But, what is a #GlobalSelfie? Put simply, it's a selfie. As NASA explains it, a #GlobalSelfie is a self-taken portrait of Earth's inhabitants. Snap a selfie and post it to one of five social media networks, including Twitter, Instagram, Facebook, Google+ and Flickr, with the hashtag #GlobalSelfie.
Anyone with a camera and an internet connection can participate. When you take a picture of yourself and your friends for NASA's #GlobalSelfie Earth Day celebration, make sure to include a sign (NASA has several available) telling NASA and other viewers where you are on the planet.
NASA suggests trying to get a glimpse of your surroundings in the picture: "Get outside and show us mountains, parks, the sky, rivers, lakes -- wherever you are, there's your picture."
Of course, you don't have to use a NASA-designed paper sign. You can get creative: spell out your location in the sand, or by arranging rocks or sticks.
"Once those pictures stream around the world on Earth Day," NASA explained on their website, "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."
Below are some of the best #GlobalSelfie pictures posted to Twitter so far: