Fixed stars

In astronomy, the fixed stars (Latin: stellae fixae) are the luminary points, mainly stars, that appear not to move relative to one another against the darkness of the night sky in the background. This is in contrast to those lights visible to the naked eye, namely the planets and comets, which appear to move slowly among those "fixed" stars. 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 (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.

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. 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 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. 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 (c. 429–347 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. 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.