In the stillness of the early morning, when all the birds are singing but before the world has awakened, take a step away from the city lights into the cool, dark night. Find a clear space where you can gaze without obstruction into the east, toward the place where the sun will rise. Let the song of the birds fill your spirit as you wait for the dawn, for the faintest crescent of the old moon, for the six brilliant blue stars soon to rise above the eastern horizon, fleeing the brightening sun.
The Pleiades, or seven sisters, is most familiar as an open cluster of six tightly-grouped young blue stars sitting on the shoulder of the constellation Taurus. Together they form the second brightest stellar object in that constellation, behind Aldebaran, the red eye of the bull which takes its name from following the Pleiades. In optimal viewing conditions up to 14 stars can be seen with the naked eye; and with a low-power telescope over 500 are visible, all packed together within 12 light years of space. The cluster lies within our galaxy, roughly 440 light years away from our solar system. Its Messier classification is M45.
The nine brightest stars are named for Atlas and Pleione and their seven daughters. Since only six stars are commonly visible (apparent magnitude 5.0 or lower), many people have tried to explain the “missing sister”. Some stories hold that Electra hides herself away so as not to see the terrible end of Troy, which her son had founded. Others say that Merope hides her shame at having married a mortal husband. So teasingly pervasive was this story behind the stars that in 1610 Galileo Galilei turned his new telescope to the cluster to search for the missing sister, recording the results in his book “Sidereus Nuncius” (The Sidereal Messenger). In contrast to the Greek mythological explanation, Hindu mythology names only the six most commonly visible stars, for the six mothers of the war god Skanda. Modern Japanese corporate culture has immortalised the Pleiades, or “Subaru” in Japanese, into the six-star automobile manufacturer’s logo.
Traditionally the rising of the Pleiades is tied to the agricultural calendar. Its celestial relationship to the earth is such that in the northern hemisphere, it rises in the east directly with the sun in late May or early June (its heliacal rise), and directly opposite the sun’s setting at the end of October (its achronical rise). This coincidence created a strong association with planting and harvest rituals in both hemispheres. The heliacal rising of the Pleiades above the horizon marks the beginning of the Maori new year, but in the Cook Islands just north of New Zealand, it was the achronical rise that marked the change of year. In Peru and Bolivia, near the equator, ancient Incan belief links the midsummer brightness of the Pleiades with future rainfall for that planting season. When rainfall was forecast to be scarce, the planting season would be delayed. In an unusual collaboration of atmospheric science and anthropology, current and ongoing data from the ground and from space has confirmed the link between the high cloud cover dimming the Pleiades and the early signs of an approaching El Nino event, which would cause the local climate to become dryer in direct proportion to the severity of the El Nino.
This June the Pleiades are forecast to be very bright, possibly even to the point that the stars’ reflection nebulae will be visible to the naked eye. In the northern hemisphere above the tropic of Cancer, the heliacal rise will take place in mid-June (June 13 in most of the United States and other areas of similar latitude); with the very old crescent moon passing less than a degree away. So close is the pass that a few of the Pleiades stars will be occulted by the moon. Best viewing will be just before dawn, before the Pleiades are obscured by the sun’s brightness. The two brightest planets Venus and Saturn are also slowly moving toward each other, and by the end of June will have joined the full moon in a dazzling three-way conjunction. Mercury, Jupiter, and Mars will also figure prominently in what promises to be a spectacular June sky.
References:
Orlove BS, Chiang JCH, Cane MA. Forecasting Andean rainfall and crop yield from the influence of El Nino on Pleiades visibility. Nature 403:68, 2000.
