Star Chart is under 20 MB - so that you can download it when out and about looking at stars.The developer pointed out the key features.

The iPad shows you what you’d see if the iPad weren’t already displaying the view.

You can hold, say, an iPad up to the sky, and it will be like looking through a chunk of clear glass. One of the major features of this app, like some others, is the ability to exploit the compass in the iPhone or iPad and slave the view to the actual sky. This app, priced at only US$2.99 will do that nicely. Or perhaps one would like to know when Mars will rise or set on a particular night, in the context of the rest of the sky. Or an amateur astronomer may wish to identify an forgotten, seldom seen constellation. (Or fantasize what’s behind all the pollution.) Casual observers may be out on a starry night, camping, wondering what a bright light is. The most important thing to know about a star chart app like this is that it can help one identify what’s seen in the night or even day time sky. It’s a beautiful app, always being improved, a great introduction to the night sky, but suffers from a few astronomical errors and has limited settings. Star Chart is a universal app for iOS that charts the sky for any date from the ancient past well into the future.

Star Atlas for mac download free