Pleiades

The term "Pleiades" derives from Greek πλέω (pleiu), meaning ‘to sail’. In the Mediterranean, the sailing season (gentle winds and warm weather) was given from the heliacal rise to the opposition of this star cluster. The association of "seven" with the cluster is of unknown origin, but it is a fact that (a) Greek mythology created a tale of seven daughters of the nymph Pleione (the sailing nymph), and that (b) Mesopotamian religions even before this had associated seven (male) gods with the cluster. However, the number "seven" has never been part of the name of the cluster (unlike modern English).

Etymology and History

The significance of the cluster in almost all cultures on Earth is related to its usefulness in several calendar functions: Due to the different length of the solar year (365 days) and the lunar year (354 days), a leap month had to be inserted approximately every three years in Mesopotamia. Before these leap years could be set regularly, the rising of bright stars was consistently observed. The so-called Pleiades leap rule in MUL.APIN states that a month must be inserted as soon as the Pleiades rise at the beginning of the third month (heliacal), as they should normally do so in the second month. This switching rule was also handed down in the Arab world.

For the Phoenicians and Greeks, shipping between the numerous islands was of enormous importance. The Greeks therefore understood the name of the star ‘Pleias’ from πλέω (pleiu), meaning ‘to sail’, and Hesiod and Eratosthenes mention its significance as an indicator of the shipping season in the farming calendar.

The alternative name ‘Pleiades’ is also documented by Eratosthenes, who states: ‘One does not see seven stars, but only six.’ Greek mythology invents an unconvincing story of seven sisters, one of whom hid or left the group. Although the sisters have proper names in mythology, these names were not applied to the stars of the star cluster in ancient times. The modern naming is also confusing because two of the free-eyed stars were named after their parents, Pleione and Atlas.

Origin of Constellation

Babylonian

The standard name of the star in mathematical astronomy is a Sumerian loanword indicating an indefinite number of stars - loosely translated as ‘the star cluster’. In MUL.APIN, which is already written in Akkadian, only this Sumerian name is used, but later astronomical texts of the -1st millennium pass on ‘the bristle hairs’ of the bull as the Akkadian name. In MUL.APIN, where (almost) all celestial bodies are assigned to a deity, the Pleiades are associated with the so-called Seven Gods. These are seven sacred weapons that can speak and are perceived as deities. They are sometimes associated with the underworld. A relatively recently discovered fragment of the Gilgamesh Epic fits in with this, according to which seven underworld deities are the sons of Gilgamesh's first defeated opponent and the historical fact that before the earthly king Gilgamesh (with G) there was a god of the dead called Bilgamesh (with B). He was one of seven underworld gods before the myth of the king eclipsed this figure in importance. To summarise: We don't know, but a connection between the number seven and an early Sumerian death cult is obvious.

Greco-Roman

Aratos

Near his left knee all in a cluster [255] the Pleiades move. The space that holds them all is not great, and they are individually faint to observe. Seven in number they are in the lore of men, although there are only six apparent to the eye. No star at all has been lost from our ken in Zeus [260] since our oral tradition began, but this is just what is said. Those seven are called by name Alcyone, Merope, Celaeno and Electra, Sterope, Taygete and honoured Maia. All alike they are small and faint, but they are famous [265] in their movements at morning and evening, and Zeus is the cause, in that he authorised them to mark the beginnings of summer and winter and the onset of ploughing time. (Kidd 1997)

Hipparchus

..

Almagest

In Ptolemy's Almagest, the Pleiades are only sketched and not listed with all members. They form part of the constellation of Taurus:

id	Greek (Heiberg 1898)	English (Toomer 1984)	ident.
	Ταύρου ἀστερισμός	Constellation of Taurus
30	τῆς Πλειάδος τὸ βόρειου πέρας τῆς ἠγουμένης πλευρᾶς	The Pleiades: the northern end of the advance side	19 Tau
31	τὸ νότιον πέρας τῆς ἠγουμένης πλευρᾶς	The Pleiades: the southern end of the advance side	23 Tau
32	τὸ ἐπόμενου καὶ στεηότατον πέρας τῆς Πλειάδος.	The Pleiades: the rearmost and narrowest end of the Pleiades	27 Tau
33	ὁ ἔκτος καὶ μικρὸς τῆς Πλειάδος ἀπ’ ἄρκτων	The Pleiades: the small star outside the Pleiades, towards the north	HR 1188

Note that Ptolemy's star catalog is given as part of the construction instruction for ancient globe makers. Thus, it is not meant to be a complete list of all observable stars. In particular in star clusters, he typically gives only some that allow for drawing of the dimension of the cluster. Therefore, the fact that Ptolemy lists only four stars does not allow any conclusion concerning the number of stars in the cluster or in the visual pattern.

IAU Working Group on Star Names

In antiquity, the stars of the cluster were unnamed, but modern pop-culture named them. These names were approved by the IAU WGSN in 2016:

The parents:

1. Pleione for 28 Tau in 2016
2. Atlas for 27 Tau in 2016

The daughters:

1. Alcyone for η Tau in 2016
2. Asterope for 21 Tau in 2016
3. Celaeno for 16 Tau in 2016
4. Electra for 17 Tau in 2016
5. Maia for 20 Tau in 2016
6. Merope for 23 Tau in 2016
7. Pleione for 28 Tau in 2016
8. Taygeta for 19 Tau in 2016

Reference

• References (general)
• References (early modern)
• Ian Ridpath's website (Star Tales )