Before printed calendars, human beings read time in the sky. The Sun and the Moon gave the main rhythms, but it was the stars that precisely marked the arrival of the seasons and the right moment to sow, reap or put to sea. Of all star groups, none has had such a widespread calendrical role as the Pleiades, the small bright cluster visible in the constellation of Taurus. Learning their history means discovering a wheel of the year written in the firmament.
What the Pleiades are
The Pleiades are a cluster of young, bright stars. To the naked eye they appear as a small compact group, like a bunch of grapes or a tiny sparkling cloud. Tradition speaks of seven stars, but most people can make out about six with the naked eye, depending on their eyesight and the quality of the sky. This "missing star" has inspired many stories in different cultures.
In Japanese the cluster is called Subaru, a name also given to a well-known car maker, whose logo shows precisely a group of stars.
Heliacal rising: when a star reappears
To understand the star calendar we need to know a phenomenon called heliacal rising. During part of the year a star is invisible, because it lies in the sky close to the Sun and is drowned by its light. Then comes a day when the star becomes visible again for a brief moment in the east, just before dawn. That first return is the heliacal rising.
Because this event recurs every year at roughly the same time, the ancients used it as a true seasonal clock.
Hesiod and farm work
The most famous text on the agricultural use of the Pleiades is Works and Days by the Greek poet Hesiod. Hesiod advises beginning the harvest when the Pleiades, daughters of Atlas, rise, and ploughing when they set. He also notes that the Pleiades stay hidden for forty days and nights before reappearing as the sickles are being sharpened.
The poem shows how the farming calendar was interwoven with observation of the sky. The stars were not an object of abstract contemplation but the concrete guide of daily life. Navigation, too, was regulated by the Pleiades: Hesiod advises against putting to sea when they, fleeing Orion, plunge into the misty sea, that is, at the arrival of the bad season.
The myth of the seven sisters
In Greek mythology the Pleiades were the seven daughters of the Titan Atlas and the nymph Pleione. Pursued by the hunter Orion, they were turned into stars by Zeus. Even today the constellation of Orion seems to follow them across the sky without ever catching them.
Different versions of the myth explain the less visible star: according to some, one of the sisters, Merope, hides out of shame for having married a mortal; according to others, Electra veiled her face in grief at the fall of Troy. The myth turns an observational fact, one star fainter than the others, into a story of human emotions.
Sirius and the Nile flood
The Pleiades are not the only example of a star calendar. In ancient Egypt the heliacal rising of Sirius, the brightest star in the night sky, roughly coincided with the start of the annual Nile flood, the event on which the country's whole agriculture depended. The reappearance of Sirius at dawn thus announced the return of water and fertility.
This example shows a general principle: where observation of the sky and the rhythms of nature coincided, the star became the very symbol of the season it announced.
Matariki and the Nebra sky disc
The role of the Pleiades is found far from the Mediterranean as well. In New Zealand, among the Māori, the reappearance of the cluster, called Matariki, marks the beginning of the new year and is now celebrated as a public holiday.
In central Europe the Nebra sky disc, a Bronze Age bronze artefact found in Germany, shows the Sun or full Moon, a lunar crescent and a group of seven closely spaced dots that many scholars interpret as the Pleiades. If the interpretation is correct, it would be one of the oldest known depictions of this cluster.
A sky-watching practice
- In autumn and winter, on clear evenings, look in the east for a small group of stars near the constellation of Taurus.
- Count how many stars you can make out with the naked eye.
- Note the date of the first time you see them and of the last time, in spring, before they disappear into the light of sunset.
- Repeat the observation every year: over time you will build your own personal star calendar.
In short
- The Pleiades are a star cluster used as a seasonal clock in many cultures.
- Heliacal rising is a star's first return at dawn after its period of invisibility.
- Hesiod linked the rising and setting of the Pleiades to harvesting and ploughing.
- In Egypt the heliacal rising of Sirius announced the Nile flood.
- Matariki and the Nebra sky disc show the universal importance of the Pleiades.
