Another week, another planet. This time we talk about our own home world: Earth. You might think you know the planet beneath your feet, but it's actually one of the most interesting and dynamic places in the Solar System. ...

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It is also the one that people neglect the most when asked "How many planets can be seen with the naked eye?"

Thanks for ANOTHER great show!

On the episode about Earth, Pamela mentioned that we are already in the twilight of life on Earth, and that in merely 50 million years or so Earth will become inhospitable for large animals to live on - the Sun will change, it will get hotter, the oceans will boil away, the atmosphere will be blown off etc.

Is it really only 50 million years until that happens? I've read very different numbers on this in other places. Wikipedia says on the page End of planet Earth for example that in 1.1 billion years the Sun will get 10% hotter, and that the oceans will evaporate away in 3.5 billion years, when the Sun is 40% hotter.

50 million or 3,500 million years, that's a big difference!

Where did Pamela get the 50 million years figure from?

I enjoyed this episode but felt some areas were missed out, such as the composition and structure and different layers in Earth's atmosphere.

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Mike
. IceInSpace - The Australian Amateur Astronomy Community
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Thanks for your feedback Iceman - I'll add some of that kind of thing into the show notes (I'm a little behind).

-Rebecca B-F
Astronomy Cast
Assistant Producer

I'm a little behind listening to the show, but I have the same question as Jesper. 50 million years seems to be an awfully short time, given that life has been around for perhaps 3.5 billion.

"The Life and Death of Planet Earth" fixes the figure at 500 million years before the Sun becomes too bright for us, though I see no reason why we might start to become very uncomfortable long before then.

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I think fish is nice, but then I think that rain is wet, so who am I to judge?

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

I heard 700 million years.

This may just be the type of question that will get me slapped around, but I will ask it anyway.

Every particle with mass has some form of gravity.

If I am standing at the center of our planet...the gravity of the planet is because of its mass. If all its mass is around me...then would't I be pulled apart at the center?

I am guessing this would mean that I would be weightless at the center. And I would get lighter the closer to the center.

Apparently yes:

http://hyperphysics.phy-astr.gsu.edu.../earthole.html

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

At first I thought of this in terms of mass following curved spacetime and "falling" in to Earths gravity well, suggesting (to me at least) gravity would increase at the centre, but apparently not...

...good question!

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

this explanation is good too:

"... Earth caused gravity would disappear at earth's center of gravity.
But the gravity caused from other space objects would still be there, like the gravity which is acting on earth as a whole: particularly important would be the gravitational influences of the moon and the sun.

The result would be the same force which is acting on Earth as a whole, resulting in no acceleration relatively to Earth itself. But acceleration relatively to other space objects."
by iwnit http://www.answerbag.com/profile/?id=147540

I'm trying to find about pressure at the centre of the earth if there is 'no' gravity?

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

I kind of thought so.

But, the deal with pressure...I was putting thought into this...and it kind of made my head spin.

As you get further down...is more atmospheric pressure. There is more atmosphere above you...causing more pressure.

If you are stading at the center of the earth...you would be crushed by the pressure of the atmosphere.

However...if there is no atmosphere...say like on a journey to the center of the moon...you would not have that problem.

At least...I think that is how it would work.

Perhaps, in the case where you dig a tunnel, and were somehow able to keep it from collapsing?

I haven't had much luck so far with a description of the actual conditions at the centre of the earth - what are the conditions for the iron particles that are actually there? They experience weightlessness it seems - but what of the pressure? Intuitively the pressure would be tremendous - or - does the action of gravity somehow also act in opposition to the pressure?

If the gravity is halved, half-way to the centre, and zero at the centre, wouldn't the pressure also halve and zero? At the centre gravity would 'pull' in all directions and there would be no pressure? A vacuum?

Time for a lie down.

...still looking.

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

hmm..

OK...from what I remember about pressure....

There is atmospheric pressure....which is only what is above us. this is part of the reason why pressure as you get higher up (on everest) pressure is less then when you are in death valley...where it is more.

So, the way I look at it...if I were able to dig a way to the center of the earth...a solid hole...I would be at higher pressure. Not because of the gravity (as determined before)...but because I have that much more atmosphere above me....

But...wait...my head is hurting....

the atmosphere is there probably from the gravity of the earth...right?

So...as you get closer to the center of the earth....would the pressure diminish the same as my own weight?

ARRRGGGHHH!

Logically...the air particles that make up the atmosphere would be affected the same as someone going to the center.

If that is the case...then yes...a vacuum...and weightlessness....at the center of the earth.

Right....time to lie down

Ok. I think I've got it.

Imagine you're at the centre of the earth.

Imagine a column of iron, molten rock, solid rock etc above you in any direction, say towards the north pole.

Now. You are weightless.

The layer of iron immediately above you in the column is nearly weightless, but not quite. Because there is more mass 'below' that layer (towards the centre and then on to the surface opposite) than above, what nett weight that layer has is vectored by gravity towards the centre. Same for the molten rock halfway up the column, which we could say has a nett 'half-weight' (due to half gravity) also vectored towards the centre. And the crust at the surface has full gravity, full weight, also vectored towards the centre.

The gross effect on you at the centre is to ADD the full weight of the top layers, the half weight of the middle layers, and the not-very-much weight of the bottom layers... add it ALL up and it is ALL pushing on you towards the centre. That's heavy. And there is a similar column in every direction.

Thats gotta hurt!

__________________
If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

I think I follow now.

I was only thinking about the column on top of me...not the column below me. Each of these columns are pulling towards each other. That creates the gravitational pressure.

I may be weaightless...just like everythign around me...but the gravitational pressure is pulling everything together!

Thanks for bringing me through that! My head is not so much hurting anymore!

Yep, that was fun!

Another moment of clarity for me (I have them from time to time, usually after a Bourbon) was that although you would be weightless (kind of) at the centre of the earth (or at least the centre of gravity) that doesn't mean gravity-less. You are still subject to the entire gravity of the earth (just like on the surface) it's just that it is acting in every direction, so every direction is cancelled out by its opposite.

And the pressure you feel isn't so much about weight (which is just the effect of mass and gravity) but more about the atoms and particles that make you up pushing back against the forces acting against you.

So you're jam in other words!

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

... if you want another headache you could always try imagining the centre of a neutron star. Or black hole.

Pass the aspirin please.

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If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it... of the manifestations of the profoundest reason and the most radiant beauty, which are only accessible to our reason in their most elementary forms...
Albert Einstein

The math shows that we would be weightless near the mass center of Earth. The moon and Sun gravity make hardly any difference as Earth is accellerated in the direction of their pull, mostly cancelling the slight gravity. We have not traveled even 1% of the way to the center. A slight increase (on the average) has been observed, thought to be because the average density increases with depth. The pressure is thought to be enormous due to the weight of the rock etc above. Neil