Two Sphere Universe

The "Two Sphere Universe" is a term coined by Thomas Samuel Kuhn, in his book "The Copernican Revolution".

The two sphere universe is a conception of the astronomical heavens by ancient Greece. It was one of several conceptions of the universe and the one taken most seriously by the largest number of people, particularly by astronomers. Before the end of the second millennium BC, the Babylonians and Egyptians started methodical observations of the motion of the sun. This was partly motivated by agricultural purposes to predict seasons and the flooding of the Nile. But the earliest solar calendars were based on a 360 day year, a new round number in the sexagesimal number system of the Babylonians. But this year would creep, as over time the seasons moved by 5 days every year. The motion of the stars are much simpler and more regular then the sun's.

One key characteristic of the stellar motion, and observations thereof is that the North Star remains very nearly stationary. Careful observations show that it is not quite stationary, but the actual stationary point is only 1 degree away. This point is known as the north celestial pole. Because the stars and celestial pole retain the same relative positions hour after hour and night after night, they can be permanently located on a map of the heavens. This is called a circumpolar star map. Together observations of the heavens made by ancient astronomers provided no direct structural information. They told nothing about the composition of the heavenly bodies or their distance, no explicit information about size, position, or shape of the earth. They do not even indicate that the celestial bodies really move. Anyone can be sure that the angular distance between a celestial body and the horizon changes continually. The change might as easily be caused by a motion of the horizon as by a motion of the heavenly body. Terms like sunset, sunrise, diurnal motion are interpretations of the data.

The tradition that detailed astronomical observations supply the principal clues for cosmological thought is native to Western civilization. It is one of the most significant characteristic novelties that we inherit from the civilization of ancient Greece.

In the 6th century BC, Anaximander of Miletus taught that the stars are compressed portions of air, rotating wheels of fire emitting flames from small openings. The sun is 28 times the size of the earth, like a chariot wheel, the hollow rim full of fire emitting fire from a nozzle-like opening. Eclipses of the sun occur because the fire vent is shut up. The moon is a circle 19 times as large as the earth, like a chariot wheel, the rim hollow and full of fire, placed obliquely. Its eclipses depend on the turnings of the wheel. Astronomically, the ancient Greece conception by Anaximander were far in advance of the Egyptians'. The gods have vanished in favor of mechanisms familiar on the earth. The size and position of the stars and planets are discussed. The problems raised and discussion of diurnal circles are handled with some success by treating celestial bodies as orifices on the rims of rotating wheels.

For most Greek astronomers and philosophers, from the fourth century on, the earth was a tiny sphere suspended stationary at the geometric center of a much larger rotating sphere which carried the stars. That is, the Earth was the center of the universe, immobile, stationary and fixed. The sun moved in the vast space between the earth and the sphere of the stars. Outside of the outer sphere there was nothing at all. No space, no matter, nothing. These two spheres (earthly and celestial) comprise the two-spheres, in the "two-sphere universe". This was the only theory of the universe, but it was the one that gained the most adherents. It was this developed version of this theory that the medieval world inherited from the ancients.There were many two sphere universes. But, after its first establishment, the two-sphere framework was almost never questioned. For nearly two millennia it guided the imagination of all astronomers and most philosophers.

One of the earliest problems with the two sphere universe was the problem of the planets. Plato is said to have asked in the early fourth century BC: "What are the uniform and ordered movements by the assumption of which the apparent movements of the planets can be accounted for?" and the first answer was provided by his pupil Eudoxus (c.408-c.355 BC). In Eudoxus' planetary system each planet was placed upon the inner sphere of a group of two or more interconnected, concentric spheres whose simultaneous rotation about different axes produced the observed motion of the planet. These are now known as homocentric spheres, because they have a common center, the earth. Two or three such spheres can approximately represent the total motion of the sun and moon. Eudoxus' greatest genius as a geometer was displayed in the modification of the system for handling the other five known planets. Though not used very long homocentric spheres played a major and important role in the development of astronomical and cosmological thought.