The corona is the sun's outer atmosphere. It stretches many thousands of kilometers (miles) beyond the Sun's apparent "surface," gradually converting into solar wind, which travels outward across our solar system. The material in the corona is a tremendously hot yet tenuous plasma. Although the corona can be as dense as the solar wind, it usually isn't. People have wondered about the existence of the corona for centuries. In 1610, William Harvey described what he called the "coronam agitur"--or "the moon is agitated by the sun"--a statement that has been interpreted to mean that there is a fluid surrounding the moon that gets stirred up when the moon comes into contact with the sun's gravity. In 1715, James Ferguson observed that some stars appear to move across the night sky and concluded that these must be planets orbiting the sun outside of Earth's orbit. He suggested that if other stars were found to move similarly they might be drawn out toward the edge of the solar system where they would be able to see a different part of the sun's surface. It wasn't until 1877 that another scientist came up with the idea that we are not alone. On August 2 of that year, German astronomer Richard Carrington published his findings about a major solar event that had occurred on August 1st.