Around the Earth in 200ms A month or two back, i saw an interesting figure on Bram Cohen's blog:

“The speed of light in a fiber optic cable around the earth’s circumference is about 200 milliseconds.”
(from here)

I clipped it for my ever expanding Evernote tech tips, thinking it's one of those useful metrics to know. I've referred to it a few times now, but I always like to verify things myself, so this morning I looked up the relevant data -

So - speed of light in a vacuum is 186,000 miles per second. However according to this wikipedia article, the index of refraction for the cladding of an optical fiber is 1.52. “From this information, a good rule of thumb is that signal using optical fiber for communication will travel at around 200 million meters per second”.

Ok, so 200, 000,000 meters / second = 200, 000 meters / ms

“The circumference of the earth at the equator is 24,901.55 miles (40,075.16 kilometers).” // from here

40,075.15km = 40,075,000 meters

With all figures then, Earth Circumference is 40,075,000 meters, and the speed of light in fiber is 200,000 meters per ms:
40,075,000 /2 00,000 = 200.375 ms

// or to be even smarter, I could have just followed the Wolfram Alpha link from Bram's blog here - gotta love the Wolfram //