Bah. I have to answer BOTH these questions, because I have both an alternate Earth and that alternate Earth's magic world, which isn't Earth at all. XD
Q: (Alternate Earth)
How similar are the history and culture of the alternate earth to real history and culture? Why is it so similar/different?
A: Identical to the history and culture of our Earth, at least on the surface. The events that led to the creation of the magic world occurred either prehistory or were swallowed up and obscured in myth. On this Earth, originally the planet was a world of magic, until a group of very powerful beings with now-obscured agendas took it upon themselves to create another world, a world built on magic, one they could control, separated from the currently developing nonmagic cultures. Some say they did it to protect the nonmagic beings. Some say they did it to gain power indescribable. Some say they did it to recreate an old, fallen world; believing they could recreate an Eden.
Whatever their motivations, these beings succeeded in creating the magic world. What they failed in doing was to retain any sort of control over their creation.
The magic "firmament" of the Earth was divided from the nonmagic "firmament"; a schism was created and Earth became two worlds. Earth's magic world (in modern times called the Aether) was placed on an artificially created dimensional plane, using a combination of magic and technology beyond the grasp of the modern day's most advanced sciences.
To separate the two worlds great dimensional Barriers were built, constructed through contracted alien labor and technology of the Gallyava race. Only at certain points were natural portals created into the impassable Barriers, and the first of the Blades were created to guard them.
It seemed the creation was perfect. But something went wrong.
To this day no one knows how, but the dimensional plane of the Aether tore loose from its moorings to Earth. For an unknown span of time the Aether wandered in extradimensional space, cut lose from time, until finally it came back to rest roughly in its old moorings, partially reconnecting with the old portals and tethers that had once held.
Scant centuries had passed on Earth. But untold millennia had passed on the Aether plane, and it had evolved out of all control. To this day, no one knows what happened to the creators of the Aether. It is widely assumed by scholars who study these things that over time they and their progeny were absorbed and assimilated without a trace into the world they had created.
In current times, the Aether floats in an unstable dimensional orbit around Earth. Portal connections are ever moving, sometimes infinitely slow, sometimes blindingly fast. On several occassions since the Aether settled into its current moorings, its orbit will take it too far from Earth, and another break in time will occur. Depending on the dimensional angle, years may pass on the Aether while minutes pass on Earth, or vice versa, or but a total difference of minutes or even seconds.
The Blade Society, which survived the millennia-long schism to the present day, are currently working on stabilizing the dimensional orbit of the Aether. Unfortunately, they have another problem distracting them at the moment; the Barriers between the magic world and the human world, under unstable and varying pressures they were never designed for, have begun to form cracks.
Q: (Not Earth at All)
How does this world differ physically from earth? Is it the same size (same density, same gravity), same ratio of land/water, same atmosphere, etc.? Does it have more than one sun or moon? Rings? Are there spectacular constellations/comets, etc. visible at night or by day?
The known surface of the Aether (and the section that connects with Earth) is shaped roughly like a great disk. It is possible that the Aether is a huge globe, but as travel is difficult to impossible beyond the immense central disk of the Aether, the entire world is often referred to as a "disk".
If the Aether is
literally a disk, no one knows how large it is, or where the edges are. It is known that no one has ever made it to any of the theoretical edges, even with judicious use of warp-space where possible.
The gravity, atmosphere, seasons, etc are designed to mimic Earth's, through self-replicating magic processes. The sun, moon and sky of the Aether match Earth's in most places. The Aether is certainly larger than Earth, but besides the ever-presence of magic, mimics it in the area of physics. In the middle of the central disk, in the central continental spiral, the ratio of land to water is actually higher than Earth's in favor of land.
Many types of alien celestial phenomena occassionally decorates the sky at night. This is generally a side effect of magic forces moving on the ground below. Sometimes, this may cause stars, constellations, etc in the sky to become slightly distorted, but generally the sky mimics what would be seen from Earth in places that correspond to locations on the Aether.