Moon and Mercury 
Mercury is one of our closest planetary neighbors, yet it's still not all that well known. If you look at the planet in the evening sky this month, you can see why.
Mercury looks like a fairly bright star. But it's quite low in the west at sunset, and drops from view by dark. It's immersed in the twilight for the entire time it's in view, so it's hard for ground-based telescopes to see any detail. And to make things worse, we're viewing it through a thicker layer of air than if it were up high, which distorts the view.
All is not hopeless, though -- there are ways to see the planet without looking through an optical telescope.
For one thing, you can use a radio telescope, which bounces a signal off the planet's surface. Radio telescopes were used to determine how fast Mercury turns on its axis. They can also produce images of its surface, and help probe its interior; more about that tomorrow.
Another way to explore Mercury is from up close -- with a spacecraft. Only two craft have ever visited Mercury. The first one made three passes back in the 1970s. The other, known as Messenger, made three passes in the last decade, then entered orbit around the little planet this year. It will give us by far the best look yet at this close but hard-to-study world.
Again, look for Mercury low in the west beginning about a half-hour after sunset. This evening, it's just to the upper right of the fingernail crescent Moon.
Script by Damond Benningfield, Copyright 2011