Earth images from NASA (PHOTOS)

Earth Day, which is celebrated each year on April 22, is a day to promote awareness and appreciation for the Earth's environment. It was founded by U.S. Senator Gaylord Nelson (D-Wisconsin) as an environmental teach-in in 1970 and is celebrated in many countries every year.

At present, Earth Day is observed in 175 countries, and coordinated by the nonprofit Earth Day Network. The passage of the Clean Air Act, the Clean Water Act, and the Endangered Species Act are considered to be products associated with the 1970 Earth Day.

Earth Day is the annual celebration of the environment and a time to assess the work still needed to protect the natural gifts of our planet. It affirms that environmental awareness is part of our consciousness and that the idea of protecting the environment has moved into the mainstream.

In 2011, Earth Day's theme is A Billion Acts of Green. In honor of the Earth Day, Internet search engine Google Inc. has created an animated doodle depicting Earth as a green planet with flora and fauna, clear water and blue sky. Ticklish pandas, frolicking penguins, a growling lion, a cascading waterfall, waterfall climbing fish, a fish devouring bear, birds, butterflies, a koala and a jumping frog get together on the Google doodle.

On this Earth Day, it is time for us to lay a new foundation for economic growth by beginning a new era of energy exploration in America, President Barack Obama said Wednesday, according to remarks prepared for delivery in Newton, Iowa, where he traveled to mark Earth Day and tour a former Maytag plant that is now used to produce towers for wind-energy production.

Earth Day is every day at NASA. NASA's Earth Science Mission seeks to understand Earth's systems and their responses to natural and anthropogenic (human-made) changes.

A fleet of satellites in NASA's Earth Observing System gives scientists the global, long-term measurements they need to connect the atmosphere (air), lithosphere (land), hydrosphere (water), cryosphere (snow/ice), and biosphere (life) as a single system.