Blue Planet puts an image of the Earth on your desktop. It determines what time zone you are in, and, by default, centers the map on that location. Then, it establishes whether it is day time in your location and it mixes a satellite image of the Earth with NASA's Visible Earth: Earth at Night image. So, what you basically see on your desktop is where in the world it is daytime and where it is nighttime. The image looks very convincing and it doesn't look like a mix at all. You can choose the day image, but you can't customize the night one. There are four different images that you can choose from, but I thought the default one was the better-looking one.
As I mentioned above, the app automatically centers the map on your time zone, but you can also choose to have it centered on the Sun, the Prime Meridian, America or Asia.
Another interesting thing you can do with this app is see what the day looks like throughout the year. The app tracks the current hour to determine where the sun is in your area, but you can choose any day of the year and any hour, and the image will show which areas of the planet are dark and which ones are lit by the sun. It is a very interesting feature.