Then, in 1676, a Danish astronomer named Ole Roemer saw something interesting in the sky. He was watching the moons of Jupiter go around — something fairly easy to see even with a primitive telescope. He started marking down the exact times when one moon went behind Jupiter and when it came back into view again. To his amazement he found that these eclipses occurred later and later as the year went on. Can you guess why? Roemer correctly concluded that Jupiter’s orbit took it farther away from the earth at some parts of the year. Because it took time for light to travel from the moons to Roemer’s telescope, the farther away they were, the later the eclipses seemed to be taking place! Roemer’s observations were among the first to demonstrate that light does indeed take time to travel.