Celestial Lights
By Phyllis Pitluga, Aug 4, 2006

Jupiter dominates the evening sky

During the next two months, the brilliant planet Jupiter will be increasingly closer to the western horizon after sunset (by October, we will lose this planet in the glare of the Sun until December, when it will emerge in the dawn sky ahead of the rising Sun). Jupiter is the largest planet orbiting our star, the Sun. It orbits five times farther from the Sun than the Earth orbits. Jupiter is 10 times the diameter of our Earth. Because of its relatively large size in the sky and its white cloud tops, Jupiter is brighter than all celestial objects after the Sun, Moon and Venus.

Through binoculars, the four largest moons orbiting Jupiter can be seen changing positions from night to night. You can track these changes each night by making a sketch showing Jupiter as a small circle and the moons as dots separated by "Jupiter-diameters" apart on either side. These four moons are similar in brightness, but the moon Ganymede is the brightest because it is the largest.

By following it for seven nights, you can see Ganymede make a complete orbit of Jupiter-which includes passing in front of Jupiter and behind it. 

The moon Io orbits in less than two nights and is second-brightest. It is so close to Jupiter that the gravitational squeezing and relaxing on the little moon causes the interior to remain liquid. Io is the most actively volcanic world in our Solar System. 

The next closest moon, Europa, orbits in three and a half nights and is third-brightest. It, too, is undergoing gravitational squeezing by Jupiter, so that its world of rock and ice has liquid water beneath its crust. Europa is one of the places where we hope to land a robotic submarine to explore its ocean and check for life forms.

The moon Callisto is much farther away from the planet, and it takes 16 and a half nights to orbit Jupiter. It is also the dimmest of the four moons. By viewing these four moons through binoculars or a small telescope, you will follow in the path of Gallileo 400 years ago. From observing the miniature system of moons orbiting Jupiter, he became passionately convinced that Copernicus had been correct 50 years earlier in proposing that the Earth and planets orbit the Sun, instead of the planets and Sun orbiting the Earth. 

Through larger telescopes, details in the cloudy atmosphere can be observed. Last month, I alerted readers to the drawing together of two large red hurricanes, and the event is continuing as we go to press. The Great Red Spot is a huge oval storm swirling at 350 miles per hour and is larger than two Earths side-by-side. It is located in the South Equatorial Belt of clouds and is about five miles higher than the surrounding clouds.

Picture courtesy of Christopher Go] Between 1998 and 2000, three white oval clouds gradually merged into a large white oval. Then, this past February, Christopher Go of the Philippines discovered that the spot had turned brick red.

Now dubbed Red Spot Junior, perhaps the merger intensified the stormy area and dredged up new material that was already red in color or became red by being chemically altered by the Sun's ultraviolet light. At any rate, we are interested to see whether there are further changes as these two red spots pass by one another. This is something you can follow yourself with a telescope. Because Jupiter rotates in almost 10 hours, the Great Red Spot may be centered, toward one edge or the other, or even behind the planet. So, keep trying.



Sky Calendar, August 2006

By following the Moon as the biggest and brightest "pointer" in the sky, during the month you can identify different planets and bright stars. On following nights you can relocate them but without the Moon, which moves about 25 times its own diameter from one night to the next.

Friday, August 4: The Moon passes just below Antares, a Red Giant star that is 604 light years away and has a companion star; Antares irregularly varies in brightness. 

Wednesday, August 9: The Full Moon rises at sunset; for the next two weeks the Moon rises about 50 minutes later each night. 

Thursday, August 10: Mercury and Venus are about one extended finger's width apart in the northeastern dawn sky.

Saturday, August 12: This year, the annual Perseid meteor shower will be drowned out by the light of the nearly full Moon not far away from where the shower meteors collide into our atmosphere (the best shower will be December 14 and 15 of this year).

Tuesday, August 15: The Last Quarter Moon rises at midnight and passes just above the Pleiades Star Cluster. 

Monday, August 21: The Moon passes above Venus.

Wednesday, August 23: New Moon (not visible because the dark side faces Earth).

Friday, August 25: The Moon passes just above Mars.

Saturday, August 26: Venus and Saturn are very close in the northeastern dawn sky-so close that they will appear to nearly merge.

Monday, August 28: The Moon passes just below the star Spica.

Tuesday, August 29: The Moon passes below Jupiter.

Thursday, August 31: First Quarter Moon (halfway across the sky at sunset).


Phyllis Burton Pitluga is Astronomer Emerita of the Adler Planetarium and Astronomy Museum, Chicago, and is now a San Miguel resident.