What is a constellation


The title of this article is ambiguous. Further meanings are listed under Constellation (disambiguation).

As constellation becomes apparent in visual astronomy and astronomical phenomenology position bright celestial bodies refer to each other as they appear from the position of an observer on earth. The term is used to this day in both astronomy and space travel as well as astrology.


The term is made up of the Latin parts stella ("Star") and the prefixcon ("Together"), loosely translated as "meeting of stars". Its original meaning goes back to astrology (see below - Chapter Constellations of ...) and is here synonymous with the Greek word έποχή (epoch) = Breakpoint. It means "holding on" or the "stopping point" of the image of a temporary meeting of celestial bodies.[1]


The term mainly describes special angular positions of the large ones Planets relative to the sun (conjunction, opposition, etc.), but also des Earth moon to the sun and the planets. A constellation is sometimes also used to describe their encounters with bright fixed stars or the position of bright moons (e.g. the moons of Jupiter) in relation to their central body.

These are apparent views of the sky for the location (reference system) of the observer. As Planet stand, Moon position etc. they refer to a coordinate system in the fixed star sky or to the position of the celestial bodies in relation to one another (aspect, v. lat .: aspectus = Sight, view).

Constellations in a broader sense relate to relative star locations (Constellation of stars) and the position of celestial bodies in relation to celestial mechanical reference points such as the equinox, periapsis or celestial pole.

Special constellations have their own names, although they are still used today - in astrometry ("star measurement") or as astronomical events in the ephemeris calculation ("astronomical diary") - according to the historical terms "wandering stars" (planets, planetoids, moons) and " Fixed stars ”(“ real ”stars).

Constellations of "wandering stars" (planets, moons, comets, etc.)

The elongations (angular distances) of planets in relation to the sun as seen from earth

The most common of these constellations relate to the angle between the object and the sun that is visible from Earth. His name is Elongation and is the difference between the ecliptical lengths of the sun and the object:

  • opposition ("Facing") a celestial body, the elongation is 180 °
  • conjunction ("Encounter"), the elongation is zero
  • quadrature ("Crossing"), an elongation of 90 °
  • In astrology too Sextile and Trine of planets (mutual angular distance 60 ° or 120 °)
  • Half phase (Dichotomy “separation in two halves”), a phase angle of 90 °
  • the largest or smallest elongation with the inner planets
  • Planetary series or. Line up - along the ecliptic, or with other stars or the moon in relation to the horizon
  • Passage (passage, transit, Eclipse / occultation, cover, Near Encounters), various events that relate to a small angle difference between two objects.

Constellations of "fixed stars"

Apparent star clusters

So-called Asterisms without physical (gravitational) connection:

  • the 88 constellations that the International Astronomical Union (IAU) has set as the standard for sky mapping
  • historical constellations of western astronomy
  • Constellations in other cultures (Sumerians, Ancient China, India, Mayans, etc.)
  • bright Asterisms - especially with a conspicuous shape such as B. Star rows: they are mostly only apparent clusters, whereby the stars can have very different distances.

True star clusters

Star groups and clusters whose members are spatially closely spaced:[2]

  • Open star clusters: stars formed at the same time, which still form a loose group of 20-200 suns - z. B. Pleiades (seven stars) and Hyades in Taurus, Praesepe in Cancer, etc.
  • Close star groups in star clusters: Trapeze of the Pleiades, bull's head of the Hyades, star chains in nearby star clusters such as Messier 36
  • Globular clusters: very compact "nebulae" with 50,000-500,000 stars, which can only be resolved into single stars in the telescope at the edge
  • Galaxies:
    • the Milky Way and its nearby spiral arms, the brightest stars of which can appear as part of some constellations or as star clouds
    • nearby spiral nebulae, of which individual giant stars can be seen in large telescopes
  • Groups of galaxies (Galaxy clusters) and the large structures of the universe: they cannot be seen with open eyes and are therefore only rarely referred to as a constellation.

In practice, because of the difficulties in measuring fixed stars at great distances, it is not always possible to determine with certainty whether a grouping of fixed stars is an apparent or a real cluster.

Web links

Individual evidence

  1. ↑ έποχή. In: Gustav Eduard Benseler et al .: Greek-German school dictionary. 13th edition. B.G. Teubner, Leipzig 1911, p. 341
  2. ↑ Albrecht Unsöld: The new cosmos. Springer-Verlag, Berlin 1967 and 2005