This composite image of observations by NASA and the ESA's Solar and Heliospheric Observatory shows the path of Mercury during its November 2006 transit. On Monday, May 9, 2016, the solar system's smallest, innermost planet will resemble a black dot as it passes in front of the Sun. NASA says the event occurs only about 13 times a century. (Photo by Solar and Heliospheric Observatory/NASA/ESA via AP Photo)
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