Earth size, gravity & Moon orbit.

[michel123456]

So as i was spending my time, i had a look at this blog debunking the idea that the large size of dinosaurs was caused by a weaker gravity.

 

And i have a question:

 

In the blog, we see the following diagram

 

The relationship of gravity to size: if two objects have the same mass, but one is smaller than the other, the smaller object will have correspondingly higher surface gravity.

 

And lower in the blog the following statement at the end of "Blue plate tectonic special: (maybe it was intended to be "blue planet")

 

The truth is that Earth’s tectonic plates, on which the continents rest, are always in motion, rearranging themselves very slowly over tens of millions of years. Yet the Moon reliably orbits, which wouldn’t be true if plate tectonics made a huge difference to Earth’s gravity. In fact, there’s another sign Earth’s gravity hasn’t changed much in the last 100 million years: the Moon is actually moving away from Earth, albeit very slowly. If Earth’s gravity had doubled since the time of the sauropods, we would expect the opposite effect.

 

 

(enhancing mine)

 

Which statement raises my question:

Since the overall MASS of the Earth remains the same, why would the Moon's orbit change at all?

The phrase "If Earth’s gravity had doubled since the time of the sauropods, we would expect the opposite effect." looks dubious to me, because the doubling of gravity should be understood as "doubling of the effect of gravity at Earth's surface", not doubling of the overall gravity of the planet since the mass remains constant.

 

 

I hope my question is clear.

Edited May 25, 2014 by michel123456

[Acme]

...Which statement raises my question:

Since the overall MASS of the Earth remains the same, why would the Moon's orbit change at all?

...

 

I hope my question is clear.

Because of tidal forces.

 

Tidal acceleration @ Wiki

Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon), and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of the smaller first, and later the larger body. The EarthMoon system is the best studied case. ...

PS Because the Earth collects meteors and meteoric dust at a higher rate than it loses gasses to space, it's mass is actually slightly increasing over time.

[swansont]

So as i was spending my time, i had a look at this blog debunking the idea that the large size of dinosaurs was caused by a weaker gravity.

 

And i have a question:

 

In the blog, we see the following diagram

 

And lower in the blog the following statement at the end of "Blue plate tectonic special: (maybe it was intended to be "blue planet")

 

 

(enhancing mine)

 

Which statement raises my question:

Since the overall MASS of the Earth remains the same, why would the Moon's orbit change at all?

The phrase "If Earth’s gravity had doubled since the time of the sauropods, we would expect the opposite effect." looks dubious to me, because the doubling of gravity should be understood as "doubling of the effect of gravity at Earth's surface", not doubling of the overall gravity of the planet since the mass remains constant.

 

 

I hope my question is clear.

 

You can only have both be true if Newton's law of gravity is wrong.

 

g = GM/r^2

 

The only variables are M and r. If the mass stays constant, then the earth's diameter has to decrease. Which then requires a corresponding increase in the rotation rate, since the moment of inertia will change, and is contrary to the evidence we have, and a frikkin' change in the diameter of the earth. So there's no way to accommodate this conjecture within known physics or geology.

[michel123456]

 

You can only have both be true if Newton's law of gravity is wrong.

 

g = GM/r^2

 

The only variables are M and r. If the mass stays constant, then the earth's diameter has to decrease. Which then requires a corresponding increase in the rotation rate, since the moment of inertia will change, and is contrary to the evidence we have, and a frikkin' change in the diameter of the earth. So there's no way to accommodate this conjecture within known physics or geology.

Do you say that as we have evidence that the earth was spinning faster in the past it means that the Earth was smaller?

[Endy0816]

No, the spin is slowing down due to the Moon's influence.