Fixed stars
In astronomy, the fixed stars are the lights, mainly stars visible to the naked eye, that appear not to move relative to each other against the dark background of the night sky. They are defined in contrast to lights that appear to move relative to each other and to the fixed stars, which include classical planets and comets. The fixed stars include all the stars visible to the naked eye other than the Sun, as well as the faint band of the Milky Way. Due to their star-like appearance when viewed with the naked eye, the few visible individual 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 either never had a belief in a sphere of fixed stars, or constructed it in different ways.
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
Pythagoreans
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 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 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
's 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. 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.Eudoxus of Cnidus
, a student of Plato, was born around 400 BC. 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. 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.Aristotle
, who lived from 384 to 322 BC 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. He claimed that all things have some way of moving, 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. He stated that everything was moved by something and started exploring a concept similar to gravity. He was one of the first to argue that the Earth was round, drawing on observations of eclipses and the movements of the other planets relative to the Earth. 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. Aristotle also proposed a fifth element called "aether," which is purported to make up the Sun, the planets, and the stars. 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. 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". 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
, 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. 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.Claudius Ptolemy
, 100–175 AD, summarized ideas about the cosmos through his mathematical models and his book Mathematical Syntaxis, much more commonly known as the Almagest. 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. He utilized a method using eclipses to find the star distances and calculated the distance of the Moon based on parallax observations. Shortly after, he wrote a follow-up called Planetary Hypotheses.Ptolemy used and wrote about the geocentric system, drawing greatly on traditional Aristotelian physics, 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. 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.
Martianus Capella
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.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 and the quadrivium, that structured early medieval education. Nonetheless, his single encyclopedic work, De nuptiis Philologiae et Mercurii, also called De septem disciplinis was read, taught, and commented upon throughout the early Middle Ages and shaped European education during the early medieval period and the Carolingian Renaissance.