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Adam Savage_How Simple Ideas Lead to Scientific Discoveries
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I totally get how the circumference of the earth was calculated but the speed of light one made my head hurt.

I rewatched the vid and I have to agree, circumference discovery I get easily, while the speed of light discovery I vaguely do. Whether I can even explain that is a whole other issue.

They wanted to know if light had a speed and in order to find that out they needed to know how fast it traveled over a the constant of the distance from the light source to the mirror and back. First though, they needed to make the light, which is basically a steady stream, pulse so, it could be measured so they used the toothed wheel to break it into pulses. The wheel must be a variable meaning that for the light to only pass through an opening, the spin rate would vary depending on the number of teeth on the wheel. Maybe like more teeth-slower spin rate and less teeth-high spin rate?

So, they sent the light through the wheel and back over their known distance correcting the spin rate until it was just right for the light to only pass through an opening, not hitting a tooth, thereby timing the spinning of the wheel to match the pulsing of the light.

By knowing the spin rate of the wheel, multiplying that by the number of teeth on the wheel and multiplying that by the distance traveled to the mirror and back, they could calculate the speed at which the light had to have traveled in order to have gone there and back without hitting any of the wheel's teeth but only hitting the next opening and then the next opening and so on.

Did I only make it worse?

You did not make it worse. You actually did a really good job of explaining it which is a feat considering the subject matter and the lack of a visual to demonstrate - you did it all with words and did it very well. Thank you, I understand it now. But if you test me, it will have to be an open book test where I'm allowed to refer to your explanation!

no test will be required, I'm just glad you think my verbals were up to the task, thank you.

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