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ividual nebulae and other deep sky objects are also counted among the fixed stars approximately 6 000 stars are visible to the naked eye under optimal conditions the term fixed stars is a misnomer because those celestial objects are not actually fixed with respect to one another or to earth due to their immense distance from earth these objects appear to move so slowly in the sky that the change in their relative positions is nearly imperceptible on human timescales except under careful examination with modern instruments such as telescopes that can reveal their proper motions hence they can be considered to be fixed for many purposes such as navigation charting of stars astrometry and timekeeping due to the large distances of astronomical objects human vision is unable to perceive the three dimensional depth of outer space giving the impression that all stars and other extrasolar objects are equidistant from the observer in the astronomical tradition of aristotelian physics which spanned from ancient greece to early scientific europe the fixed stars were believed to exist attached on a giant celestial sphere or firmament which revolves daily around earth hence it was known as the sphere of fixed stars which acted as the supposed limit of the whole universe for many centuries the term fixed stars was a synonym for that celestial sphere many ancient cultures observed new stars now called novas which provided some clue the heavens were not completely unchanging but as novae fade in a few weeks or months the phenomenon was not understood then as well as that of comets in european scientific astronomy evidence that disproved the firmament was gathered gradually the copernican revolution of the 1540s fueled the idea held by some philosophers in ancient greece and the islamic world that stars were actually other suns possibly with their own planets the definitive discovery of proper motion was announced in 1718 and parallax was suspected in the 1670s but shown definitively in the 1830s other cultures such as chinese astronomy either never had a belief in a sphere of fixed stars or constructed it in different ways see cosmology historical cosmologies people in many cultures have imagined that the brightest stars form constellations which are apparent pictures in the sky seeming to be persistent being deemed also as fixed that way constellations have been used for centuries and still are today to identify regions of the night sky by both professional and amateur astronomers astronomical models which included fixed stars edit pythagoreans edit 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 bc 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 1 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 1 another pythagorean ecphantos of syracuse c 400 bc 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 1 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 1 plato edit plato s c 429 c 347 bc 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 2 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 3 eudoxus of cnidus edit eudoxus a student of plato was born around 400 bc 4 a mathematician and an astronomer he generated one of the earliest sphere centric models of the planet systems based on his background as a mathematician eudoxus s model was geocentric with the earth being a stationary sphere at the center of the system surrounded by 27 rotating spheres 4 the farthest sphere carried stars which he declared to be fixed within the sphere thus though the stars were moved around the earth by the sphere which they occupied they themselves did not move and were therefore considered fixed 5 aristotle edit aristotle who lived from 384 to 322 bc 4 studied and published similar ideas to plato and based on the eudoxus system but he improved on them through his books metaphysics and on the heavens written around 350 bc 4 he claimed that all things have some way of moving including heavenly bodies or planets but he denies that the movement could be caused by a vacuum because then the objects would move much too fast and without sensible directions 4 he stated that everything was moved by something and started exploring a concept similar to gravity he was one of the first to argue and prove that the earth was round drawing on observations of eclipses and the movements of the other planets relative to the earth 4 he proceeded to conclude that most planets navigated in a circular motion his cosmos was geocentric with the earth at the center surrounded by a layer of water and air which was in turn surrounded by a layer of fire which filled the space until reaching the moon 5 aristotle also proposed a fifth element called aether which is purported to make up the sun the planets and the stars 4 however aristotle believed that while the planets rotate the stars still remain fixed his argument was that if such a massive body was moving there must surely be evidence that is noticeable from the earth 6 however one cannot hear the stars moving nor can they really see their progress so aristotle concludes that while they may be shifted by the planets they do not move themselves he writes in on the heavens if the bodies of the stars moved in a quantity either of air or of fire the noise which they created would inevitably be tremendous and this being so it would reach and shatter things here on earth 7 his theory that the stars may be carried but were fixed and do not autonomously move or rotate was widely accepted for a time aristarchus of samos edit aristarchus 3rd cent bc proposed an early heliocentric universe which would later inspire the work of copernicus in his model the sun entirely stationary lay at the center and all planets revolved around it 8 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 9 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 claudius ptolemy edit ptolemy 100 175 ad 5 summarized ideas about the cosmos through his mathematical models and his book mathematical syntaxis much more commonly known as the almagest 4 it was written around 150 ad and ptolemy declared that the stars placement in relation to each other and distances apart remained unchanged by the rotation of the heavens 5 he utilized a method using eclipses to find the star distances and calculated the distance of the moon based on parallax observations 10 shortly after he wrote a follow up called planetary hypotheses 10 ptolemy used and wrote about the geocentric system drawing greatly on traditional aristotelian physics 10 but using more complicated devices known as deferent and epicycles he borrowed from previous works by geometer apollonius of perga and astronomer hipparchus of nicaea 11 he declared that the stars are fixed within their celestial spheres but the spheres themselves are not fixed the rotations of these spheres thus explain the subtle movements of the constellations throughout the year 5 martianus capella edit martianus capella fl c 410 420 describes a modified geocentric model in which the earth is at rest in the center of the universe and circled by the moon the sun three planets and the stars while mercury and venus circle the sun all surrounded by the sphere of fixed stars 12 his model was not widely accepted despite his authority he was one of the earliest developers of the system of the seven liberal arts the trivium grammar logic and rhetoric and the quadrivium arithmetic geometry music astronomy that structured early medieval education 13 nonetheless his single encyclopedic work de nuptiis philologiae et mercurii on the marriage of philology and mercury also called de septem disciplinis on the seven disciplines was read taught and commented upon throughout the early middle ages and shaped european education during the early medieval period and the carolingian renaissance 14 nicolaus copernicus edit nicolaus copernicus 1473 1543 created a heliocentric system composed of orbs carrying each of the heavenly bodies 15 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 15 the perceived motion of the stars therefore is created by the daily rotation of the earth about its axis tycho brahe edit tycho brahe s 1546 1601 system of the universe has been called geo heliocentric due to its twofold structure 9 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 16 this sphere rotates about the stationary earth creating the perceived motion of the stars in the sky 16 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 16 johannes kepler edit johannes kepler 1571 1630 was a devoted copernican following copernicus s models and ideas yet developing them 4 he was also an assistant of tycho brahe and he could access his patron s accurate measurements in his observational database kepler s mysterium cosmographicum 1596 a strong defense of the copernican system still pictures an image labelling the outmost celestial sphere as sphaera stellar fixar latin for sphere of fixed stars following the long held belief in such sphere this view was later superseded in his book astronomia nova 1609 where he established his laws of planetary motion 17 the mathematical basis for his own rudolphine tables which are working tables from which planetary positions could be shown 10 kepler s laws were the tipping point in finally disproving the old geocentric or ptolemaic cosmic theories and models 18 what was backed by the first uses of telescope by his contemporary galileo galilei also an advocate of copernicus estimated radius edit first greeks as many other ancient cultures thought of sky as it was a giant dome like structure only a few meters above the highest mountains the myth of atlas tells that this titan held the whole heavens on his shoulders 19 around 560 bc anaximander was the first to present a system where the celestial bodies turned at different distances but erroneously he thought the stars were closer to earth about 9 to 10 times the earth s size than the moon 18 19 times and the sun 27 28 times 20 nonetheless later pythagorians as philolaus around 400 bc also conceived a universe with orbiting bodies 21 thus assuming the fixed stars were at least a bit farther than the moon the sun and the rest of the planets meanwhile circa 450 bc anaxagoras was the first philosopher to consider the sun as a huge object larger than the land of peloponnesus 22 and consequently to realize how far from earth it might be he had suggested that the moon is rocky thus opaque and closer to the earth than the sun giving a correct explanation of eclipses 23 as far as the sun and the moon were conceived as spherical bodies and as they do not collide at solar eclipses this implies than the outer space should have some certain indeterminate depth eudoxus of cnidus in around 380 bc devised a geometric mathematical model for the movements of the planets based on conceptual concentric spheres centered on earth 24 and by 360 bc plato claimed in his timaeus that circles and spheres were the preferred shape of the universe and that the earth was at the centre and the stars forming the outermost shell followed by planets the sun and the moon 25 around 350 bc aristotle modified eudoxus model by supposing the spheres were material and crystalline 26 he was able to articulate the spheres for most planets however the spheres for jupiter and saturn crossed each other aristotle solved this complication by introducing an unrolled sphere by all these devices and even assuming the planets were star like single points the sphere of the fixed stars should implicitly be farther than previously thought around 280 bc aristarchus of samos offered the first definite discussion of the possibility of a heliocentric cosmos 27 and by geometrical means he estimated the moon s orbital radius at 60 earth radii and its physical radius as one third that of the earth he made an inaccurate attempt to measure the distance to the sun but sufficient to assert that the sun is much bigger than earth and it is much further away than the moon so the minor body the earth must orbit the major one the sun and not the opposite 28 this reasoning led him to assert that as stars do not show evident parallax viewed from earth along a single year they must be very very far away from the terrestrial surface and assuming they were all at the same distance from us he gave a relative estimation following the heliocentric ideas of aristarcus but not explicitly supporting them around 250 bc archimedes in his work the sand reckoner computes the diameter of the universe centered around the sun to be about 10 10 14 stadia in modern units about 2 light years 18 93 10 12 km 11 76 10 12 mi 29 in archimedes own words his aristarchus hypotheses are that the fixed stars and the sun remain unmoved that the earth revolves about the sun on the circumference of a circle the sun lying in the mi...
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