PHOENIX-- In honor of Earth Day, NASAput together a special photo gallery of some of the most incredible pictures it has of our planet. While most people celebrate Earth Day once a year, every day is Earth Day for the teams at NASA.

The agency has a fleet of 13 Earth-orbiting satellites and many research aircraft. NASA's collective eyes on Earth are pioneering scientists' ability to see our home planet around the clock in order to better understand how it works.

More than just showcasing inspiring views of our the planet, NASA's advanced scientific spacecraft are dedicated to unraveling the complex factors that drive Earth's climate system and providing far-reaching benefits to society in areas such as disaster management, weather forecasting and agriculture.

For over 40 years, Earth Day -- April 22 -- has inspired and mobilized individuals and organizations worldwide to demonstrate their commitment to environmental protection and sustainability.

The first Earth Day, on April 22, 1970, activated 20 million Americans from all walks of life and is widely credited with launching the modern environmental movement. The passage of the landmark Clean Air Act, Clean Water Act, Endangered Species Act and many other groundbreaking environmental laws soon followed.

As part of Earth Day 2011, is encouraging people around the world to take action and pledge A Billion Acts of Green.

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