both in the southern and northern hemispheres in the 50s and 60s respectively.

We conducted our own experiment on the radio to show how variable this observation can be, and then asked Australia's Dr Karl to explain it all. Here's a link to that show and transcript:

http://www.thenakedscientists.com/HTML/shows/2005.12.18.htm#3

Chris
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #12 on: 21/11/2006 13:44:38 »
I would point out that I referred whirpools that are less subject to geometric influences than wash basins,the point I was making that as one is upside down compared to what one is in the northern hemisphere ones perspective is distorted.
In the outback one often hears tales of hikers getting lost and I am sure that the sun going round in an anti clockwise manner and being at its zenith in the north is a contributary factor
« Last Edit: 21/11/2006 13:50:51 by syhprum »
 

Offline Heliotrope

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Re: Why does the moon always show us the same face.
« Reply #13 on: 22/11/2006 21:02:56 »
Upside down ?
No you aren't.

Gravity pulls in toward the centre of the planet not from the top to the bottom.
We're all the same way up.

 

Offline Heliotrope

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Re: Why does the moon always show us the same face.
« Reply #14 on: 22/11/2006 21:08:07 »
Everybody's "down" is the same as eveyrone else's "down".
Down only has meaning if you're in a gravity well anyway.
Floating out in space there is no down.
There are downs on other planets but to get to them you have to go up and then float around a bit in directionless space before you fall down into another gravity well. The Moon's for example.

 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #15 on: 25/11/2006 09:28:58 »
I am certainly correct in saying I am upside down (why else would the moon appear to be inverted, see post 1), but on further reflection I realise that my conception of what is clockwise and anti clockwise would not alter , the Sun still rises in the East!.
 

Offline Heliotrope

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Re: Why does the moon always show us the same face.
« Reply #16 on: 25/11/2006 18:26:02 »
You're not upside down when you're in New Zealand.

Down is defined as the direction in which objects fall when acted upon by gravity.
You drop the ball and it falls towards your feet.
Towards the centre of the planet.
Down the gravity well.
It doesn't matter where on a sphere you try this the results are the same.
The ball falls down towards your feet.
Down is towards the centre of the planet.
Everyone has the same down.
Everyone has the same up.
No one on a sphere is upside down relative to anyone else.
Assuming they're not standing on their head of course.

If you happen to be floating out in space where you can see the people in the UK standing at an angle relative to you then the people in New Zealand will be at the opposite angle. Approximately 180 degrees from those in the UK.
It looks to you floating out there that the peeps in NZ are upside down relative to those in the UK but they are not.

It's all relative.
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #17 on: 25/11/2006 19:46:57 »
If I board the elevator that travels from the antipodeon islands via the centre of the Earth to the UK, I start with my feet pointing towards the centre of the earth but after 42 minutes when I arrive in the UK my head points towards the centre of the earth, I call this being converted from rightway up to upsidedown.
 

Offline Soul Surfer

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Re: Why does the moon always show us the same face.
« Reply #18 on: 26/11/2006 16:33:14 »
The reasin that the moon has settled down with one side facing the earth is that it  although it is approximately spherical it has slight asymmetries and mass concentrations with in it that make it settle down in one position.  It has probaly been this way for quite a long time because its back side shows more signs of recent collision damage than the side facing us which hase the well known Maria or flattish plains on it.  this is what you would expect to hapen for a body that is rotation locked to its orbital period.  I have however read papers suggeting that shorthly afer it was formed it wass probably in a 3 to 2 resonance with the earth like the planet Mercury is with the sun.
 

Offline Heliotrope

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Re: Why does the moon always show us the same face.
« Reply #19 on: 28/11/2006 20:00:59 »
If I board the elevator that travels from the antipodeon islands via the centre of the Earth to the UK, I start with my feet pointing towards the centre of the earth but after 42 minutes when I arrive in the UK my head points towards the centre of the earth, I call this being converted from rightway up to upsidedown.

Duh !
Sorry.
I forgot about the elevator.
I don't get out much. What can I say ?
:D
 

Dr. Praetoria

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Re: Why does the moon always show us the same face.
« Reply #20 on: 28/11/2006 21:01:05 »
An interesting question ???  I was wondering how Earth looks from the moon?
see:
http://www.smeter.net/propagation/views/earth-from-moon.php

I ;Dt appears to have phases as it revolves around the Moon once a year--relative to the Sun.

 

Offline science_guy

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Re: Why does the moon always show us the same face.
« Reply #21 on: 29/11/2006 15:43:41 »
Quote
You're not upside down when you're in New Zealand.

I think he was referring to a plane perpindicular to the equator where "up" equals north.  You are upside down relative to where you are now when you are in New Zealand.

Agreements, disagreements, death threats?
 

Offline eric l

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Re: Why does the moon always show us the same face.
« Reply #22 on: 01/12/2006 15:11:25 »
If I board the elevator that travels from the antipodeon islands via the centre of the Earth to the UK, I start with my feet pointing towards the centre of the earth but after 42 minutes when I arrive in the UK my head points towards the centre of the earth, I call this being converted from rightway up to upsidedown.
This brings to my mind an other question :  what happens to your weight (not your mass) if you penetrate as deep as 1000 km (not meters), or to make things easier to 1/4 of the earth's diameter ?  You distance to the earth's center of gravity will be halved, does that mean that your weight (force of attraction from the earth) will be 4 times higher ?  What is the influence of the part of the earth above you ?
 

Offline Soul Surfer

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Re: Why does the moon always show us the same face.
« Reply #23 on: 01/12/2006 15:30:56 »
Inside the earth the gravitational field goes down linearly with your distance from the centre of the earth.  At the centre of the earth your weight is zero.  This is because the gravitational field inside a uniform thick spherical shell is zero at all points inside the shell.
 

Offline syhprum

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Re: Why does the moon always show us the same face.
« Reply #24 on: 01/12/2006 19:54:37 »
Am I correct that in free fall your trip would take 21 minutes down to the centre and at that point your speed would be the same as if you were doing a ground level orbit (84 mins)(7.937KM/s),
I notice that if you accelerated at 1 G for 21 mins your speed would be 12.36KM/s does this confirm there is some reduction in G as you go down.
I think to calculate the reduction of velocity due to a linear reduction in G with distance would need a knowledge of calculus which I do not have but I could probably do a numerical approximation 

P.S
I am puzzled by your assertion that that G falls off in a linear manner as one descends, this I could understand for a cubic or cylindrical body but for sphere it does not seem logical!!!

This post seems to have arrived on the scene rather late due to some technical problem, I have tried to calculate how the varying gravitational field affects the transit time but it is rather beyond my GSE level maths the only thing I worked out was the average value of G for the descent was 8.02/Ms/s which is reasonable as the elevator travel relatively slowly through the regions of high G but zips through the low G regions  
« Last Edit: 05/12/2006 10:32:16 by syhprum »
 

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Re: Why does the moon always show us the same face.
« Reply #24 on: 01/12/2006 19:54:37 »

 

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