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December 28, 2010
By Aileen O'Donoghue

Jupiter has been shining brightly in the southeastern sky during early evening since mid-September. This week, it can be found almost due south at 10 p.m., about half way between the horizon and the zenith, the point directly overhead.

As shown in Figure 1, it serves as a bright guide to part of the rather obscure constellation of Pisces, with its distinctive ringlet 5 degrees to 10 degrees above the planet. Above the ringlet are four bright stars forming a square that your palm, held at arm's length, should just fit within. This is the Great Square of Pegasus, an easily recognized "asterism" of the autumn and early winter sky. An asterism is a recognized grouping of stars that is not one of the official 88 constellations of the International Astronomical Union. The Big and Little Dippers are also asterisms in the constellations of Ursa Major and Ursa Minor.

The diameter of the giant planet is about 11 times that of Earth and, at its closest this year, was about 368 million miles away. That was on Sept. 21, when Jupiter was opposite the sun in the sky, an arrangement conveniently called "opposition." Thinking about the solar system as a racetrack, this was the day that Earth passed Jupiter so the two planets were at their closest. Since then, the distance has increased to 392 million miles as Earth has raced past the larger, more distant world.

Lurking very close to Jupiter, but much harder to see, is Uranus. Not only is it less than half the size of Jupiter, with a diameter only about four times that of Earth, but it is much farther away. This week it will be 1.80 billion miles away. At its closest, also on Sept. 21, it was still an incomprehensible 1.77 billion miles away. That may not seem like much of a change, but for the distance to change by 25 million miles in that time, we have to be leaving Uranus behind at about 25,000 mph.

Uranus is quite faint compared to Jupiter, at the very edge of visibility with an unaided human eye. Fortunately the brighter world is currently a wonderful guide for spotting it. If you have terrific eyesight and extremely dark skies, you might be able to see it just east-northeast, that is, left and slightly above Jupiter. The rest of us need binoculars.

Figure 2 shows the field of view of my 8 x 25 compact Celestron binoculars, with which I can easily spot Uranus to the left of Jupiter forming the base of an upside-down triangle with slightly brighter 20 Piscium and slightly dimmer 24 Piscium as the peak. As you use these to locate Uranus, keep in mind that both are giant stars at the end of their lives. 20 Piscium is 11 times larger than our Sun, shining 64 times brighter from a distance of 294 light years. 24 Piscium is 20 times larger than Sol, 156 times brighter and 502 light years away. Both planets will move up and to the right (northeast) relative to the stars until Nov. 19 when Jupiter will pause then zoom back toward Uranus and 20 Piscium, passing between them on the night of Dec. 29.

Jupiter's Galilean moons should also be visible in binoculars. Their positions at 10 p.m. in November are shown in the inset. Galileo was the first to observe these worlds that range from the size of our moon (Io and Europa) to 1.5 times larger (Callisto and Ganymede). Over the course of a month, you can repeat Galileo's observations of January 1610 by watching these worlds change positions as they orbit Jupiter. These observations of objects orbiting a planet other than Earth were some of the evidence he cited in Sidereus Nuncius supporting Copernicus' heliocentric arrangement of the solar system.

At the "Physics" and "Astronomy" Applets site, located at, you can find the positions of the moons for any date and time. So grab your binoculars, a blanket and a warm cup of tea, settle into your chaise lounge, and commune with Galileo as you observe Jupiter, its moons, Uranus and a couple of impressive stars.



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