One of the default pictures included with the iPhone is a beautiful image of the Earth as seen from space. Unless you weren't paying attention, you saw this picture when you first purchased your iPhone, and you can still view it by going to Settings --> Wallpaper --> Wallpaper; it is the second image provided by Apple. Here is what it looks like as a wallpaper on an iPhone:
If you have ever wondered where this photograph came from, a recent post from Gizmodo sheds some light on the subject, although to get the full story you need to go to some more sources, including this page from the Institute for Atmospheric and Climate Science in Switzerland. Here is a short version.
First, a little history. One of the most famous pictures of the Earth was taken on December 7, 1972 from a distance of about 18,000 miles out by the crew of Apollo 17 who were fortunate enough to have the sun directly behind them so that the Earth was fully illuminated. One of the astronauts, Eugene Cernan, quipped when the picture was taken: "I know we're not the first to discover this — but we'd like to confirm, from the crew of America, that the world is round." The image was called Blue Marble and is one of the most widely distributed photographs in existence:
[UPDATE 12/11/2013: You can view the original Blue Marble 1972 picture on the NASA website here, with many different versions available.]
In 2000, climatologist Reto Stöckli was working with the NASA Earth Observatory team and decided to update that famous picture. By compiling satellite data and images taken over several months, he created an artificially colored image that he also called Blue Marble:
Then in 2002, Stöckli worked with Rob Simmon of the NASA Earth Observatory and others to update that image and create a so called "true-color image" that was based on data which closely reflects the full spectrum range of visual perception. In other words, this is what the Earth might really look like if you were in space. This image, called Blue Marble 2002, was the most detailed true-color image of the entire Earth to date. To be clear, this is not a single snapshot of Earth, but instead is a composite of months of satellite-based observations. Most of the data came from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) which flew on a satellite over 400 miles above the Earth, so much closer than the 18,000 miles out of the Apollo 17 astronauts. Here is Blue Marble 2002:
You can download the full size, 2048 x 2048 pixel, version of Blue Marble 2002 from the Flickr page for the NASA Goddard Space Flight Center. You can also view this animation of the Earth spinning created from the same images. NASA released the Blue Marble 2002 image to the public, and that was the image that Apple decided to use on the iPhone.
Although the Gizmodo article cited above says that this is this is the highest resolution image of the earth, that is actually no longer true. Stöckli, Simmon and the rest of the gang got the band back together to work on a project called Blue Marble: Next Generation (BMNG). Using MODIS data from 2004, the BMNG team created this image in 2005:
The full size version of Blue Marble 2005 is 4096 x 4096 pixels, so it is four times the size of the 2002 image used by Apple on the iPhone. You can download full versions of all of the images mentioned above — the 1972 Apollo 17 image and the Blue Marble versions dated 2000, 2002 and 2005 — on this page from the Institute for Atmospheric and Climate Science. [UPDATE 12/11/2013: That page doesn't work anymore, but there are other links to these photos above.] You can also get more information on all of the different versions of Blue Marble on Wikipedia.
Although Blue Marble 2005 is the more recent and more detailed version, in my opinion the 2002 version is the most beautiful of all of them, although of course the original 1972 version is also wonderful because it is "real" and not a composite. I suppose that someone at Apple felt the same way since the 2002 version is the one included with the iPhone. If you changed your iPhone's default wallpaper then you probably won't see this image on your iPhone again unless you have a serious crash and the iPhone returns to a default state and commands you to connect your iPhone to a computer running iTunes. If that does happen to you and you find yourself starting to panic, perhaps you can distract yourself by thinking of all of the time and care that went into creating the beautiful image of Earth on your iPhone screen.
[UPDATE 1/26/12: NASA has now released Blue Marble 2012, an even higher definition (8000 x 8000) view of Earth created by combining several images taken by a NASA satellite on January 4, 2012. Looks like it is time for Apple to update the iPhone wallpaper!]
[UPDATE 5/23/16: In February of 2015, NASA launched the U.S. Deep Space Climate Observatory (DSCOVR) (Wikipedia link), a satellite which stays in the correct position to always see the sunlit side of the Earth. The camera on DSCOVR is called the Earth Polychromatic Imaging Camera (EPIC), and it takes a picture about once every two hours. Thus, we now have 12 new Blue Marble-style pictures of the Earth every single day. The first released set of images were taken on July 6, 2015, and you can see all of the pictures on NASA's website here. Some of them are particularly amazing, such as this series of pictures (and video) of the moon passing between EPIC and the Earth, taken July 16, 2015. It is fascinating to me that an image that originally was breathtaking in its uniqueness back in 1972 is now literally an everyday occurrence.]