Scientific Data Surah 85 · Ayah 1
A study of the properties and astronomical positions of Constellations and Asterisms
A study of the properties and astronomical positions of Constellations and Asterisms
Constellations are groups of stars which have been designated as representing a particular figure in the sky. Different cultures have done this in different ways, and many ancient and modern constellations are no longer in use. Eighty-eight of the traditional and modern Western constellations (based on Babylonian and Greek constellations of two to five thousand years ago) have been designated as "official" constellations by the IAU (the International Astronomical Union).
Constellations can be represented by stick figures, some of which are traditional, and others which are created by the illustrator of one book or another. The latter are often copyrighted (e.g., those used in The Stars by H.A.Rey). In the past, constellations were usually depicted by artistic drawings based on allegorical figures, and different books typically had different drawings, which didn't always include exactly the same stars. In 1930 the IAU defined constellation boundaries which enclose the regions traditionally occupied by allegorical figures, so that every part of the sky is inside one constellation or another. Stars which are inside the boundary of a constellation are usually said to be "in" that constellation, meaning that they are in a direction enclosed by that boundary.
Asterisms are groups of stars such as the Little Dipper, Big Dipper and the Pleiades, which are not constellations but have well-known names of their own. The Big and Little Dippers are part of the Big and Little Bears (Ursa Major and Ursa Minor). The Pleiades are an example of a cluster of stars which happens to be visible to the eye in the constellation of Taurus (the Hyades is another asterism, also in Taurus, which is also a cluster).
Even though a well-known group of stars, the Pleiades is not a constellation, but simply part of the constellation of Taurus. Many people confuse the Pleiades with one of the Dippers (which are also asterisms). The misty glow around the stars is light reflected from clouds of dust that the cluster happens to be passing through, and has nothing to do with the dust which surrounded them during their formation.
Until fairly recently maps in celestial atlases showed allegorical figures which more or less encompassed the region and stars traditionally associated with the constellations in that region.
Since the allegorical figures used for older constellation maps have little if any relationship to the stars "in" the constellations, some attempts were made as early as the mid 1800's to replace allegorical figures with stick figures that connected the stars. In many cases the stick figures bear no more similarity to the constellation name than the older allegorical figures, but in some cases they do give an impression of what the constellation is supposed to "look like" that is more satisfactory; and in most modern celestial atlases stick figures are used, instead of allegorical figures.
Since both allegorical and stick figures only give a general idea of where constellations are located and rarely show the "boundaries" of the constellations (and when they do, those boundaries vary from one atlas to another), in 1930 the International Astronomical Union "regularized" the constellations by assigning specific boundaries to each constellation. The boundaries jogged in and out to try to place stars that traditionally belonged to a given constellation inside that constellation's boundary, but were otherwise aligned with the hour circles of right ascension and parallels of declination according to the way the Earth rotated in 1875 (that being the approximate date when the idea was first proposed). Because of precession, the hour circles and parallels are gradually moving and tilting relative to their position more than a century ago, so the boundaries do not align with the current hour circles and parallels, and will become more and more misaligned as time goes on.