There's an App for That
IfA astronomer Richard Wainscoat has an amazing stargazing application on his iPhone, "Star Walk." Made by Vito Technology Inc., it requires an iPhone 3GS to be fully functional. It uses the phone's compass, GPS, and tilt sensors to show you what you can see in the sky in the direction the phone is facing. As you look around the sky, your view of the sky on the iPhone changes accordingly. You can even look below the horizon to see when an object will rise and zoom in to get a close-up of a particular part of the sky. If you select a celestial body and click on the i, you activate the magnifier and can read information about the object. Planning a star party? You can use the Time Machine feature to see what will be visible at the time and date you choose. All this for $4.99.
Other planetarium applications available for the iPhone include StarMap 3D (Sanville Software, $1.99), and Starmap and Starmap Pro (Fredd, $11.99 and $18.99).