Complex sentence: The speed of light was first measured in 1676 by the Danish astronomer Ole Christensen Roemer, who observed that the time it took light to come from Jupiter's moons varied depending on their distance from the earth.
Split into simpler sentences: The speed of light was first measured in 1676 by the Danish astronomer Ole Christensen Roemer. Roemer observed that the time it took light to come from Jupiter's moons were longer or shorter depending on their distance from the earth.