Fixed star

The fixed stars (Latin: stellae fixae) is the background of celestial objects that do not appear to move relative to each other in the night sky compared to the foreground of Solar System objects that do. Generally, the fixed stars are taken to include all stars other than the Sun. Nebulae and other star-like objects may also be counted among the fixed stars.

Exact delimitation of the term is complicated by the fact that no celestial objects are in fact fixed with respect to each other. Nonetheless, extrasolar objects move so slowly in the sky that the change in their relative positions is nearly imperceptible on typical human timescales, except to careful examination, and thus can be considered "fixed" for many purposes. Furthermore, distant stars and galaxies move even slower in the sky than comparatively closer ones.

People in many cultures have imagined that the stars form pictures in the sky called constellations. In Ancient Greek astronomy, the fixed stars were believed to exist on a giant celestial sphere, or firmament, that revolves daily around Earth.

The phrase originated in classical antiquity, when astronomers and natural philosophers divided the lights in the sky into two groups. One group contained the fixed stars, which appear to rise and set but keep the same relative arrangement over time. The other group contained the naked eye planets, which they called wandering stars. (The Sun and Moon were sometimes called planets as well.) The planets seem to move and change their position over short periods of time (weeks or months). They always seem to move within the band of stars called the zodiac by Westerners. The planets can also be distinguished from fixed stars because stars tend to twinkle, while planets appear to shine with a steady light.

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Wikipedia