NASA invites you -- and everyone else on the planet -- to take part in a worldwide celebration of Earth Day this year with the agency's #GlobalSelfie event.  While NASA satellites constantly look at Earth from space, on Earth Day we're asking you to step outside and take a picture of yourself on Earth. The individual pictures tagged #GlobalSelfie will be used to create a mosaic image of Earth -- a new "Blue Marble" built bit by bit with your photos. For more information on how to participate, go to http://1.usa.gov/PfjXln