The largest and smallest planets in the solar system slide past each other this week. They're quite low in the west as twilight begins to fade. Without a program, though -- or a chart like one we've posted on our website -- it's hard to tell them apart. The only thing that distinguishes them is brightness.
Jupiter is the brighter of the two. It outshines everything else in the night sky at that hour except the Moon.
With a diameter about 11 times that of Earth, Jupiter is the solar system's largest planet. It shines so brightly because of that great size, and because the clouds that top its atmosphere reflect a lot of sunlight. It doesn't shine brighter because, on average, it's about half a billion miles away.
Mercury, the smallest planet since the demotion of Pluto, creeps up toward Jupiter tonight, passes it tomorrow night, and pulls away later on. As it climbs higher, though, it grows a little fainter, although it still outshines almost everything else in the night sky.
Mercury is less than half the diameter of Earth, and its surface is much darker than Jupiter's. But it's also much closer to the Sun and Earth, so it receives far more sunlight, and reflects more in our direction.
Planetary scientists are paying special attention to Mercury this week. That's because a probe is scheduled to enter orbit around Mercury for a year-long exploration of the solar system's smallest planet. More about Messenger tomorrow.
Script by Damond Benningfield, Copyright 2011
For more skywatching tips, astronomy news, and much more, read StarDate magazine.