Good reply, Geo. I used Earth because certain aspects are in place for our planet, and those aspects are not fully in place for the Moon and what-have-you.

I keep it simple and flatten Earth's gravity into a clock, presenting in 2D therefore.

The pull experienced at 4 o'clock is from the entire planet. Maximum gravity experienced is found at the surface, because that is where the entire planet delivers its pull.

When I move halfway toward the center of the planet from that 4 o'clock position, then all matter from 7 o'clock via 12 to 1 o'clock is pulling on me, collectively toward the center, while all matter from 1 o'clock to 7 o'clock is pulling me in the opposite direction. As such, I experience a weaker gravitational field.

I told you I'd keep it simple.

In the center, all matter is pulling on me in equal strength, which means there is a net-zero position there. Team Lidenbrock was there before I got there, and they told us that their compasses were misbehaving wildly. I concur.

This was the story about gravity. Now the story about pressure, which is distinct.

The pressure increases the further we move toward the center. After so many billions of years, pressure helped separate matter and the heaviest parts are found in/near the center.

The pressure is such that for the inner core we find two different realities. In the center, all is stuck; the pressure is simply too high for any motion to occur. Any motion that was possible in the past caused heavier parts to sink toward the center and for light matter to move up.

Right where the pressure is a single tad shy of establishing an additional layer of stuck environment, we actually find the most motion on our planet. The simplest level of friction causes the second part of the inner core to be fluid.

Note how the pressure is still of an extreme high level at the fluid inner core, and we find a churning that simply cannot get stopped, all the way until the pressure further out above it has diminished and we reach the edge of the fluid inner core.

The fluid inner core is in motion like nothing else, and this heats up the planet.

Here is a bit of fun I have with Mars, because we have a much smaller planet with Mars, and when started out the same way as planet Earth, it could have had a solid inner core and a fluid inner core. However, size does matter, and the solid inner core was not substantial enough to withstand the churning of the fluid inner core.

Yes, I am fitting the story to the outcome, so it is not a strong story, perhaps just a fantasy I have fun with. Because we have the tallest mountain in the solar system on Mars, and some decent cracks in the surface as well, the idea is that the solid inner core got ejected through the fluid inner core due to the wild churning of the fluid inner core. It is then a bit like a bullet from a gun, with the bullet the solid inner core and the gun the fluid inner core. Like a child letting go of a discus that a professional Olympian would hold onto.

That internal action did not kill off everything inside Mars, but it did undermine the inner workings of keeping the planet warm like ours.

Even when this is a fantasy story, do recognize that it supports the idea that size matters. The level of pressure buildup helps to deliver the outcomes. Water at 100 degrees Celsius boils, so when there is just energy to get water to 98 degrees then it will not boil. Mars was just a bit too small to sustain that solid inner core and the beneficial fluid inner core process.

Glad we agree there is zero gravity in the center. For gravity, we can indeed have a from-outside-to-inside story and kind of keeping it simple.

For pressure, however, we have something very special happening. A line in the sand got drawn by nothing else but pressure, and this line has two different outcomes. One side things are stuck, the other side things are fluid, and wildly so.

Same for the Eye of the Storm.

The Eye is 'stuck' if we were to compare this to the enormous amounts of winds of the Storm. The Wall of the Eye shows us the strongest action, wind-wise.

Same for the Black Eye in which the collective of all masses in a galaxy create a gravitational center in which we find net-zero right smack in the middle (0, minimum action, 'stuck') and we find 1 as the maximum right next to it, with the strongest gravitational action anywhere in that entire galaxy.

0, 1, 0.9, 0.8, 0.7, 0.4, 0.2, 0.1, 0.0, 0.0.

0 = minimum, 1 = maximum.