User:TCOU53378/Fixed Stars

Pythagoreans
Pythagorean philosophers held a number of different views on the structure of the universe, but each included a sphere of fixed stars as its boundary. Philolaos (c. 5th cent. BCE) proposed a universe which had at its center a central fire, invisible to man. All of the planets, the moon, sun, and stars rotated about this central fire, with the Earth being the nearest object to it. In this system, the stars are contained in the furthest sphere, which also rotates, but too slowly for motion to be observed. The motion of the stars is instead explained by the motion of the Earth about the central fire.

Another Pythagorean, Ecphantos of Syracuse (c. 400 BCE) proposed a system quite similar to that of Philolaos, but without a central fire. Instead, this cosmos was centered on the Earth, which remained stationary but rotated on an axis, while the moon, sun, and planets revolved about it. This system’s final boundary was a fixed sphere of stars, and the perceived motion of the stars was thought to be caused by the rotation of the Earth.

Plato
Plato’s (c. 429-347 BCE) universe was centered on a completely stationary Earth, constructed with a series of concentric spheres. The outer sphere of this system consisted of fire and contained all of the planets (which according to Plato, included the moon and sun). The outermost portion of this sphere was the location of the stars. This sphere of fire rotated about the earth, carrying the stars with it. The belief that the stars were fixed in their place in the sphere of fire was of great importance to all of Plato’s system. The stars’ position was used as a reference for all celestial motions and used to create Plato’s ideas of planets possessing multiple motions.

Aristarchus of Samos
Aristarchus (3rd cent. BCE), proposed an early heliocentric universe, which would later inspire the work of Copernicus. In his model, the sun, entirely stationary, laid at the center, and all planets revolved around it. Beyond the planets was the sphere of fixed stars, also motionless. This system presented two more unique ideas in addition to being heliocentric: the Earth rotated daily to create day, night, and the perceived motions of the other heavenly bodies, and the sphere of fixed stars at its boundary were immensely distant from its center. This massive distance had to be assumed due to the fact that stars were observed to have no parallax, which can only be explained by geocentricity or immense distances which create a parallax too small to be measured.

Copernicus
Nicolaus Copernicus (1473-1543) (see illustration at top of pages) created a heliocentric system composed of orbs carrying each of the heavenly bodies. The final orb in his model was that of the fixed stars. This final orb was the largest of his cosmos, in both diameter and thickness. This orb of stars is entirely fixed, as the stars are embedded in the sphere, and the sphere itself is immobile. The perceived motion of the stars, therefore, is created by the daily rotation of the Earth about its axis.

Brahe
Tycho Brahe’s (1546-1601) system of the universe has been called “geo-heliocentric” due to its novel structure. At its center lies the stationary Earth, which is orbited by the moon and sun. The planets then revolve about the sun while it revolves about the Earth. Beyond all of these heavenly bodies lies a sphere of fixed stars. This sphere rotates about the stationary Earth, creating the perceived motion of the stars in the sky. This system has an interesting feature in that the sun and planets cannot be contained in solid orbs (their orbs would collide), but yet the stars are represented as being contained in a fixed sphere at the boundary of the cosmos.