Two giant planets are crossing paths in the pre-dawn sky this week. One of them is bright and easy to find. The other hides in the distant reaches of the outer solar system, so you need some help to pick it out.
The bright one is Jupiter. It rises in the wee hours of the morning, and is in the south at first light. It looks like a brilliant cream-colored star, so you won't have any trouble spotting it.
The other planet is Neptune. Tomorrow, it's about half a degree to the upper left of Jupiter -- less than the width of a pencil held at arm's length. But it's more than two billion miles farther than Jupiter is, so it isn't nearly as bright. In fact, you need a telescope to pick it out.
Jupiter is the largest planet in the solar system. Neptune ranks fourth. It's less than half Jupiter's diameter.
In decades past, planetary scientists described both worlds as "gas giants." Today, though, Neptune is often described as an ice giant. Like Jupiter, its outer layers consist of mostly hydrogen and helium gas. But the layers of gas on Neptune aren't as deep. They give way to layers of frozen or liquid water and other ices, which in turn surround a rocky core that's about as massive as Earth.
Jupiter is moving past Neptune. By Sunday morning, Neptune will be directly above Jupiter. After that, Jupiter will slowly move away from its fellow giant. They won't meet up again for more than a decade.
Script by Damond Benningfield, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.