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Author Topic: How was the speed of light measured 400 year ago?  (Read 4891 times)

Offline waytogo

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Tell me something please...


 

Offline imatfaal

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Re: How was the speed of light measured 400 year ago?
« Reply #1 on: 12/09/2012 11:27:02 »
http://en.wikipedia.org/wiki/R%C3%B8mer%27s_determination_of_the_speed_of_light

This is the first measurement that bears close historical and scientific scrutiny
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #2 on: 12/09/2012 16:12:49 »
Thanks, I know wiki and that link, but it does not explain the Q.
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #3 on: 12/09/2012 16:45:44 »
I forgot this: I guess that it was used a math formula to calculate the stuff, so where is it?
« Last Edit: 12/09/2012 16:48:13 by waytogo »
 

Offline Soul Surfer

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Re: How was the speed of light measured 400 year ago?
« Reply #4 on: 13/09/2012 00:13:05 »
The results do not come from a precise formula but precise observation and prediction.

You must first realise that astronomers have been able to make quite precise measurements of times and positions of events in the sky for many centuries.  The greeks well BCE were able to measure the size of the spherical earth and the approximate distance to the moon.

Romer's determination of the speed of light depended on reasonably accurate time measurements of precisely timeable events in the motions of the four large satellites of Jupiter.  These events can be predicted quite accurately using  standard orbit and geometrical theory.

However the relative motions of the Earth and Jupiter mean that jupiter is observable at a range of distances equal to the diameter of the earth's orbit.  this results in the timing of events varying by around 16 minutes and gives a first approximate measurement of the speed of light
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #5 on: 13/09/2012 00:22:08 »
That does not explain how.
« Last Edit: 13/09/2012 00:26:05 by waytogo »
 

Offline imatfaal

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Re: How was the speed of light measured 400 year ago?
« Reply #6 on: 13/09/2012 11:35:51 »
waytogo - you are gonna have to put a bit of effort in yourself.  we can explain and help you get over sticking points - but the wikipage does explain Romer's idea
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #7 on: 13/09/2012 13:11:17 »
Again, wikipedia is useless in that case (and many others as well).

 Why that stuff is not explained at all? that's the point.
« Last Edit: 13/09/2012 14:27:52 by waytogo »
 

Offline neilep

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Re: How was the speed of light measured 400 year ago?
« Reply #8 on: 13/09/2012 18:28:33 »
This is so frustrating....I only read a few days ago  the most wonderful article regarding the entire history of the measurement of the speed of light and I was in two minds about reposting it here......at the time...I decided no because I was pushed for time...........I wish i did now because I just can't locate it anywhere............arrrrghhhhhhhh !!
 

Offline yor_on

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Re: How was the speed of light measured 400 year ago?
« Reply #9 on: 13/09/2012 18:35:57 »
You will find it Neil..

Or we will make you an offer you can't refuse.
 

Offline neilep

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Re: How was the speed of light measured 400 year ago?
« Reply #10 on: 13/09/2012 18:48:17 »
You will find it Neil..

Or we will make you an offer you can't refuse.

I've been searching high and low chum !!

It was on one of the many science communities on facebook and it's since scrolled out of view and I can't recall whcih community it was (I'm still looking now)...

....ewe can still make me the offer though !  ;)
 

Offline neilep

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Re: How was the speed of light measured 400 year ago?
« Reply #11 on: 18/09/2012 23:29:04 »
FOUND IT :

SOURCE: https://www.facebook.com/fromquarkstoquasars

Light is something we often take for granted. In fact, the only time we ever really think about light is when there isnít any, a thought that usually precedes running into something. But what about light itself?

As one of the most fundamental ways of expressing energy, light is extremely important in physics. One of the most elementary properties of light is itís speed, usually expressed by using the letter ďcĒ. Even though Einstein found what is perhaps the most useful property of the speed of light with his famous equation E = mc^2, the search for lightís speed started long before Einstein.

The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a humanís ability to distinguish a discrepancy in time.

Near the late 17th century, the astronomer RÝmer conclusively proved that light had a finite speed, but he underestimated the speed of light by approximately twenty-six percent. RÝmer started by assuming that the speed of light was so massive that it would take less than a second to span a distance equivalent to the Earthís diameter. He was correct of course, as light can travel many times that distance in a single second. RÝmer used his approximations for the direction and velocities of multiple bodies (though primarily Jupiter, Io, and the Earth) to conclude that it would take light approximately twenty-two minutes to span the distance equivalent to the diameter of Earthís orbit around the sun. This leads to a value of about 220,000 kilometers per second, as opposed to lights actual speed of just under 300,000 kilometers per second.

A few decades later James Bradley devised another way to test the speed of light, and estimated it with a much higher degree of accuracy than had been done before. Bradley was the first to discover the aberration of light, a phenomenon where objects appear to be in a different location due to the time delay between seeing the light and when the light was reflected. By using this principal Bradley estimated the speed of light to be 301,000 kilometers per second by comparing the speed of light to the speed of Earth in its orbit. According to his research, light travels 10,066 times faster than the Earth does when orbiting the sun, and from there he calculated the speed.

In the early 19th century, Hippolyte Fizeau found a new way of attempting to measure the speed of light by using time-of-flight measurements of Earth. His study was substantially less accurate than Bradleyís, with an estimated speed of 315,000 kilometers per second, however his method lead to the more precise attempt of Lťon Foucault. When Foucault tried his modification of Fizeauís method he came up with an estimated 298,000 kilometers per second.

Just after the start of the 20th century, yet another attempt at measuring the speed of light was undertaken by Rosa and Dorsey. Their method allowed them to measure lightís speed without directly depending on a measurement of the propagation of electromagnetic waves (In other words, they werenít directly observing light itself). Instead they used an electrical principal explained in one of Maxwellís equations. They were able to estimate lightís speed at 299,710 kilometers per second, which is off by a mere 82 kilometers per second, and was by far the most accurate measurement of the day.

From 1926 to 1950, there were three additional attempts to measure lightís speed. Albert Michelson was the first to test lightís speed in that time, and he did so using a test remarkably similar to the very first proposed test by Isasac Beeckman. However Michelsonís test had a distance of approximately 22 miles and substantially more precise equipment. His test had an outcome of 299,796 kilometers per second. A few years later Louis Essen and A.C. Gordon-Smith calculated the speed of light in a cavity resonator. They had extremely accurate results and calculated lightís speed to be 299,792.5 kilometers per second. This was off by 0.042 kilometers per second.

K. M. Evenson is primarily credited with created the most accurate means of testing the speed of light, which he demonstrated in the 70s. Evensonís method relies on the separate measurements of wavelength and frequency emitted by a stabilized laser. Evenson was able to get a value of 299,792.4562 meters per second, the most precise approximation of lightís speed to this day. That value is off by 0.0018 meters per second.

~Alan

Sources:
Huygens, C (1690) (in French). Traitťe de la LumiŤre. Pierre van der Aa. pp. 8Ė9.
Bradley, J (1729). "Account of a new discoved Motion of the Fix'd Stars". Philosophical Transactions 35: 637Ė660.
Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Retrieved 2010-01-13.
Gibbs, P (1997). "How is the speed of light measured?". Usenet Physics FAQ. University of California, Riverside. Retrieved 2010-01-13.
Essen, L; Gordon-Smith, AC (1948). "The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator". Proceedings of the Royal Society of London A 194 (1038): 348Ė361. Bibcode 948RSPSA.194..348E.
Rosa, EB; Dorsey, NE (1907). "The Ratio of the Electromagnetic and Electrostatic Units". Bulletin of the Bureau of Standards 3 (6): 433. Bibcode 1906PhRvI..22..367R. DOI:10.1103/PhysRevSeriesI.22.367
Michelson, A. A. (1927). "Measurement of the Velocity of Light Between Mount Wilson and Mount San Antonio". The Astrophysical Journal 65: 1. Bibcode 927ApJ....65....1M. DOI:10.1086/143021.
Evenson, KM; et al. (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters 29 (19): 1346Ė49. Bibcode 1972PhRvL..29.1346E.
 

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Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #12 on: 22/09/2012 23:02:30 »
Shrunk
FOUND IT :

SOURCE: https://www.facebook.com/fromquarkstoquasars


The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a humanís ability to distinguish a discrepancy in time.


That's simply ridiculous and even do not explain nothing, the other one is the same story posted here but again, it is useless (the correct term would be disinfos).
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #13 on: 24/09/2012 15:05:26 »
FOUND IT :

SOURCE: https://www.facebook.com/fromquarkstoquasars

The first known proposed test was by Isaac Beeckman, who suggested in the early 1600s that a man with a stopwatch could sit atop a hill with a cannon, with a large mirror facing him approximately one mile away. Beeckman believed that when the cannon was fired, the man could start the stopwatch and stop it again after seeing the flash of light off of the mirror. Obviously this method is extremely flawed, and when a similar test was conducted by Galileo it yielded no results. Since then, we have calculated the time delay between the cannon firing and the mirror returning the flash to be approximately 11 microseconds, well below a humanís ability to distinguish a discrepancy in time.


Hi there, thanks but that does not explain how, don't you? and with which tools they've misured 11 microseconds?
 

Offline yor_on

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Re: How was the speed of light measured 400 year ago?
« Reply #14 on: 27/09/2012 23:43:18 »
Thnx Neil..
 

Offline waytogo

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Re: How was the speed of light measured 400 year ago?
« Reply #15 on: 21/11/2012 10:50:35 »
Anyone?
 

Offline Phractality

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Re: How was the speed of light measured 400 year ago?
« Reply #16 on: 21/11/2012 21:55:21 »
In 1612, scientists were still debating over whether the speed of light is finite or infinite. The cannon flash reflecting back to its source by a mirror was too short an interval to be measured with a stop watch. So the simple answer to the question is, "It wasn't."

With 20/20 hindsight, we can now devise an experiment which could have been done back then, using a spinning cylinder to send short pulses of sunlight which reflect back at the source by a corner reflector. (The corner reflector wasn't invented until 1938, but they could have easily made one in 1612. A big mirror would work well enough, though.) A very bright light source would shine thru a slit in the spinning cylinder and travel to the corner reflector and back. They didn't have carbon arc lamps, back then. Burning magnesium would have worked, except magnesium hadn't even been discovered, let alone refined. I'm not sure what light source they had that would have been bright enough. Sunlight might work if they could block out the ambient light. It's difficult enough to see a heliograph several kilometers away; try seeing a reflection of a heliograph thru a crude telescope.

Let's assume you would be able to see the reflected light by looking thru the same slit with Galileo's newly invented telescope, after the slit has rotated thru a certain angle. If the cylinder rotates 100 revolutions per second, and the reflected light is seen 1/100th revolution away from the light source, then the delay is 1/10,000 second. If the reflector is 15 km away, that would tell you that the speed of light is roughly 300,000 km/s.

Sounds easy with today's technology that we take for granted, but a lot of it hadn't been invented, yet. Today, we can use a laser light and count interference fringes to measure the delay in picoseconds.
 

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Re: How was the speed of light measured 400 year ago?
« Reply #16 on: 21/11/2012 21:55:21 »

 

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