Mercury could be seen crossing the face of the sun on May 7, 2003. This video compresses the actual transit time of about five hours. Mercury aligns with the Earth and the sun 12 or 13 times a century. Far rarer: Venus could be seen crossing the sun last month for the first time since 1882. The transit of Venus in 1769 found observers around the globe—including Capt. James Cook in Tahiti—taking measurements in a joint effort to figure the distance from Earth to the sun. They were off by only two million miles. It's 93 million miles away, on average, and its light makes the trip in about eight minutes.