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Be on the lookout tonight: Pleiades is in sight

April 3, 2012
By AILEEN O'DONOGHUE , Adirondack Daily Enterprise

The weather forecast says we should enjoy clear skies tonight. So just as it darkens after sunset, grab a pair of binoculars and look to Venus in the western sky. As the dusk fades, the glittering star cluster of the Pleiades will emerge to embrace the bright planet in its midst.

The brighter stars should become visible in binoculars by 8 p.m. when they will be about 30 degrees (three widths of your fist at arm's length) above due west. As shown in Figure 1, Venus will be to the south (left) of the six bright stars forming the tiny dipper that many mistake for the Little Dipper of Ursa Major.

Venus' position in our sky repeats every eight years and its next four passages through Taurus, in 2020, 2028, 2036 and 2044, will take it closer to the center of this cluster. In 2052 and 2060, Venus will pass through the northern edge of the Pleiades. Though most of us see six stars with the unaided eye, there are nine stars named for the titan Atlas, his wife, the sea-nymph Pleione (PLEE-uh-nee), and their seven daughters.

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In order of birth, the seven sisters are Maia (MY-uh or MAY-uh), the mother of Zeus' son Hermes; Electra (ee-LEK-truh, meaning "amber" or "shining", the Greek root of our word "electricity"), Taygeta (tay-GEE-tuh), Alcyone (al-SEE-uh-nee or al-SIGH-uh-nee), Celaeno (suh-LEE-noh), Sterope (STERR-uh-pee) and Merope (MERR-uh-pee), who is known as the lost Pleiad because she married the mortal, Sisyphus. All these stars are younger and hotter than our sun, having surface temperatures between 21,000 degrees and 25,000 degrees F compared to Sol's 10,000 degrees F.

Since hotter stars burn through their fuel more quickly, this means these stars are all much younger than the sun and all but two of them, Pleione and Sterope, are already moving into the end stages of their lives. The full cluster of around 500 stars, however, has many more small red stars than these bright giants. A study using the Hubble Space Telescope determined the distance to the cluster to be about 440 light years.

Other studies have revealed its age to be about 100 million years, so it's a very young cluster compared to our Sun's age of five billion years. When these stars were born, dinosaurs ruled the Earth, though the only dinosaur fossils found in New York state, those from near Nyack, were already over 100 million years old.

On Friday, the moon will be full at 3:19 p.m., when it is exactly opposite the sun in our sky. This full moon falls on Good Friday with Easter following on Sunday. In fact, it is this occurrence of the full moon that determined the date of Easter which occurs on the first Sunday after the first full moon after the vernal equinox.

The Ecclesiastical full moon is determined from tables based on patterns in the 400-year Gregorian calendar as opposed to astronomical determinations, but they track closely.

As shown in Figure 2, tonight's moon is a waxing gibbous and about 10 degrees south (below and right) of Mars and Regulus in Leo. Since the moon orbits the Earth once relative to the sun every 29.5 days, that's 360 degrees/30 days or 12 degrees per day. Thus it will take four days as the moon waxes toward full for it to cross the span of sky between Mars and Saturn, passing Purrima (POUR-ee-ma) on the way and, as a full moon on April 6, passing very near to Spica. As the sun sets that Friday evening, the Jews of the world will celebrate the first night of Passover on a date determined by that same lunar event determining Christian Easter two days later. So pause this week to look at the planets, the Pleiades and moon. Then gather over a meal with your family and friends, no matter what your religious persuasion, to savor their company on the curious paths of our human lives with weeks, months and years marked by the endless cycles of the sun and moon, the planets and the glittering stars.

If you have questions about the any astronomical topic, please visit the Adirondack Public Observatory website at or email Aileen at



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