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Astronomy Picture of the Day

Airplane Twilight

Photo of the Week.. Deepening twilight fills the skies from 35,000 feet.

Astronomy news for the short week starting Sunday, October 27, 2002.

The Moon approaches its last quarter early in the week, reaching the phase on the night of Monday the 28th, about midnight in the Americas, just about the time of Moonrise. In the third century BC, the great Greek astronomer Aristarchus of Samos tried to measure the distance to the Sun (relative to the Moon) by timing the exact moment of the quarter. The angle between the two would be 90 degrees if the Sun were infinitely far away. If it was not, then the angle would be less. From his estimated angle of 87 degrees, Aristarchus found the Sun to be 20 times farther than the Moon. The experiment is impossible, and the ratio is actually 400. But Aristarchus was on the right track, clearly locating the Sun much farther away than the Moon. The night of Tuesday, the 29th, the Moon will have passed just to the north of bright Jupiter .

The time to see the two giants of the Solar System, Jupiter and Saturn, is still early morning. At that time, Saturn lies to the south, just above Orion, while Jupiter is to the southeast, to the east of the Beehive Cluster in Cancer. In its great majesty, Jupiter rolls around the heavens in a dozen years, at the rate of one constellation of the Zodiac per year. Saturn takes over twice as long.

On Sunday the 27th, little Mercury passes north of Spica in Virgo, quite the impossible observation, while Venus passes invisibly south of the Sun, going through its inferior conjunction with the Sun on Thursday, the 31st. Because of the tilt of the orbit, the passage is far to the south of the Sun. Look for this beautiful planet, the brightest in the sky, in eastern morning twilight in late November. Venus, the brilliant one, is the classic evening and morning "star" that all cultures have noted and honored, the Mayans even making it part of their calendar.

As the Moon departs the evening sky, watch now for the autumn constellations. Aquarius, with its "Y"-shaped "Water Jar" gives way to the "Circlet" of Pisces, above which lies the striking Great Square of Pegasus, a centerpiece of fall. To the northeast is Andromeda, made of two graceful strings of stars. Go a bit to the north of central Andromeda to find the "Andromeda Nebula," Messier 31, a "nearby" (all things relative) galaxy much like ours some two million light years away, and the most distant thing visible to the unaided eye. Such galaxies abound into the distance, over a trillion of them potentially visible with the most sophisticated telescopes. Near it in the constellation Triangulum (the eponymous Triangle) is Messier 33, another, though smaller, galaxy at about the same distance that is also visible, though just barely, to the naked eye.

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