system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How the speed of light was calculated.

How would a scientist calculate or theorize the unmaginable speed of light?
gpt
The first *direct* measurement involved a rapidly spinning cog and a mirror very far away.  The idea is to pass a beam of light between the gap between two teeth, have it travel back.  If the cog is spinning fast enough, the light hits the next tooth instead of the gap.  And if it spins even faster, it hits the next gap.  If you know the precise distance and speed of rotation, you can compute the speed of light.

This was first done in 1849, and computed the speed to be 315,000 km/s, which is about a 5% error.