2 Answers

186 thousand miles per second how they managed to measure the speed though is beyond me

Here is an interesting article.

Trying to Measure the Speed of Light

James Clerk Maxwell's theory of electromagnetism tells us that light is an electromagnetic wave traveling at 300,000 kilometers per second (kps). Maxwell's equations tell us that changing electric and magnetic fields create and sustain each other even in regions where there are no electric charges to accelerate or magnets to move. Maxwell showed how these two fields, interlocked in a dance, create their own light show. The fields spread out through space as light or as any other electromagnetic wave.

But before Maxwell, other scientists made their own attempts to identify the nature of light and to calculate its speed. 

Now read on - 

http://www.dummies.com/how-to/content/trying-to-measure-the-speed-of-light.html