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Comment by wwarner | Hacker News Reader

In 1676 Roemer estimated the speed of light by timing the orbit of Jupiter’s moon Io, noting that as the Earth approached Jupiter, Io emerged from behind Jupiter a little earlier every day, and as the Earth traveled away from Jupiter it appeared a little later every day, with the time of day varying by 22 minutes over a year. Knowing the difference between the two distances, he reckoned that light travels that distance in 22 minutes, or 227 thousand km/s. The actual speed is about 300 thousand km/s. Not bad!

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I always appreciate these stories about how very specific observations that most people would miss can give away far deeper details of the universe that many wouldn't even consider. Eratosthenes using shadows and figuring out the size of the earth within a few percent is another well known one.

https://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_o...

https://en.wikipedia.org/wiki/Speed_of_light#First_measureme...

Another interesting thing about using the timing of moon eclipses:

> Galileo proposed a method of establishing the time of day, and thus longitude, based on the times of the eclipses of the moons of Jupiter, in essence using the Jovian system as a cosmic clock. The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude.

https://en.wikipedia.org/wiki/Galilean_moons#Determination_o...

It's amazing to think that with nothing more than a telescope and careful timing, he managed to get so close to the actual speed of light.

Well, also knowledge of the distances between the objects involved (at least the Earth and Jupiter), which in turn depended on a series of further investigations.

Which is not to denigrate the achievement, but if I were to drop you on an alien world with only a telescope and an accurate time keeper, you're not going to be able to recreate it.

> if I were to drop you on an alien world with only a telescope and an accurate time keeper, you're not going to be able to recreate it.

But you can relatively easily derive the distances too with timekeeper and telescope, by applying Keplers laws?

One can, in principle. Most people lack the talent. (It's a skill that almost anyone could learn, but that doesn't make it a skill everyone has.)

That’s not the definition of talent

From what I've seen, talent is the tendency for a person to naturally develop a skill, if left alone to do so. It isn't some kind of intrinsic capability.

The speed of light, because it is so fast, is the most precise physical constant known : 299 792 458 m / s. Less than 7/1000ths away from 300,000,000 m / s. I am not going to sweat this in the least.

So light travels only 0.3 m / nano second, or 11.802 inches.

https://www.youtube.com/watch?v=9eyFDBPk4Yw

Armed with this fact thinking about electronic devices. How the front of the signal travels and how suddenly the distance matters. Was blown away when I first thought about this.

Therefore in one cycle of a 3GHz processor, light travels about 4 inches. Wild.

Which is maybe part of the reason RAM needs to be so close to the CPU? Granted though most RAM access takes several CPU cycles.

This is exactly it, the longer your traces (or any sort of write I guess) between processor and memory, the longer delay you have to have for things to "settle". Additionally increasing capacitance means that it takes more effort (read, even longer) for the line to settle.

Does make me realise I don't know so much about the super low level parts of it beyond electrons and holes. One end of the wire needs to be at a lower potential to encourage electrons to flow from that side to the other. Hmmm.

My favorite riff on this: at a previous job we worked on a 12GHz SERDES, which meant each inch of the cable had a different data bit on it

Speed of light has recently been redefined, and is now _exactly_ 299792458 m/s. It’s no longer a measurement, it’s a definition.

That's a little misleading: either the meter or the second has been redefined to make c = 299792458 m/s.

The second is (and has been) defined independent of length for a while. It's the time it takes for a certain number oscillations of a caesium atom.

The meter was redefined as the distance light travels in a specific time. So you could say that either the meter or the speed of light was redefined to make the speed of light a round constant, but not the second.

Since 1983, the meter is defined in terms of the distance traveled by light in a vacuum during 1 second, see https://en.wikipedia.org/wiki/Metre#Speed_of_light_definitio...

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