Pleiades: The Seven Sisters Aratus Said Show Only Six

Seven named, six seen

English Wikipedia places the writing of the Phaenomena at the court of the Macedonian king Antigonus II Gonatas, where Aratus went about 276 BC. In the texts checked here, the name is older than the poem. Homer's Iliad (18.486) and Odyssey (5.272) name the Pleiades among other stars, with no number and no father in those lines. English Wikipedia dates both poems to around the 8th century BC and notes arguments for a date in the mid-7th.

Hesiod's Works and Days, dated to around 700 BC, already calls the Pleiades daughters of Atlas, about four centuries before Aratus. What Aratus adds is the count, and the trouble with it:

ἑπτάποροι δὴ ταί γε μετʼ ἀνθρώπους ὑδέονται, ἓξ οἶαί περ ἐοῦσαι ἐπόψιαι ὀφθαλμοῖσιν.

"Seven-pathed they are called among people, though only six of them can be seen with the eyes." (Translation: Where Tales Began)

Those are lines 257–258 in G. R. Mair's 1921 Greek text. In the next three lines Aratus insists that no star has gone missing from the sky in living memory; the number seven is simply what is said. He gives no reason for the gap, and he names no hidden sister.

Apollodorus's Library, in J. G. Frazer's 1921 translation, spells out the family: "Atlas and Pleione, daughter of Ocean, had seven daughters called the Pleiades." Frazer's note adds that Aratus gives the same seven names in the same order.

The name itself is not settled in the sources checked. Frazer's note reports that some derived "Pleiades" from the mother's name, Pleione. To him "the most probable view appears to be" that it comes from the Greek πλεῖν, "to sail," because in the Mediterranean the cluster was visible at night from mid-May to the beginning of November, the ancient sailing season. The cluster stayed in the night sky after November, though; Hesiod has it hidden only forty days a year.

Which seven? The nine names on the IAU list

The International Astronomical Union set up its Working Group on Star Names (WGSN) in 2016 to catalogue official names for stars. Its list as of January 1, 2021 includes nine stars in the Pleiades, each with a J2000 position and a V magnitude. Magnitude is a brightness scale on which a lower number means a brighter star, as English Wikipedia defines it. The nine names were approved between June 30 and August 21, 2016.

Table data: pleiades-wgsn-named-stars.csv Data source on record: IAU WGSN, List of IAU-approved Star Names (as of 2021-01-01), CC BY 4.0, https://iauarchive.eso.org/public/themes/naming_stars/, with the designation, magnitude, position and approval date of each star.

Ranked by magnitude, the six brightest named stars are Alcyone (2.85), Atlas (3.62), Electra (3.72), Maia (3.87), Merope (4.14) and Taygeta (4.30), by this page's count. One of those six is Atlas, the father, not a sister. The two faintest names, Celaeno at 5.45 and Asterope at 5.76, both belong to sisters. The list gives names to stars; it does not say which star any ancient poet had in mind.

The list spells two sisters differently from Aratus: Asterope for his Sterope, and Taygeta for Taygete. Frazer's 1921 note already writes of "Sterope or Asterope, as she is also called." Alcyone's designation in the list is eta Tauri (η Tau), the same Greek letter that sits beside the cluster on Bayer's 1603 chart at the top of this page.

The lost sister, explained twice

Ovid's Fasti, a Latin poem on the Roman calendar made public in AD 8, about 280 years after Aratus, repeats the count and then explains it. In book 4, lines 169–170, he writes of the Pleiades:

quae septem dici, sex tamen esse solent

"which are said to be seven, but are usually six" (Translation: Where Tales Began)

Ovid gives two reasons in lines 171–178. Six sisters had gods as lovers: Sterope had Mars, Alcyone and Celaeno had Neptune, and Maia, Electra and Taygete had Jove. Merope, the seventh, married Sisyphus, a mortal, and hides in shame; or else Electra could not bear to watch Troy fall and put her hand before her eyes.

Frazer, annotating Apollodorus in 1921, called these "alternative and equally probable theories" and gave Ovid as one writer who states them. He added a third candidate: the hidden Pleiad "is identified by modern astronomers with Celaeno, a star of almost the seventh magnitude," citing A. von Humboldt's Cosmos. The WGSN list gives the star now named Celaeno a V magnitude of 5.45, so the 1921 description and the 2021 table do not match. Ovid's two candidates are among the brighter stars in that table: Electra at 3.72 and Merope at 4.14.

Table data: pleiades-missing-sister-sources.csv Data source on record: IAU WGSN, List of IAU-approved Star Names (as of 2021-01-01), CC BY 4.0, https://iauarchive.eso.org/public/themes/naming_stars/, with the passage and link for every source.

Galileo counts again, 1610

Galileo turned a telescope on the cluster and printed what he saw in Sidereus Nuncius, published in Venice with a dedication dated March 12, 1610. Edward Stafford Carlos's 1880 English translation reads:

As a second example I have depicted the six stars of the constellation Taurus, called the Pleiades (I say six intentionally, since the seventh is scarcely ever visible) [...] Near these there lie more than forty others invisible to the naked eye, no one of which is much more than half a degree off any of the aforesaid six; of these I have noticed only thirty-six in my diagram.

His text still speaks of six stars and a seventh "scarcely ever visible," nearly 1,900 years after Aratus. The telescope adds the fainter neighbors, and the passage names no missing sister.

A farmer's calendar, and when to look

Hesiod used the cluster as a farm clock. In H. G. Evelyn-White's 1914 translation of Works and Days, lines 383–387:

"When the Pleiades, daughters of Atlas, are rising, begin your harvest, and your ploughing when they are going to set. Forty nights and days they are hidden and appear again as the year moves round, when first you sharpen your sickle."

The notes in that 1914 edition place the rising in early May and the setting in November. Aratus gives the faint cluster the same job, saying that Zeus made it the sign for the start of summer, the start of winter and the coming of plowing (lines 264–267). The date of Easter is another calendar set against the sky, with a difference: its rule follows a table moon rather than the moon in the sky.

Illustration

The Pleiades stand opposite the Sun around November 21–22 and cross the meridian near midnight, by this page's calculation from Alcyone's WGSN position and the Sun's position worked out with two independent formulas. Around May 19–20 the cluster lines up with the Sun and sits in the daytime sky (dates for 2026–2030, UTC). English Wikipedia describes the cluster as a prominent winter sight in the Northern Hemisphere.

Subaru, Matariki and other names

In Japanese the Pleiades are called Subaru. Sei Shōnagon's Makura no Sōshi (The Pillow Book) puts the cluster at the head of a list of stars; English Wikipedia says the work was completed in 1002, and Japanese Wikisource dates it to 1013. In the 1903 Kokubun taikan printing, page 324:

星は すばる、ひこぼし、明星、ゆふづゝ。

"Stars: Subaru, Hikoboshi, the morning star, the evening star." (Translation: Where Tales Began)

The Māori name for the cluster is Matariki. William Williams's Māori dictionary, in its 1871 third edition, glosses Matariki as the Pleiades and as the sign of the first month. As English Wikipedia summarizes it, the cluster's first rising in late June or early July marks the start of the new year in the Māori lunar calendar, though some Māori use Puanga (Rigel) or other stars instead; Matariki became a public holiday in 2022. Employment New Zealand's holiday table puts the 2027 holiday on Friday, June 25.

The first-ink date for each name below is the date of the earliest written record of it that can be checked in the sources for this entry; it marks where the paper evidence starts, not where the name began (how this site dates records).

Table data: pleiades-names-earliest-ink.csv Data source on record: IAU WGSN, List of IAU-approved Star Names (as of 2021-01-01), CC BY 4.0, https://iauarchive.eso.org/public/themes/naming_stars/, with the passage, scope and link for every row.