Most maps are designed to help you look down on the Earth from space. But sometimes you want to do just the opposite, which is why star maps have always been designed to help you look up into space from the Earth. Since the launch of Google Sky in 2007, we've been helping users to see the view above as well, and now we're happy to welcome the newest tool in the sky mapping family: Sky Map for Android.
Most new phones today can sense their location using GPS and network signals, so they could construct a map of the stars and constellations that you might expect to see overhead. But Sky Map goes even further: Android-powered phones today have access to a compass reading as well as the other sensors, and so they can also determine the direction you're facing. Once you have a phone with a compass ("magnetometer"), a plumbline ("accelerometer"), and you can pinpoint your position (using GPS) and your time (using a clock), that's enough to work out which direction you're pointing in the Universe.
The result is a whole new sky mapping experience. Point the phone at any part of the sky, and Sky Map gives you a specially constructed map telling you what stars, planets and constellations you're looking at. You can even search for stars and planets by name, and the phone displays an arrow telling you how to move to face the object you're interested in. The phone can't "see" the stars as such: it works out their positions mathematically from the sensor readings. One quirky consequence of this approach is that Sky Map doesn't need a line-of-sight to find the stars and planets. It will just as reliably display stars you can't see on a cloudy night, stars you can't see because the Sun is shining, stars you can't see because you're indoors, and even stars in the opposite hemisphere that you can't see because the Earth itself is in the way. So you can even use Sky Map to work out that the Southern Cross is currently somewhere beneath your right foot!
As well as being orientation aware, another way that Sky Map differs from most mapping applications is that its core data model is very small. There are only a few planets in our solar system, and 88 recognized constellations in our night sky - but more surprisingly, there are only some 6000 to 9000 stars that are visible from the Earth witch the naked eye, even under the best viewing conditions. And apart from the occasional supernova once every few centuries, star data is pretty stable. This meant that we could fit all of Sky Map's core data into a tiny 60K, all of which ships freely with the application and might never need a single update in the lifetime of your phone. When you're out in the country at night, miles and miles from the nearest city light or network connection, Sky Map is one mapping application that still works perfectly, just when you want it the most!
To try Sky Map, visit the Android market at http://www.android.com/market, and search for "sky map", or you can find the application listed by popularity in the Applications > Reference section.
For more information, have a look at our post on the Google Mobile Blog and at the Sky Map landing page.
Update (5/13): We have updated the version of Sky Map for Android in Android Market. We realize that some users were experiencing crashes or slow start-up times. This new version fixes these issues, so Sky Map should now work fine on all Android-powered phones.
