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Looking up to the cold

November 13, 2012
By Aileen O’Donoghue , Adirondack Daily Enterprise

Welcome back to our exploration of the wilderness above, a deep wilderness that stretches our imaginations with each new object we discover. From our tiny terrestrial base, our instruments and theories carry our minds to worlds of gas, stars so dense a sugar-cube sized piece would weigh billions of pounds on Earth and black holes where matter, energy, space and time are crushed together into a quantum foam that our brightest minds struggle to understand.

One world of gas will dominate our sky through the coming winter, treating us to a beautiful beacon gliding around the horns of the bull through the long Adirondack nights. Jupiter will rise tonight just after 6 o'clock in an eastern sky already dark, joltingly early at 4:40 p.m. after our return to Standard Time over the weekend.

As shown in Figure 1 (for 11 p.m.), the giant world is between the horns of Taurus, having continued the eastward movement we saw last winter as it moved beneath the cosmic hockey stick of Aries. On Oct. 4, however, it stopped in its path and began retrograde, westward, motion as Earth catches up to and passes the more distant world with its faster orbital motion.

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As shown in Figure 2, we'll be able to watch the retrograde motion until the end of January when it will again pause to resume prograde, eastern motion. This will take it back through the horns of the bull until it's lost in the glare of dusk in June. We will pass Jupiter at 9 p.m. on Dec. 3 when it will be exactly opposite the sun in our sky.

This is also when we will be closest to it, closing to distance of a mere 378 million miles, giving us our best view of this world. Jupiter is the largest planet, 11 times larger than Earth with a thick hydrogen-ammonia atmosphere whipped into raging winds and storms 1,000 times more powerful than those on Earth by rotation so fast that the Jovian day is only 10 hours long.

With Jupiter at is closest, even moderate binoculars can reveal the four large Galilean moons, Io (EYE-oh), Europa, Ganymede (GAN-uh-mead) and Callisto (Cal-LISToh).

They are named for the famed astronomer because he was the first to observe them, describing three of them in a letter on Jan. 7, 1610. He watched their motion for months, discovering the forth, and determining that they orbited Jupiter, placing another chink in the armor of Ptolemy's system with everything orbiting the Earth.

When the Voyager probes passed Jupiter in 1979, we got our first startling images of these worlds. Since then, the Galileo mission orbited Jupiter 34 times between 1995 and 2003. These probes have shown us that active volcanoes are erupting on Io and a liquid water ocean lurks beneath Europa's icy crust. With its fuel supplies running out, Galileo was deliberately crashed into Jupiter to eliminate the possibility of its crashing into Europa and contaminating its ocean with microbes from Earth. That ocean is currently thought to be the other place in the solar system most likely to harbor life.

Io and Europa are both about the size of Earth's moon, Luna. The other two Galilean Moons, Ganymede and Callisto are larger than Luna. Ganymede, the largest moon of any planet, is actually slightly larger than Mercury. Like Europa, these worlds are icy, but show no signs of liquid water. If you have an opportunity to observe Jupiter and its moons, a copy of Astronomy Magazine or Sky and Telescope Magazine will help you identify them. Lacking either of those, simply Google "Sky and Telescope Jupiter" to find a JavaScript utility that will show you the positions of the moons for any date and time between 1900 and 2100 as observed from any time zone.

At Jupiter's opposition at 9 p.m. on Dec. 3, it will be more than four times brighter than Sirius, the brightest star in the sky, rising in the east. As stars rise (and set), their light passes through many more miles of our atmosphere than when they are overhead. Just as the air above a lit candle or a hot roadway makes objects appear to shimmer, the complex fluid motions of the atmosphere make the stars appear to change in color and brightness. This is due to different parcels of air at slightly different temperatures acting like lenses and prisms. The effect can be startling as bright Sirius rises and I generally get calls and questions about it in November and December when it

rises in the early evening. If you happen to see it, I hope you'll take a moment to pause and enjoy the show.

This year, other astronomers of the Adirondack Public Observatory will be contributing to this column. So look for articles with different viewpoints and styles to enhance your enjoyment of our wonderful dark skies.

Beyond this column, local astronomers are anxious to share our love of the sky with you. Check out the Adirondack Public Observatory web site at www.apobservatory.org for events. Listen for me on North Country Public Radio about once a month during "The Eight O'clock Hour" or email me with any questions at odonoghue@stlawu.edu.

 
 

 

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