# The Moon plays integral part in our existence.



## Chinook (May 30, 2010)

I watched a special the other night on the National Geographic channel (by satellite) that tended to stress the relevance of the moon's importance (to Earth), and also made a lot of faulderal about the fact that the moon is slowly drifting (in a spiral) away from Earth. I did some quick calculations. The moon is moving away from Earth at ~4 CM/ yr. That's 1 Meter in 25 years. They stated that the various types of protection that Earth receives from the moon's presence would begin to dissipate when the Moon's orbit reach 110% of it's current orbit. It's current distance is 384,400 km. So the danger will occur when the moon is close to 38,400 km further away. So, it will take 38,400 X 1000 m/km X 25 years = 960,000,000 years before we're in trouble. (please feel free to correct my math if it's wrong) Do you suppose other things will have changed a bit by then?  

SPACE.com -- Moon Mechanics: What Really Makes Our World Go 'Round

Another interesting fact:  [FONT=arial, helvetica, sans-serif]The Sun happens to be 400 times the Moon's diameter, and 400 times as  far away. That coincidence means the Sun and Moon appear to be the same size when viewed from Earth. A total solar eclipse, in which the Moon is between the Earth and Sun, blocks the bright light from the Sun's photosphere, allowing us to see the faint glow from the corona (the Sun's outer atmosphere.)[/FONT]


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## HareBrain (May 30, 2010)

Chinook said:


> Another interesting fact: [FONT=arial, helvetica, sans-serif]The Sun happens to be 400 times the Moon's diameter, and 400 times as far away. That coincidence means the Sun and Moon appear to be the same size when viewed from Earth. A total solar eclipse, in which the Moon is between the Earth and Sun, blocks the bright light from the Sun's photosphere, allowing us to see the faint glow from the corona (the Sun's outer atmosphere.)[/FONT]


 
It's often struck me how neat this is. Almost _too _neat ...

Almost.


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## Pyan (May 30, 2010)

> Do you suppose other things will have changed a bit by then?



Well, in very rounded terms, H.sapiens has been around for 0.5% of this time, and it's longer than we've taken to evolve from unicellular eukaryotes... so I'm putting it a _loong _way down my worry list...


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## goldhawk (May 30, 2010)

Chinook said:


> Another interesting fact:  [FONT=arial, helvetica, sans-serif]The Sun happens to be 400 times the Moon's diameter, and 400 times as  far away. That coincidence means the Sun and Moon appear to be the same size when viewed from Earth. A total solar eclipse, in which the Moon is between the Earth and Sun, blocks the bright light from the Sun's photosphere, allowing us to see the faint glow from the corona (the Sun's outer atmosphere.)[/FONT]



Most of the time.  The Moon follows an elliptical path and sometimes it's so far away during an eclipse that it doesn't fully block the sun; you can see a ring of the photosphere around the Moon.

Another fact:  the Moon orbits in a different plane than the Earth.  This means eclipses, both solar and lunar ones, can only happen at certain times for the year:  March/April and September/October.


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## ColdBurn (Jun 7, 2010)

I know the moon's presence keeps the earth relatively stable on it's axis but does the moon rotate on it's own axis or is it locked in place on the  end of an ever-lengthening gravitational tether?


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## HareBrain (Jun 7, 2010)

Ooh! I can answer a science question!

We only ever see the same face of the moon, so no, it doesn't rotate on its own axis.

As to why, I assume some kind of god is at work.

(D'oh!)


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## goldhawk (Jun 7, 2010)

HareBrain said:


> We only ever see the same face of the moon, so no, it doesn't rotate on its own axis.



Actually, the Moon does rotate on its own axis.  It happens that its rotational axis is the same one as its orbital axis.  This is called Tidal Locking.

And I don't think the Earth's rotation is stabilized by the Moon.  In fact, it changes; this is called Precession.


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## Nik (Jun 7, 2010)

IIRC, the Moon *does* stabilise the Earth's spin axis angle at the current 23-some degrees. The slow precession affects which way the axis points, not how far off 'normal' to orbital plane-- Unlike Mars, which wobbles wildly...


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## The Ace (Jun 7, 2010)

So that's why Martians are green.  All that wobbling makes them sea-sick.


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## Stephen Palmer (Jun 8, 2010)

Nik said:


> IIRC, the Moon *does* stabilise the Earth's spin axis angle at the current 23-some degrees.


 
This is true. Briefly:

Stabilization of the Earth's obliquity by the Moon


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## goldhawk (Jun 8, 2010)

Stephen Palmer said:


> This is true. Briefly:
> 
> Stabilization of the Earth's obliquity by the Moon



Very nice but not very useful.  It does not describe how the mechanism works.


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