Constellation

See also: Constellation (homonymy)

A constellation is a whole of star S to which the Projection S on the vault of heaven are sufficiently close so that a given civilization decided to connect them by imaginary lines, thus tracing a figure on the vault of heaven. A constellation is thus a particular Astérisme.

In three-dimensional space, the stars of a constellation usually are very dispersed, but they appear to be gathered on the imaginary plain of the night sky.

Various cultures recognized different constellations, although some of most visible tend to frequently reappear, such as for example Orion and the Scorpion, because of their brightness, of the proximity of the projection of stars, the passage of the Planet S.

The Western constellations are gathered in two parts, dividing the sky into following the two terrestrial hemispheres more or less, the southern sky for the south and the boreal sky for north. The boreal constellations are oldest and correspond to the visible patch of sky since the areas of the the Mediterranean by the astronomers of the Antiquité. The southern constellations were not named by the Western astronomers before at least the 15th century (even if, of course, the inhabitants of the southern hemisphere had observed stars of them well before this time). They were also useful, formerly, of reference mark for the sailors left at sea.

Currently, the international astronomical Union (UAI) divides the sky into 88 official constellations with precise borders, so that any point of the sky belongs to a constellation. Those are substantially based on the Hellenic tradition and pre-Hellenic, transmitted through the medieval era.

Western constellations

Ancient constellations

By an obvious lack of historical documents, it is impossible to know the precise origin of the oldest Western constellations. It seems that the Lion, the Taureau and the Scorpion existed already (not inevitably under these names) in Mésopotamie towards 4  000 years front J. - C.

Among the 88 constellations adopted by the UAI, about half come the Greek astronomers . Homère mentioned Orion in the Odyssée as of. The Zodiaque appears towards, divided into 12 constellations. Aratos de Soles fixed the main part of the names of constellation taken again by Ptolémée at our era.

The exhaustive compilation of constellations oldest that one knows goes back with Ptolémée, the 2nd century, and its Almageste when it grouped 1  022 stars in 48 constellations. This work will be the base of work of the Western astronomers until the end of the Moyen-âge. It includes/understands however only visible stars of Alexandria where Ptolémée made its observations.

Zodiac

See also: Zodiac

The constellations of the Zodiaque, band of sky containing the layout of the orbits of the Sun and the Planet S, are generally oldest, having an obvious importance for the location and the observation of the movements of the Moon, that it is in Astrologie or Astronomie (these two disciplines then were largely confused). It is also in the zodiac that one finds the twenty eight stations lunar traditional, asterisms which were used as calendar with open sky to follow the movements of the Moon.

The constellations present in the Zodiaque are: the Ram, the Bull, the Gemini, the Cancer, the Lion, the Virgin , the Balance, the Scorpion, the Sagittarius, the Capricorn, the Aquarius and the Poisson, which complete the cycle.

Astronomically speaking, one can notice that the Ophiuchus forms integral part of the Zodiaque, since the Sun crosses its current limits definite by the UAI from November 30th to December 17th. The old ones however did not raise it for purely esthetic or astrological conditions: only the south of the constellation is crossed by the Sun and the brilliant stars of the Scorpion of it are close. The constellations known as of the zodiac are thus only those of the very known list of twelve constellations. Right from the start, the constellations thus did not merge with the signs of the Zodiaque.

Constellations of Ptolémée

In addition to the twelve constellations of the Zodiac, Ptolémée made an inventory of 36 other figures:

The 48 constellations registered by Ptolémée in its Almageste will be used during more 1  000 years in occident without any change nor addition. Put aside the immense Argo Ship, cut out later in four constellations, they all will be adopted without any modification by the UAI.

Modern constellations

The Almageste of Ptolémée passed in the hands of the astronomers Arab S who supplemented his observations, adding some constellations which are not currently used any more, lengthening some (like the Éridan) in order to mention visible stars since the Latitude S more southern than that of Alexandria.

The Almageste being a work then lost in Europe, the Western astronomers obtained from them copies only in the last part of the Moyen-âge, starting from translations of Latin Arabic, at the same time as a certain number of observations of the Arab astronomers.

As from the 16th century, when the European countries left to explore the seas of the southern hemisphere, they discovered new stars which were mentioned in no known constellation. It was thus necessary to invent news of them.

Proposals of Johann Bayer

The German astronomer Johann Bayer published in 1603 the Uranometria , the first astronomical atlas entirely covering the celestial Sphère. He contained, in addition to those of Ptolémée, 12 new constellations visible since the southern hemisphere. These constellations probably were charted by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman which made profit Bayer from their inventions:

These new constellations, at the exotic names, arrived the first on a still virgin celestial planisphere and were such a success that they are always used nowadays.

They also testify to a change of perception in what is a constellation. The former Greeks divided the sky into two parts: constellations and spaces between those which were judicious to belong to none. Johann Bayer, by producing a sky chart for each constellation, starts to attach any point of the sky to a given constellation.

Profusion of new constellations

Starting from the publication of the Uranometria , the European astronomers will try to impose a maximum of their creations, without however meeting same success as Bayer.

In 1624, the German astronomer Jakob Bartsch defines five new constellations between several already existing. Only the Unicorn, the Giraffe and the Croix of the South remained to us, the Tiger and the Jordan failing without posterity.

About the same time, Tycho Brahé raises with the row of constellation the asterism of the Chevelure of Bérénice.

In 1643, Anton de Rheita imagines a Figure of Jesus between the Lion and Hydre, a Fly close to the Ram, renamed Fleur of lily under Louis XIV: to name the constellations becomes a play of courtier. In France, Augustin Royer uses a group of stars between Andromède, Céphée and PEGASE which it names the Sceptre. In Prussia, the royal astronomer Gottfried Kirch creates a second sceptre under Éridan in order to give him the counterpart. These claims of prestige are not essential in the community of the astronomers.

Johann Hevelius

Towards 1690, Johannes Hevelius, burgomaster of Gdańsk, proposes several constellations:

  • the Gun dogs

  • the Ecu of Sobieski
  • the Lizard (instead of the Sceptre of Augustin Royer)
  • the Lynx (because its stars are, to paraphrase Hevelius, if weak that one needs eyes of Lynx to see them)
  • the Petit Lion
  • the Petit Fox (originally the Fox with goose, being described as a fox which flees by carrying a goose in its mouth)
  • the Sextant

These denominations, not attached to any sovereign, modest, will have finally more success than all the others and will remain until our time.

Additions of Nicolas-Louis de Lacaille

Nicolas-Louis de Lacaille is abbot, astronomer and mathematician. Having remained in 1750 and 1751 with the Cape in South Africa in order to proceed to systematic statements of stars of the southern hemisphere, the Ruail reference in its work Coelum southern stelliferum (published in 1763, after its death), several new constellations in order to supplement still virgin spaces of sky of any denomination:

The selected names reflect the ideas of the time, more carried towards Science and Technology that towards the Adventure and Mythology. Moreover, the Ruail dismantles the Argo Ship in three smaller constellations in order to handle it more easily.

Constellations today

In the Years 1920, the international astronomical Union decides to put order in the constellations and to define the limits rigorously of them. The official atlas of the constellations, defined in 1930 by Eugene Delporte, divides the following sky of the lines of Right ascension and variation. Delporte used the data valid for the time B1875.0; because of the Precession of the equinoxes, the limits of the constellations perfectly any more horizontal and vertical on a modern sky chart, are not usually drawn according to the coordinates of right ascension and variation of the time J2000.0.

The layout was made so as to respect the traditional memberships of various brilliant stars to their traditional constellation. As far as possible, the fastening of stars or weaker celestial objects, which had been quoted in the scientific literature, was also respected. So these limits very tortuous, are sometimes pushed on a side or other to include such star and to leave such other in the close constellation. Today, the constellations do not have any more in astronomy the same interest but before: the celestial objects being referred by their coordinates, their position in such or such constellation does not have great importance.

Nowadays, the constellations, objects of measurements and attention during centuries, are from now on more really known only amateurs. Their limits hardly any more have of importance for these observations, which stick only to the figures formed by stars to identify them.

See also: Location of the constellations

Chinese constellations

See also: Chinese Astrology

Following the example Greek astronomers, the Chinese astronomers gathered certain stars in constellations on the zone of the ecliptic and the celestial equator, in a way similar to the Western Zodiaque. This “Chinese zodiac” was however divided into four “districts” (四象, sixiang ), represented by four animals, themselves divided into 28 “houses” (宿, sù), crossed by the the Moon during one month lunar.

The Chinese constellations are in general regrouping completely different from the Western constellations. One finds successively the following constellations:

  • Dragon of azure of the East (東方青龍):

  • black Tortoise of North (北方玄武):

  • white Tiger of the West (西方白虎):

  • Bird vermilion of the South (南方朱雀):

Movement of the constellations in the sky

Owing to the fact that the Ground rotates, one observes the constellations turn around a center which point the terrestrial axis of rotation, i.e. α Ursae Minoris in the northern hemisphere, σ Octantis in the southern hemisphere. This is why on the celestial charts, the pole star is reproduced there in the center (σ Octantis is unfortunately too not very luminous to be easily observable).

Thus, in the northern hemisphere, the constellations which are close to pole star never go down in lower part from the horizon and are visible all the year starting from the latitudes where the pole star is not too low in the sky: they are called the circumpolar constellations. They are, for example, the Small one and Large Ourse, Cassiopée, Céphée and the Dragon. Contrary, the majority of the constellations are visible only in certain seasons, like Orion, visible in winter, the Quadrant in summer, the Lion in spring, or visible Andromède in autumn.

See too

Internal bonds

External bonds

  • All constellations on Astronoo
  • constellations disappeared
  • constellations from the Northern hemisphere

References

Nds-nl: Konstelloatsy Simple: Constellation Zh-yue: 星座

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