The second in the Top Eleven is the first quantitative measurement of the speed of light, by Ole Christensen Roemer (whose last name ought to contain an o-with-a-slash-through-it, that I’ve rendered as an “oe”).
Who: Ole Roemer (1644-1710), a Danish astronomer.
When: The crucial observations were made around 1675.
What: Roemer made careful observations of the orbit of Jupiter’s moon Io (which circles the planet once every two days or so), and noted that the time between eclipses of Io (times when it disappeared behind Jupiter) got shorter as the Earth moved closer to Jupiter, and got longer as the Earth moved farther away. The shift was pretty small (tens of minutes), but large enough to be accurately measured.
Roemer correctly deduced that the difference in times was a consequence of the finite speed of light. As the Earth and Jupiter get closer together, it takes less time for light to travel from Jupiter to Earth, and thus the time bewteen observations of eclipses gets shorter. As the Earth and Jupiter move farther apart, the travel time gets longer, and the time between observations increases.
Why It’s Important: Just the idea that the speed of light is finite, rather than infinite is an important development in physics (and was moderately controversial for a good many years). It’s also the first reasonably precise measurement of a constant of nature– he was hampered by the lack of a good measurement of the distance from the Earth to the Sun, but his method gives a good value for the speed of light.
Reasons to Vote for Him:: First measurement of an important physical constant.
Reasons to Vote Against Him: Didn’t actually get a single number (though Huygens took his measurements and got a pretty good value for c). Has a name with non-standard characters in it.