The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that, depending on the Earth–Sun–Jupiter geometry, there could be a difference of up to 1000 seconds between the predicted times of the eclipses of Jupiter's moons, and the actual times that these eclipses were observed. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance between these two planets varies. He obtained a value of c equivalent to 214,000 km/s, which was very approximate because planetary distances were not accurately known at that time.
In 1728 James Bradley made another estimate by observing stellar aberration, being the apparent displacement of stars due to the motion of the Earth around the Sun. He observed a star in Draco and found that its apparent position changed throughout the year. All stellar positions are affected equally in this way. (This distinguishes stellar aberration from parallax, which is greater for nearby stars than it is for distant stars.) To understand aberration, a useful analogy is to imagine the effect of your motion on the angle at which rain falls past you, as you run through it. If you stand still in the rain when there is no wind, it falls vertically on your head. If you run through the rain, it comes at you at an angle, and hits you on the front. Bradley measured this angle for starlight, and knowing the speed of the Earth around the Sun, he found a value for the speed of light of 301,000 km/s.
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The first successful measurement of c was made by Olaus Roemer in 1676. He noticed that, depending on the Earth–Sun–Jupiter geometry, there could be a difference of up to 1000 seconds between the predicted times of the eclipses of Jupiter's moons, and the actual times that these eclipses were observed. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance between these two planets varies. He obtained a value of c equivalent to 214,000 km/s, which was very approximate because planetary distances were not accurately known at that time.
In 1728 James Bradley made another estimate by observing stellar aberration, being the apparent displacement of stars due to the motion of the Earth around the Sun. He observed a star in Draco and found that its apparent position changed throughout the year. All stellar positions are affected equally in this way. (This distinguishes stellar aberration from parallax, which is greater for nearby stars than it is for distant stars.) To understand aberration, a useful analogy is to imagine the effect of your motion on the angle at which rain falls past you, as you run through it. If you stand still in the rain when there is no wind, it falls vertically on your head. If you run through the rain, it comes at you at an angle, and hits you on the front. Bradley measured this angle for starlight, and knowing the speed of the Earth around the Sun, he found a value for the speed of light of 301,000 km/s.
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