Constellations have probably existed for as long as man has put names to pictures. By drawing imaginary lines between stars and around stars, the night sky could be turned into a fresco of pictures. Over time, many different pictures and their names turned into a standard system of constellations, all of them with attached stories.
Why were constellations useful?
Pictures and stories are easier to remember than lists of data. This is because pictures are more perceptually rich than isolated data, while pictures which are also connected with stories add an additional layer of associations and mental processing. In general, it is easier to remember data when it has multiple associations than it is to remember only the list of data without any extra associations.
Thus, a specific star in the night sky is easier to identify when it is part of a constellation. Because of the ancient constellations, the stars could become the basis for ancient calendars and ancient navigation. The location of known stars in the night sky could signal the upcoming flood of the Nile River, show which way was north, and even measure distance between distant cities.
Ancient constellations were also the basis for astrology. Within a human lifespan, the constellations never changed, except to alter their position in the night sky in predictable ways throughout the night and throughout the year. Because the stars and constellations were outside the changeable world, their nature and positions were seen as being of divine origin. Thus, the cycles of the constellations and planets could be studied as a way to achieve insight into the world and the greater cosmos which surrounded it.
How do star catalogs use constellations?
Modern astronomy no longer believes in the fixed firmament. However, the old constellation names still continue to be used, even in modern astronomy.
= Ptolemy =
The earliest surviving attempt to categorize the stars by their positions in the constellations is Books VII and VIII of Ptolemy’s “Almagest.” This star catalog identifies 1022 stars within 48 constellations. Every star is sorted by brightness within the constellation to which it belongs. This 2nd century system was heavily plagarized from Hipparchus’ system 3 centuries earlier, but Ptolemy’s work is the dominant one to have survived into modern times.
= Bayer designation =
After being lost to Europe for centuries, a Greek translation of the Almagest returned to Western Europe in the 15th century by way of Arabic astronomy. In 1603, Johann Bayer updated the information in his own star atlas, “Uranometria.” This system added 12 new constellations from the southern hemisphere to the Ptolemaic constellations, but used the same system of sorting stars by brightness within the constellations to which they belonged.
Bayer also updated Ptolemy’s alphabetic ranking of stars by magnitude into a simple combination of Greek letter and constellation. For example, the brightest star in the constellation Centaurus became Alpha Centauri. If more than one star in a constellation fell into the same brightness class, its ranking was based on its position within the constellation, starting at the head of the traditional figure and going downwards. In a few cases, Bayer ignored brightness classes entirely in favor of the star’s position within the constellation.
The main weakness of the Bayer designation is that there are only 24 letters in the Greek alphabet. To get around this limit, Bayer often gave adjacent stars the same Greek letter, differentiated with numerical superscripts. For example, the stars of Orion’s shield were classified as Pi^1 to Pi^6 Orionis. This could be confusing, even with only the 1,564 stars classified by Bayer. As astronomers kept discovering more stars within the constellation, the system soon fell apart.
= Flamsteed designation =
The other major constellation-based star catalog was the “Historia coelestis Britannica,” which included 2,935 stars sorted by constellation and brightness. This massive work by John Flamsteed, based on 40 years of work at Greenwich Observatory, was nearly lost before it was ever discovered, because Flamsteed was reluctant to release unverified data. A pirated version of the catalog was published by Isaac Newton and Edmund Halley in 1712, without Flamsteed’s permission. Flamsteed managed to buy back 300 of the 400 printings, and burned them. An authorized version of the Historia was not printed until 1725, 6 years after Flamsteed’s death.
Unlike Bayer’s Greek letters, the Historia used numbers to rank stars within their constellations. The Flamsteed designation does not use brightness at all. Instead, stars are numbered from west to east in order of right ascension. For example, 140 Tauri is the star which is farthest east in the constellation Taurus.
The Flamsteed numbers may not actually be Flamsteed’s invention. They appear only in the pirated version, and were removed in the authorized version. No one knows if they were added by either Newton or Halley, or if Flamsteed’s wife removed them prior to authorized publication.
Constellations in modern astronomy
Modern astronomy recognizes 88 constellations. Most of these are derived from Ptolemy, but several were added during the 17th and 18th centuries. These constellations have been standardized internationally to define specific areas of the celestial sphere.
However, modern astronomy no longer uses constellation-based cataloguing systems. There are simply too many stars. As well, most stars in a constellation are not in physical proximity to each other. This alone makes it nearly impossible to combine any classification based on absolute location with the constellation in which a star appears to be.
Yet in everyday astronomical usage, both Bayer’s and Flamsteed’s constellation-based systems still survive. For practical purposes, very few people would refer to Alpha Centauri as anything but Alpha Centauri, or 61 Cygni as anything but 61 Cygni. In general, Bayer’s system is used where the Greek classification is straightforward, and Flamsteed’s system is used wherever Bayer resorted to numerical superscripts.
Most stars south of Europe were never formally classified in either system. These stars are sometimes referred to by a variation of Bayer’s system, using southern hemisphere constellations.
Finally, some of Bayer’s and Flamsteed’s catalogued stars fell into a different constellation after the constellation borders were standardized in 1930. In these cases, the original classifications are now useless, except for historical purposes.
