Al-Thurayya

Arabic calls the Pleiades al-Thurayya, and it also calls them simply al-Najm, the star, as though the definite article settled which one. That is a strong statement about a small faint cluster, and it reflects the cluster's position at the centre of the anwa, the rain-star calendar of pre-Islamic Arabia.

The name is usually derived from a root meaning wealth or abundance, which fits an object made of more points than a person can count. Six are easy, seven is the traditional number, and a very sharp eye under a dark sky gets more.

Its setting and rising divided the year. The period after its heliacal setting was one of expected rain, and the agricultural and pastoral schedule of the peninsula was pegged to it. Poetry uses it constantly, both as a time marker and as an image of something high, bright and out of reach.

The same cluster carries this weight almost everywhere. It opens the Maori year as Matariki, it is the rattle of a snake to the Maya, it is a hen and chicks across Europe, and it is the sisters in Australia and in Greece alike. For an object that is not bright, is not large and contains no first-magnitude star, the Pleiades have more sky lore attached to them than anything except the sun and the moon.

The Pleiades are the most widely storied object in the sky and it is worth asking why, since they are not conspicuous by brightness. Three properties account for most of it.

First, they are a genuine group. Almost everything else in the sky is a chance alignment of unrelated stars at different distances; the Pleiades are a real open cluster, physically associated and close together, and they look it. A tight knot of points reads as a thing rather than as a pattern.

Second, they sit near the ecliptic, so the moon and planets pass close to them and they are visible from nearly the whole inhabited world, north and south. A figure at the celestial pole is invisible to half the planet; this one is not.

Third, their heliacal rising and setting fall usefully in the agricultural year across a wide band of latitudes, which is why so many calendars are pegged to them.

The recurring detail that seven are named while six are easily seen has invited a great deal of speculation, including the suggestion that the stories preserve a memory of a star that has since faded or moved. That is not demonstrable and the atlas does not assert it: seven is a common ritual number, and the eye under good conditions does get more than six.