c by starlight

[BillyJoe]

How can the speed of light be measured using the light from a distant star?

(You could obviously use starlight like any other source of light to measure the speed of light in the ususal way but this method uses starlight specifically.)

[Dub]

c is only the speed of light in a vaccum. As most of space is a vaccum star light would be a good source to use. Although im not sure how its done.

[BillyJoe]

Originally posted by Dub
c is only the speed of light in a vaccum. Okay, there will be a slight error due to the Earth's atmosphere and gravity.
Originally posted by Dub
As most of space is a vaccum star light would be a good source to use. Although im not sure how its done. Do you want a clue?

[rwald]

Doesn't relate to gravitational pertebution, I guess? Maybe some weird thing with pulsars, and looking at the redshift...nah. I've got no idea. I suppose just having two stations on earth with atomic clocks and lasers or photon sensors isn't right either?

[BillyJoe]

Originally posted by rwald
....gravitational pertebution?.....Maybe some weird thing with pulsars, and looking at the redshift?..... two stations on earth with atomic clocks and lasers or photon sensors? None of those, at least not the method I'm looking for.

Want a clue?

[Skeptoid]

Is this the method you have in mind? Lifted from discovery.com (http://www.discover.com/ask/main9.html).

[ B]ritish astronomer James Bradley ... obtained a very good estimate for the speed of light. Since the speed of light is finite, starlight falling on the Earth will exhibit a slight angular displacement depending on the direction the Earth is moving (in its orbit). Bradley knew the orbital speed of the Earth to be approximately 30 kilometers per second, roughly 1/10,000 of the known value of the speed of light. By measuring, at different points in the Earth's orbit, the angular displacement of starlight falling from a given star Bradley was able to estimate the speed of light to within 1 percent of the currently accepted value.

[BillyJoe]

****, Skeptoid, you weren't supposed to go googling to find the answer :mad: .
Perhaps you can redeem yourself by explaining the "angular displacement" bit :cool: .

[BillyJoe]

.....and no googling.

[Skeptoid]

Without googling, huh? Let's see ... The angular displacement is what astronomers call the aberration of light. It is the apparent shift in position of a star due to Earth's motion around the sun. It is roughly the ratio of the Earth's speed to c. Thus, if one knows the angular displacement and the Earth's speed, one can estimate c. An astronomer has to adjust his telescope to compensate for the angular displacement in much the same way as a person tilts his umbrella forward when walking through the rain.

Best I can do from memory. Please correct me if I'm in error.

[BillyJoe]

Sounds good.

Here's another analogy...
A sailboat has the wind coming in straight over the beam. The flag at the top of the mast is flying at right angles to the boat indicating the true direction of the wind. However, as the boat sails forward, the wind seems to shift towards the direction in which the boat is heading. The faster the boat travels, the greater the shift. Of course, to set his sails, the sailor uses the apparent direction of the wind not the true direction of the wind (just as the astronomer sets his telescope towards the apparent position of the star)

I'm a bit surprised this thread didn't create more interest. Oh well.

[xouper]

bump

[Skeptical Greg]

Originally posted by BillyJoe

I'm a bit surprised this thread didn't create more interest. Oh well.

Interest?!!!

Darn, I was just preparing a paper on this ( which I thought was very original material )....


At least you have saved me a lot of trouble... Thanks..

[BillyJoe]

Obviously the best thread on the forum.....I'm still getting thanks for it after twelve months! :)