Jupiter at Opposition
The planet Jupiter is at its boldest this week. It's at opposition -- the point where it lines up opposite the Sun in our sky. It rises around sunset, and remains in the sky all night long. And it's closest to Earth for the year, so it shines brightest. It outshines everything else in the night sky except the Moon and Venus, so you can't miss it.
Opposition occurs as Earth passes Jupiter in our smaller, faster orbit around the Sun. It takes Jupiter about 12 years to complete a circuit around the Sun, versus just one year for Earth. So Earth passes by Jupiter once every 13 months.
As Earth speeds past, the giant planet moves backwards against the background of stars. The planet itself is still moving forward in its orbit. Instead, the change in direction is caused by the relative motions of the two worlds.
The effect is like passing another car on the highway. As you zip by, the other car is still moving forward. But because you're moving faster, the other car looks like it's moving backwards against the background of distant buildings, trees, or mountains.
Jupiter will continue this "retrograde" motion for another two months. After that, it'll resume its normal eastward motion against the starry background.
Look for Jupiter low in the southeast as darkness falls. It's in a region of the sky that's a bit short on bright stars. That'll make it even easier to pick out. It looks like a brilliant cream-colored star.
Script by Damond Benningfield, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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