[Dudley]

Dudley asked the Naked Scientists:
   
How can we find a datum point in order to measure anything? At the centre of the milky way galaxy ? No, that wont do : it is not at the centre of the universe, as far as we know, but is there a centre of the  Universe?

How is light speed calculated, when we are on a moving planet?

What do you think?

[MikeS]

Light speed is an arbitrary measurement as there is nothing particularly fundamental about either the second or the meter.  The essential thing to remember is that light speed is the fastest anything can go and light always travels at that speed regardless of your frame of reference.  So it doesn't matter how fast you are travelling or where you are when you take the maesurement, the speed of light will always be the same.

Modified 02-10-11
It might make more sense if I put it like this.  As light experiences no passage of time it can be thought of as travelling at infinite speed.  Infinite plus or minus a finite amount is still infinite.

[simplified]

Dudley asked the Naked Scientists:
   
How can we find a datum point in order to measure anything? At the centre of the milky way galaxy ? No, that wont do : it is not at the centre of the universe, as far as we know, but is there a centre of the  Universe?

How is light speed calculated, when we are on a moving planet?

What do you think?

Relativity thinks that a moving planet and source exist in different times.

[burning]

Being on a moving planet isn't really that much of a problem.

The first calculation of the speed of light was done in the 17th century by Ole Rømer.  He was studying eclipses of the moons of Jupiter for reasons unrelated to measuring light speed.  He noticed that despite having apparently high precision measurements of the orbital periods of the moons, they wouldn't be where he expected them to be when he went back to repeat measurements months later.  He eventually figured out that the discrepancy was due to the changing distance between Earth and Jupiter.  When Jupiter was farther away, the light he was seeing was from longer ago, so the moon he was seeing was earlier in his orbit than he expected.

He was able to take this observation and turn it into a measurement of the speed of light because there were already good measurements that had allowed astronomers to calculate the sizes of the orbits of the planets.  So Rømer knew by comparing two periods of observation (a) the change in the distance to Jupiter and (b) the delay in seeing the moon at the expected position (i.e. how long it took the light to travel the extra distance).  The ratio of the two was the speed of light, and he got a pretty accurate answer.

Now the fact that Earth moves (and that Jupiter moves for that matter) could have caused Rømer problems if it moved fast enough that he couldn't make precision measurements of the orbital periods in the short term.  If the change in distance between Earth and Jupiter changed significantly between one orbit of the moon and the next, the whole thing would have been much more confusing.

[yor_on]

Very cool Burning.