Skywatchers who are out during the late evening hours on Thursday and Friday (Sept. 15 and 16) will almost certainly be attracted toward the eastern sky, where the two brightest visible objects at that hour will be clearly visible, weather permitting.
The moon, now a waning gibbous phase, will be poised near a brilliant silvery non-twinkling "star." That object is not a star, however, but the largest planet in our solar system: Jupiter.
Jupiter comes up over the eastern horizon just before 9 p.m. local daylight time. It rises about four minutes earlier each night, so by the end of this month it will be beaming before the end of evening twilight. By then, the planet will be so bright — it will be essentially at its peak brilliance for 2011 — that it's easy to see before the sky gets fully dark.
The sky map of Jupiter and the moon here shows how they will appear to skywatchers in mid-northern latitudes on Sept. 15.
Viewing Jupiter
A couple of hours after Jupiter rises, the planet will have climbed higher above the horizon haze (or as I call it, "atmospheric schmutz") and will gleam with a brighter, purer light. It will blaze high above the southern horizon during the quiet evening hours after midnight, finally paling in the western sky during dawn. [Photos of Jupiter: Solar System's Largest Planet]
On Thursday night, Jupiter will sit about 8 degrees below and to the left of the moon. Remember that your clenched fist held at arm's length measures roughly 10 degrees. On Friday, the moon will have shifted to a position about 6 degrees to Jupiter's left.
Jupiter is still in the small and relatively dim constellation of Aries, where it will remain until early December. Every passing day brings our own world closer to scooting between the giant planet and the sun. When we finally do on Oct. 28, Jupiter will be at opposition — opposite of the sun in our sky.
Four Jupiter moons
Now is a fine time to start examining the famous Galilean satellites, the four largest moons of Jupiter. They are Europa, Ganymede, Io and Callisto.
In theory they are all bright enough to see with the naked eye. In fact, more than 30 years ago, an Education Coordinator at the Hayden Planetarium had the ability to do just that (he could also discern the crescent of Venus with his unaided eyes as well). But for most of us who lack such acute vision, optical aid is almost always required to separate the moons from the glare of the mighty king of the planets.
Steadily held binoculars can reveal all four moons, though you may have to make special plans to catch innermost Io close to the time of elongation. Telescopes nearly always show all four. At moderate powers you can detect a change in their positions in an hour or so, and sometimes, when two are close together, in just a few minutes. [Video: Top 10 Telescopes for Amateur Astronomers]
On Thursday night, depending on when you're looking, you might only get a glimpse of just three satellites, since for a while Europa will be transiting (or crossing in front of) Jupiter.
On Friday night, you should be able to readily see all four. Three will be on one side (Callisto, Ganymede and Io), while Europa will appear on the other.
Editor's note: If you snap a striking photo of the moon or Jupiter, or any other skywatching target, that you'd like to share with SPACE.com for possible stories or galleries, contact managing editor Tariq Malik at: tmalik@space.com.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
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