According to John, The New Jerusalem will descend from Heaven and land on Earth. But this giant city is supposed to be in the dimensions of a perfect, Godly cube that measures 1,500 x 1,500 x 1,500 miles. "How could a cube with 6 flat sides that's 1,500 miles wide, deep and tall land on the surface of a spherical planet that's only 8,000 miles across?" you might ask. Either the cube or the planet would have to be seriously deformed, wouldn't it?! If each side of the cubical New Jerusalem is a full 1,500 miles in length, then we simply apply Pythagorean Theorem and find the hypotenuse between it's opposite base corners thus: 1500 + 1500 = 2121.32 Let's simply round it off to 2,121 miles from corner to corner to make it simple, eh?
At those dimensions and measurements, the corners of The New Jerusalem are some 15 miles up in the sky when its center is touching the Earth unless, of course, the thing crushes and compacts the living hell out of everything under it to a depth of 15 miles deep! Ouch! If it somehow were to manage to uniformly squish the Earth even half that much though, the corners are still some 7 and a half miles above the planet, while the middle edge area between the corners is still some 4 miles high. Got your ladder handy? The really tall one? Cause the 12 doors to the place are up along there somewhere. Three on each side, one for each Hebrew goat-herding tribe, remember?
And they are 500 miles apart, then. Fantastic. While one gate is in Cornwall, say, the other gate is somewhere north of Glasgow. Getting in and out of this bizarre monstrosity isn't going to be too easy, is it? But then, who needs to travel when there's all that cringing in front of God and the various chosen ones to be done? Other problems involve the mass of the object itself. At the Bible-given dimensions, including thickness of the walls and materials used, the mass involved would cause it to deform in upon itself into a somewhat spherical nature, like a ball or a moon - not the cubical description given by John.
On top of that, the effects of The New Jerusalem's mass interacting with the mass of the Earth would cause global destruction through world-wide earthquakes, not to mention putting a SERIOUS wobble effect into the Earth's rotation that would change climates and cause untold chaos with the Earths magnetic fields, relationship with the moon and possibly even circumnavigation of the sun. Because of its sheer mass, it's a global killer, even if it settles slowly and softly onto the Earth's crust. With that much mass, the madness would begin before it even touched down.
Length of each side (with one corner) = 1500 miles - 200 feet Effective length of wall required = 6000 miles - 800 feet = 9656.30784 kilometers Footprint of walls = 635740408 m^2 Volume of walls = 1.53468752 10^15 m^3 Density of Jasper ~= 2.65 g cm^-3~= 2650 kg m^-3 Mass of walls ~= 4.066921928 x 10^18 kg Height of the Tower of Babel = 91 metres (OT attempt to 'reach heaven' which unfortunately had to be stopped by gawd - so approximately the minimum height of heaven) Velocity of an object hitting the ground when dropped from 91 metres (neglecting air resistance which in the context is TOTALLY insignificant) = (91x2xg)^1/2 ~= 42.2542 ms^-1 Kinetic energy of the walls the moment before they hit the ground = 1/2 x 4.066921928 x 10^18 x 91 x 2 x g= 3.630581874 x 10^21 J Resultant earthquake: log (3.630581874 x 10^11) ~= 11.56 on the Richter scale (~= 60 thousand billion tons of TNT) Of course, its corners are still either 15 miles up in the sky, or its crushed every hill, mountain, valley, stream, lake, fish, tree, human, building, town, and cow flat as a flit, while driving them all into the Earth's crust a staggering 15 miles deep which, by the way, is only a max in SOME places on the Earth of 30 miles deep to begin with. The deepest ocean is 6.78 miles deep, to give you some idea. To provide a little more perspective, Mt. McKinley is the tallest mountain the U.S. has to offer with it's peak at 20,320 feet above sea level. That's 3.8 miles high. The New Jerusalem, at 1,500 miles tall, would be nearly 400 times the height of our tallest mountain.
Does that sound even remotely realistic to anyone, or do you think John simply had no idea what kind of problems physics, geology and square balls would present, since he thought the Earth was flat, and no god ever told him otherwise?
Most of this material came from www.buckcash.com/opinions/nj.htm by Buck Cash, who may be contacted at buckcash@buckcash.com

